Sweetpotato (Ipomoea batatas, L.) is one of the important sources of carbohydrates and economic income in Mozambique. As with most of the food crops in Mozambique, it is usually produced by small-scale farmers under dryland conditions. Despite the importance of the crop, the storage root yields are still low and it is difficult to keep planting material (vines) for the next planting season. One of the major challenges to production is drought stress. Drought stress affects sweetpotato by retarding aboveground growth, reducing total root yield, percentage of dry mass, and reducing the quality of the roots as a result of the increase in damage caused by the sweetpotato weevil (Cylas formicarius). The objective of this study was to identify sweetpotato genotypes tolerant to drought particularly amongst the orange fleshed types which can be used in breeding programmes to improve the drought tolerance of genotypes grown in Mozambique. To this end, 48 genotypes were evaluated in both field and greenhouse studies conducted at Umbeluzi Research Station (26º 03’ S, 32ºC 23’ E; 12 masl) located about 30 km from Maputo city. The field trial was a three replicate, α-design with split-plots. Genotypes were the wholeplot treatment factor and irrigation levels were the sub-plot treatment factor. The three irrigation levels imposed were: nonstressed plants irrigated from planting to 120 DAP; moderately stressed, plants irrigated until 60 DAP; and severely stressed, plants irrigated until 30 DAP. In the greenhouse trial the 48 genotypes were grown in wooden boxes arranged in a two replicate, randomized complete block design. The plants were exposed to water stress from 10 DAP to the end of experiment at 60 DAP. Genotypes were significantly different for all traits, namely: survival %, vine vigour, aboveground biomass, total and commercial root yield, total fresh biomass, harvest index, β-carotene content, % dry mass, dry mass yield, incidence of sweetpotato virus disease, and incidence of weevil damage. Irrigation levels were significant for the traits: survival %, vine vigour, aboveground biomass, total and commercial root yield, total fresh biomass, harvest index, β-carotene content, % dry mass, and dry mass yield. Irrigation levels were not significant for incidence of sweetpotato virus disease and incidence of weevil damage. The genotypes x irrigation levels interaction was significant for: total and commercial root yield, and incidence of weevil damage; and not significant for: survival %, vine vigour, aboveground biomass, total fresh biomass, harvest index, β-carotene content, % dry mass composition, dry mass yield and incidence of sweetpotato virus disease. The mean dry mass yields across irrigation levels of the national breeding lines and introduced genotypes were higher than the landrace genotypes. Most of the national breeding lines had higher β-carotene content than the introduced and landrace genotypes. The landrace genotypes had relatively higher % dry mass composition compared to the national breeding lines. The stress tolerance index (STI) separated the 48 sweetpotato genotypes evaluated in the field trial into three groups: drought tolerant (high STI); moderate drought tolerant (intermediate STI); and drought sensitive (low STI). Under moderate stress, yield potential (Yp) and yield in a stress environment (Ys) were highly significant, positively correlated with Mean productivity (MP), Geometric mean productivity (GMP), Stress tolerance index (STI) and Tolerance index (TOL). Under severe stress the same correlations were reported. Under moderate and severe stress, the correlation between stress tolerance index (STI) and Stress susceptibility index (SSI) was significant and negative. In the greenhouse trial, differences between genotypes in vine length increment, vine diameter increment, leaf width increment and number of nodes vine-1 were significant (P<0.05). Vine length, vine diameter, leaf width and length increments either increased or were reduced due to water stress. Less than 10% increment in vine length (between 25 and 50 DAP) was recorded in MGCl01, Atacama, Cordner, Beauregard, and CN1448-49. Higher than 40% vine length increment was recorded in Jonathan and UNK-Malawi, Naspot, MUSG0614-24, Resisto, K566632, Tainung64, Ejumula and MUSG0623-09. Vine diameter decreased in Manhissane and MUSG0616-18. No change in leaf length in Tacna and Jonathan and in leaf width in Xihetamakote and Resisto-Nairobi was recorded. The longest petiole length at 30 DAP was recorded by Tacna and the shortest by Nhacutse4. The longest internode length was recorded in 199062.1. Similar to petiole length, Nhacuste4 reported the shortest internode length. The highest number of primary vines was recorded by MUSG0608-61 and lowest by Beauregard. Of the 48 genotypes exposed to water stress, 18 survived until the end of the greenhouse experiment at 60 DAP and were therefore considered to be drought tolerant. / Thesis (M.Sc.Agric.)-University of KwaZulu-Natal, 2011.

Low soil nitrogen (N) and drought impede maize production in the small-scale farming sector in Zambia; and adoption of new cultivars with improved tolerance might enhance production. This study: a) assessed farmer preferences for maize cultivars; b) determined genotype x environment interaction effects among popular maize cultivars under contrasting soil fertility levels and; c) investigated landraces for tolerance to low N and drought using S1 selection. The study was carried out in Zambia from 2004-07. Farmer preference influencing the adoption of maize cultivars was investigated using both formal and informal surveys in Luangwa, Chibombo and Lufwanyama rural districts representing the three agro-ecological regions of Zambia. Focus group discussions and personal interviews were used to collect data on issues that affected maize production in these areas. It has been found that although farmers perceived landraces to be low yielding, they believed that they were superior to improved cultivars for: tolerance to drought; tolerance to low soil fertility; grain palatability; grain storability; and poundability. The need for food security, their inability to apply fertiliser, and their need for drought tolerant cultivars significantly (p ≤ 0.05) influenced farmers in adopting cultivars. The farmers would readily adopt cultivars that address these concerns. The predominant use of certain landraces (76%) reflected their superiority in meeting some of these needs. The performance of nine popular cultivars (three for each of hybrids, OPVs and landraces) under contrasting levels of soil fertility, across six environments (ENVs) in the three agro-eological regions, was evaluated. An ENV was defined as season x location combination. The fertilizer treatments were full fertilization, basal dressing, top dressing and nil fertilization. The cultivars exhibited significant non-crossover type of genotype x fertilisation interaction effects at three ENVs, while the genotype x fertilisation interaction effects, were non-significant at the other three ENVs. The cultivars exhibited dynamic stability by increasing grain yield (GY) when fertilization was increased. Landraces yielded higher than all open pollinated varieties and were generally higher yielding than two hybrids. Based on average rank for GY, the five highest yielding cultivars were MRI724, Gankata, MM603, Kazungula and Pandawe. Superiority of landraces revealed their genetic potential for GY under low soil fertility and they should be used as germplasm in developing cultivars targeting such environments. Ninety-six local landraces were selfed to generate S1 lines (2004/05 season) which were crossed to a tester (2005/06 season). Testcrosses were evaluated under optimal, low N, and drought conditions (2006/07 season). Data on GY, anthesis-silking interval, number of ears per plant, leaf senescence, leaf rolling, tassel size and grain texture were recorded in all the trials during the study period. Testcrosses, their S1 parents and landraces that were superior under low N, drought, optimal conditions and across environments were identified; these should be used to develop varieties targeted to a particular environment. Selection for tolerance to drought also selected for tolerance to low N. Selection for low N tolerance also selected for GY under drought and optimal conditions. Therefore, in selecting for tolerance to abiotic stresses, use of optimal and managed stress environments was effective. The following landraces were superior at 10% selection intensity: LR38, LR84 and LR86 (optimal, low N and drought conditions); LR11, LR35 and LR76 (low N and drought conditions); LR12 (optimal and drought conditions); LR40 and LR93 (low N conditions only); LR79 (drought conditions only) and; LR74 and LR85 (optimal conditions only). These landraces should be used as source germplasm targeting respective environments. Significant (p ≤ 0.05) positive general combining ability effects for GY under both low N and drought conditions were found implying that additive gene action conditioned GY under the abiotic stresses. The heritability for GY under low N (0.38), and drought (0.17) conditions, was low suggesting that selection based on GY alone was not effective. The genetic correlation for GY between optimal, and either low N (rG=0.458), or drought (rG = 0.03) environments, was low (rG < 0.5) suggesting that indirect selection would not be effective either. Therefore, use of secondary traits for selection is discussed. The study established that most farmers depended on local landraces for seed and would adopt low input improved varieties that yield higher than the landraces. Some landraces were found superior to some improved cultivars under contrasting fertilisation regimes. The study also found that landraces had genetic variation for tolerance to low N and drought. Landraces, S1 lines and testcrosses superior under low N, drought, optimal conditions and across environments were selected and they should be used to develop cultivars targeting respective environments. Policy implications of these results are discussed. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2007.

Rice production in Sub-Sahara Africa is hampered by drought stress, low farmer adoption of improved varieties, un-adapted germplasm, problems of crossability between species and problems of efficient emasculation. This study was initiated to: (1) determine farmer preferences for rice varieties and production practices, (2) identify drought tolerant parental lines for use in interspecific crosses, (3) determine crossability between different rice species, (4) identify drought tolerant interspecific segregants from crosses, and (5) study the efficacy of ethrel and gibberrilic acid (GA3) as chemical hybridising agents. Participatory rural appraisal (PRA) was conducted in Sikasso region in the month of September 2005 to determine farmer preferences for varieties and production constraints. A total of 125 rice farmers from 10 villages participated. This study found that farmer preferences for rice traits did not coincide with the breeders' objectives and trait preferences varied across ecologies. In upland and lowland rice ecologies, the most preferred characteristic was short duration. In the lowland tall plants was most preferred and short duration trait ranked second. Long duration ranked as the second most preferred trait after yield in the irrigated rice ecology. The estimated grain yield reduction was 60% due to drought stress from all ecologies. Sixteen genotypes including seven Oryza glaberrima Steud, six interspecific (0. saliva x 0. glaberrima) inbred genotypes and two improved 0. saliva L. subsp Gaponica) genotypes, with OS 6, as drought tolerant check were screened for drought tolerance at the vegetative stage during the dry seasons of 2004 and 2005 at Samanko research station, in Mali. Most of the genotypes could be regarded as exhibiting drought tolerance and avoidance mechanisms for most of the morpho-physiological characters under study. High selection index was observed for WAB 450-I-B-P-103-HB in the two season experiments. Drought stress tolerance (little leaf drying) was significantly correlated with leaf rolling, leaf area index, and plant height. Fast drought recovery was significantly correlated with drought tolerance, tiller number and leaf area index. Effects of genotypes, environment and their interactions were significant for tiller number, root dry weight and relative leaf water content. The most stable genotype was RAM 3 for tiller number and relative leaf water content. Thirteen genotypes were selected on the basis of these traits for rice hybridisation. A factorial arrangement involving three levels of gametocide concentrations and five application stages of the female rice parent was undertaken for each gametocide ethrel and GA3 for two seasons. Per cent empty spikelets and male sterility increased with concentration of ethrel. Male sterility induction of 41.5% and 42.8% at 2000 ppm was achieved in 2004 and 2005, respectively, while at 4000 ppm male sterility induction was 40.5 % and 46.1% in 2004 and 2005, respectively. The best application stage of ethrel that induced highest male sterility was at P4 (panicle primordium differentiation + spikelet primordium differentiation) of 46.6% in 2004 and 49.2% at PI (panicle primordium differentiation) in 2005. There was no measurement on GA3 for sterility due to lodging. Total male sterility was not achieved therefore, these gametocides could not be recommended for use as hybridising agents. Crossability was studied in four groups: group A (0. glaberrima x 0. saliva), group B (0. glaberrima x Interspecific), group C (Interspecific x 0. saliva) and group D (Interspecifics x Interspecific). High percent seed set was obtained in group D of 19.08% and group A had 9.29%, in group B the seed set was 11.26% and group Chad 6.62 %. The three best combiners as female parents based on percentage seed set were CG 14 (0.glaberrima), WAB 450-IBP-I05-HB and WAB450-I-B-P-I03-HB (interspecifics), and for male parents were NERICA 3, NERICA 2 (interspecifics) and WAB 375-B-9-H3-2 (0. sativa). Genetic components of variance and heritability estimates were studied in F3 and F4 generation progenies from North Carolina II design mating scheme. General combining ability (GCA), manly due to female and specific combining ability (SCA) were significant for tiller number, plant height and specific leaf dry weight indicating that additive and non-additive genetic effects controlled these traits. Heritability estimates for drought tolerance and leaf rolling were highly significant for all the populations examined, ranging from 0.32 to 0.72 and 0.22 to 0.56, respectively. Drought tolerance expressed as little leaf drying was significantly associated with less leaf rolling, larger leaf area index, and taller plants. Some genotypes and progenies such as WBK 39-B-B-B, WBK 30-B-B and WAB 450-I-B-P-I03-HB were identified as being drought tolerant with the ability to recover quickly from drought stress. There was a strong desire for tall plants and short duration rice in upland and lowland ecologies, while high yield was preferred in irrigated ecology. Therefore, the best breeding strategy would be ecosystem breeding targeting each of the ecologies. Screening for secondary traits that have been identified to be significantly associated with rice grain yield could be used for direct selection for grain yield under drought stressed conditions. Progenies identified, as drought tolerant would be considered for use as source germplasm in breeding that target Sikasso region and any other similar ecologies. This study therefore suggested that participatory research approaches that involve all stakeholders in rice production would be valuable in identifying and generating drought tolerant and adoptable rice cultivars in the tropics of Africa with considerable impact on food security. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2006.

Grain yield of rice can be enhanced by breeding for drought tolerance and optimum shattering. New generations of rice, the interspecific fixed lines known to carry more drought tolerance traits, are a potential source for improvement of the Ugandan landraces but they frequently carry genes for grain shattering. Information gathered through surveys to understand rice production constraints and farmers’ preferred rice traits are limited and they are generally unranked. The objectives of this study were seven-fold. They were to 1) determine farmers’ perceptions on the key production constraints and preferences for rice in Uganda, 2) develop and appraise cheap and efficient rice breeding equipment, 3) determine anthesis and the grain shattering pattern of rice, 4) characterize for drought tolerance and grain-shattering ability in rice populations in Uganda, 5) investigate crossability between O. sativa and the new interspecific fixed lines, and 6 and 7) determine the inheritance and gene action controlling drought tolerance and grain shattering traits. The experiments were conducted at Namulonge, Uganda, from 2006 to 2009. Data on farmer’s perceptions of the key production constraints and preferences for rice in Uganda were obtained through participatory rural appraisal (PRA) methods and a formal household survey of 380 farmers from three regions in Uganda during March 2007 to June 2008. The results showed that the variety NERICA 4 was the most preferred by farmers. Rice yellow mottle virus disease, rice blast and African rice gall midge were major constraints under lowland conditions, while drought was the major constraint under upland conditions. The preferred traits in traditional varieties included aroma characteristics, grain size and the texture of cooked grains. Cultivars that combine high yield potential, early maturity, drought tolerance, aroma characteristics and special market attributes should be included in the breeding objectives. A new anther suction device that uses an ordinary household vacuum cleaner was fabricated. Its overall units include a pipette tip, silicone tubing, hose tubes, a metallic stand, a rubber stopper and a household vacuum cleaner of 1600 w capacity. It is efficient, cheaper, less bulky and has lower replacement costs than the commercial one. In addition, a hand-held single grain tester, which is efficient, cheaper and more suitable for field studies than the laboratory tester, was developed and appraised. Studies to determine the grain-shattering pattern in rice were conducted by making assessments at 10, 20 and 30 days after anthesis. The results revealed that the highest level of discrimination among the varieties was between 10 days and 20 days after anthesis. Among the genotypes tested, NERICA rice attained anthesis earlier than CT rice and O. sativa attained anthesis later than both rice groups. Appropriate methods of planting the parental lines were proposed for pollination to be synchronised. Interspecific rice and O. sativa were characterized in the field for drought tolerance and grain-shattering ability. There was high variability wihin species in grain shattering and drought tolerance traits, suggesting that selection could be effective. The distribution of grain shattering scores was not normal, suggesting that some selection had taken place for the trait, unlike the drought tolerance traits and leaf roll scores where normal distributions were observed. Results of the crossability test between O. sativa and interspecific fixed lines revealed that some genotypes had high crossability when used as males, or as females, or as both females and males. The best male genotype combiners were CT 16334(2)-CA-2-M, WAB 365-B-1H1-HB and IRAT 257, while the best females combiners were CT 16334(2)-CA-11-M, CT 16317-CA-4-M and IRAT 325. In addition, additive effects were more important that non-additive effects for pollen viability and seed set in the crosses. Genotypes with acceptable crossability and varying levels of drought tolerance and grainshattering ability were selected for further studies. The selected genotypes were crossed in a North Carolina II mating scheme, and as pairs of crosses, and used to understand the genetic mechanisms of drought tolerance at the reproductive and vegetative stages. Evaluation of the populations revealed that additive effects for filled grains were more important under drought stress and non-drought stress conditions than the non-additive effects. There were high heritability estimates for leaf roll under drought stress, implying that additive effects were more important. There was a higher contribution of additive effects for grain-shattering ability than nonadditive effects in the populations. Allelic tests for the shattering trait revealed that a single gene was in 46% of the crosses; two genes in another 46% of the crosses had duplicate dominant effects as well as complimentary dominant/recessive effects; and there were three genes in four percent of the crosses. Segregation ratios of 3:1 and 1:3 were also found in two crosses. This could be due to a reversal of the function of the genes. Another surprising finding was that the segregation ratio of 3:1 was found in a cross with loose grain attachment to the pedicel in an allelism test. This segregation was probably due to unfixed loci in some of the interspecific lines. This study identified rice breeding priorities for Uganda, prepared breeding tools and provided relevant information for generating lines that have improved drought tolerance and optimum grain-shattering ability by determining the modes of gene action for these two traits. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2010.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / A cana-de-açúcar (Saccharum spp) é uma cultura em expansão no Brasil, necessitando de práticas que aumentem a produtividade nos diferentes ecossistemas do país. Dessa forma, o presente trabalho, teve como objetivo avaliar o crescimento inicial e a tolerância à seca da variedade de cana-de-açúcar RB92579 com a aplicação de bioestimulantes e enraizadores. Para isso, foi realizado um experimento em casa de vegetação, em delineamento inteiramente casualizado. Os tratamentos utilizados foram: T1 Controle; T2 - AIB (Ácido Indolbutírico), na concentração de 1000 mg.L-1; T3 - B+Zn (Bórax, 10 Kg por hectare + Sulfato de Zinco, 20 Kg por hectare); T4 - Triptofano, 7,2 Kg por hectare; T5 - (Kymon Plus®, 1,0 L por hectare + Potamol® 0,5 L por hectare) e T6 - Stimulate®, 0,5 L por hectare. Aos 41 e 124 DAP foram coletados dados morfológicos e de biomassa para análise do crescimento inicial da planta. Aos 117 dias após o plantio foi suspensa a rega por 36 horas para avaliar o efeito dos bioestimulantes e enraizadores em mitigar o efeito do estresse hídrico. A aplicação de AIB e Stimulate® proporcionaram maiores taxas de crescimento e acúmulo de biomassa. A aplicação de Ubyfol e Stimulate® proporcionaram maiores trocas gasosas na ausência de estresse hídrico e aumentaram a eficiência quântica efetiva do PSII mesmo quando a plantas estavam sob estresse hídrico, quando comparadas com o controle. Em plantas sob estresse hídrico moderado, a aplicação de Stimulate® proporcionou a manutenção de maiores taxas de fotossíntese, transpiração e condutância estomática. O experimento deve ser repetido em campo para verificar se esses resultados se traduzem em aumento da produtividade tanto na condição de sequeiro como na condição irrigada.

Os efeitos da aplicação foliar de fósforo (Pi) na condutância estomática (gs) e na fotossíntese (A), foram avaliados em genótipos de feijoeiro, A320, Carioca e Ouro Negro, cultivados em potes numa casa de vegetação. Este estudo foi feito durante o dia após a aplicação foliar de 10 g de Pi L-1, em plantas hidratadas, e durante um período de deficiência hídrica iniciada dois ou cinco dias após a aplicação. Durante o dia após a aplicação de Pi, não foi observado nenhuma diferença, entre os tratamentos com e sem Pi, nas trocas gasosas de ambas as cultivares. Durante a desidratação, a aplicação de Pi não causou aumento de gs, mas causou aumento de A, nas medições às 0900, 1200 e 1500 horas, nas cultivares (Carioca e Ouro Negro), mas com um efeito maior na cultivar Ouro Negro, principalmente às 1200 horas. A eficiência intrínseca do uso da água (EIUA) também foi superior nas plantas com suprimento foliar de Pi. Quanto aos componentes da produção, a aplicação de Pi causou aumento do número de vagens das plantas da cultivar Ouro Negro, sob desidratação. Portanto, a aplicação de Pi foliar pode reduzir o efeito de uma desidratação quando aplicado dois dias antes da suspensão da irrigação. Além disso, o potencial fotossintético (Ac) de Ouro Negro com Pi foi superior ao de A320, com e sem Pi, na seca máxima, e ao A320 sem Pi, na reidratação. A dissipação térmica, não fotoquímica (NPQ), do genótipo A320 sem Pi foi maior que com Pi e que a do Ouro Negro, com e sem Pi, na reidratação, provavelmente devido aos menores valores de A neste tratamento sem Pi. O uso de fosfato de amônio substituindo a uréia ou o sulfato de amônio, que são aplicados aos 25 DAE, é recomendável. / The effects of foliar Pi supplying (Pi) on stomatal conductance (gs) and photosynthesis (A) were measured in genotypes, A320, Carioca and Ouro Negro, grown under greenhouse condition. Measurements of gs and A were taken one day after Pi application (10 g L-1) on well-irrigated plants, and during drought stress period beginning two or five days after Pi supplying. During the day after Pi application, it was not observed any difference on gas exchange of genotypes (Carioca and Ouro Negro) due to the treatments with and without Pi. During water stress, the Pi supplying did not cause increase of gs, however, caused increase of A at 0900, 1200 and 1500 h in both genotypes. This effect was higher when considered Ouro Negro at 1200 h. The intrinsic water use efficiency (IWUE) was also increased in Pi-supplied plants. The Pi supplying caused increase on pod number of Ouro Negro plants under water deficit. These results suggest that the foliar Pi application may reduce the effect of water deficit on photosynthesis when supplied two days before the water withholding. In addition, the O2 evolution (Ac) on Ouro negro with extra Pi was higher than for A320, with or without Pi, at the last day of the mild water deficit, and it was higher than A 320 with extra Pi at rehydration. The non-photosynthetic quenching (NPQ), was higher for A320 without Pi at recovery, probably due to the smaller values of A at this treatment. The use of ammonium phosphate, instead of urea or ammonium sulphate to be applied as usual 25 DAS, is recommended.

Les recherches récapitulées dans cette thèse de doctorat ont porté sur les causes de l’organisation spatiale des végétations périodiques. Ces structures paysagères aux motifs réguliers, tachetés, tigrés ou labyrinthiques, d’échelle décamétrique à hectométrique, couvrant des étendues considérables sur au moins trois continents, constituent un cas d’école dans l’étude des processus endogènes présidant à l’hétérogénéité du couvert végétal. Ces structures prennent place sur un substrat homogène, mis à part la rétroaction du couvert lui-même, et sont marquées par des écotones abrupts et la persistance d’une proportion considérable de sol nu. Plusieurs modèles ont mis en avant l’existence possible d’un phénomène d’auto-organisation du couvert, qui verrait une structure d’ensemble émerger des interactions locales entre individus. Ces modèles se basent sur le jeu simultané de la consommation de la ressource (compétition) et de l’amélioration de l’un ou l’autre des éléments du bilan de la même ressource par le couvert (facilitation). La condition à l’existence d’une structure d’ensemble spatialement périodique et stable réside dans une différence entre la portée de la compétition (plus grande) et celle de la facilitation. L’apparition de ces structures est modulée par le taux de croissance biologique, qui est le reflet des contraintes extérieures telles que l’aridité, le pâturage ou la coupe de bois. Le modus operandi des interactions spatiales supposées entre individus reste largement à préciser.<p>Nos recherches ont été menées au sud-ouest de la République du Niger, à l’intérieur et dans les environs du parc Régional du W. Trois axes ont été explorés :(i) Une étude de la dépendance spatiale entre la structure de la végétation (biovolumes cartographiés) et les paramètres du milieu abiotique (relief, sol), sur base d’analyses spectrales et cross-spectrales par transformée de Fourier (1D et 2D). (ii) Une étude diachronique (1956, 1975 et 1996) à large échelle (3000 km²) de l’influence de l’aridité et des pressions d’origine anthropique sur l’auto-organisation des végétations périodiques, basée sur la caractérisation de la structure spatiale des paysages sur photos aériennes via la transformée de Fourier en 2D. (iii) Trois études portant sur les interactions spatiales entre individus :En premier lieu, via l’excavation des systèmes racinaires (air pulsé) ;Ensuite, par un suivi spatio-temporel du bilan hydrique du sol (blocs de gypse) ;Enfin, via le marquage de la ressource par du deutérium. <p>Nous avons ainsi pu établir que les végétations périodiques constituent bien un mode d’auto-organisation pouvant survenir sur substrat homogène et modulé par les contraintes climatiques et anthropiques. Un ajustement rapide entre l’organisation des végétations périodiques et le climat a pu être montrée en zone protégée. La superficie et l’organisation des végétations périodiques y ont tour à tour progressé et régressé en fonction d’épisodes secs ou humides. Par contre, en dehors de l’aire protégée, la possibilité d’une restauration du couvert semble fortement liée au taux d’exploitation des ressources végétales. Ces résultats ont d’importantes implications quant à la compréhension des interactions entre climat et écosystèmes et à l’évaluation de leurs capacités de charge. La caractérisation de la structure spatiale des végétations arides, notamment par la transformée de Fourier d’images HR, devrait être généralisée comme outil de monitoring de l’état de ces écosystèmes. Nos études portant sur les modes d’interactions spatiales ont permis de confirmer l’existence d’une facilitation à courte portée du couvert végétal sur la ressource. Cependant, cette facilitation ne semble pas s’exercer sur le terme du bilan hydrique traditionnellement avancé, à savoir l’infiltration, mais plutôt sur le taux d’évaporation (deux fois moindre à l’ombre des canopées). Ce mécanisme exclut l’existence de transferts diffusifs souterrains entre sols nu et fourrés. Des transferts inverses semblent d’ailleurs montrés par le marquage isotopique. L’étude du bilan hydrique et la cartographie du micro-relief, ainsi que la profondeur fortement réduite de la zone d’exploitation racinaire, jettent de sérieux doutes quant au rôle communément admis des transferts d’eau par ruissellement/diffusion de surface en tant que processus clé dans la compétition à distance entre les plantes. L’alternative réside dans l’existence d’une compétition racinaire de portée supérieure aux canopées. Cette hypothèse trouve une confirmation tant par les rhizosphères excavées, superficielles et étendues, que dans le marquage isotopique, montrant des contaminations d’arbustes situés à plus de 15 m de la zone d’apport. De même, l’étude du bilan hydrique met en évidence les influences simultanées et contradictoires (facilitation/compétition) des ligneux sur l’évapotranspiration.<p>/<p><p>This PhD thesis gathers results of a research dealing with the causes of the spatial organisation of periodic vegetations. These landscape structures, featuring regular spotted, labyrinthine or banded patterns of decametric to hectometric scale, and extending over considerable areas on at least three continents, constitute a perfect study case to approach endogenous processes leading to vegetation heterogeneities. These patterns occur over homogeneous substratum, except for vegetation’s own feedbacks, and are marked by sharp ecotones and the persistence of a considerable amount of bare soil. A number of models suggested a possible case of self-organized patterning, in which the general structure would emerge from local interactions between individuals. Those models rest on the interplay of competitive and facilitative effects, relating to soil water consumption and to soil water budget enhancement by vegetation. A general necessary condition for pattern formation to occur is that negative interactions (competition) have a larger range than positive interactions (facilitation). Moreover, all models agree with the idea that patterning occurs when vegetation growth decreases, for instance as a result of reduced water availability, domestic grazing or wood cutting, therefore viewing patterns as a self-organised response to environmental constraints. However the modus operandi of the spatial interactions between individual plants remains largely to be specified.<p>We carried out a field research in South-West Niger, within and around the W Regional Park. Three research lines were explored: (i) The study of the spatial dependency between the vegetation pattern (mapped biovolumes) and the factors of the abiotic environment (soil, relief), on the basis of spectral and cross-spectral analyses with Fourier transform (1D and 2D). (ii) A broad scale diachronic study (1956, 1975, 1996) of the influence of aridity and human induced pressures on the vegetation self-patterning, based on the characterisation of patterns on high resolution remote sensing data via 2D Fourier transform. (iii) Three different approaches of the spatial interactions between individuals: via root systems excavation with pulsed air; via the monitoring in space and time of the soil water budget (gypsum blocks method); and via water resource labelling with deuterated water.<p>We could establish that periodic vegetations are indeed the result of a self-organisation process, occurring in homogeneous substratum conditions and modulated by climate and human constraints. A rapid adjustment between vegetation patterning and climate could be observed in protected zones. The area and patterning of the periodic vegetations successively progressed and regressed, following drier or wetter climate conditions. On the other hand, outside protected areas, the restoration ability of vegetation appeared to depend on the degree of vegetation resource exploitation. These results have important implications regarding the study of vegetation-climate interactions and the evaluation of ecosystems’ carrying capacities. Spatial pattern characterisation in arid vegetations using Fourier transform of HR remote sensing data should be generalised for the monitoring of those ecosystems. Our studies dealing with spatial interaction mechanisms confirmed the existence of a short range facilitation of the cover on water resource. However, this facilitation does not seem to act through the commonly accepted infiltration component, but rather on the evaporative rate (twice less within thickets). This mechanism excludes underground diffusive transfers between bare ground and vegetation. Inverse transfers were even shown by deuterium labelling. Water budget study and micro-elevation mapping, along with consistent soil shallowness, together cast serious doubts on the traditional mechanism of run-off/diffusion of surface water as a key process of the long range competition between plants. An alternative explanation lies in long range root competition. This hypothesis find support as well in the excavated root systems, shallow and wide, as in isotopic labelling, showing contaminations of shrubs located up to 15 m of the irrigated area. Water budget study also evidenced simultaneous contradictory effects (facilitation/competition) of shrubs on evapotranspiration.<p> / Doctorat en sciences agronomiques et ingénierie biologique / info:eu-repo/semantics/nonPublished

La faible représentation de la biodiversité dans les modèles de végétation a longtemps été un obstacle à la compréhension et à la projection des processus écosystémiques. La forte biodiversité des forêts tropicales, leur rôle clé dans les cycles biogéochimiques globaux, ainsi que leur vulnérabilité aux perturbations anthropiques directes et indirectes, amplifient les difficultés et enjeux de ces questions de recherche. En particulier, l'augmentation prédite de la fréquence et de l'intensité des sécheresses pourrait impacter la structure et composition floristique de ces forêts, comme dors et déjà observé au cours d'expériences naturelles et artificielles. Cette thèse explore ces questions de recherche à travers deux approches complémentaires, de modélisation et de mesures écophysiologiques. Dans le premier chapitre, je décris un simulateur de croissance forestière individu-centré et spatialement-explicite, TROLL, qui intègre les progrès récents en physiologie des plantes. Les processus sont paramétrés à l'aide de traits fonctionnels espèce-spécifiques, pour une forêt tropicale amazonienne. Une régénération forestière est simulée, et validée par des observations faites en Guyane française. La sensibilité du modèle à plusieurs paramètres globaux clés est évaluée. Enfin, l'influence de la variation de la richesse et composition spécifiques sur les propriétés écosystémiques est explorée. La réponse des forêts tropicales à la sécheresse est mal connue, empêchant la représentation pertinente des processus en jeu dans les modèles de végétation. Les chapitres 2 à 5 de cette thèse ont ainsi pour but de documenter la tolérance à la sécheresse et sa diversité dans une forêt amazonienne. Une méthode récente et rapide de détermination d'un trait de tolérance des feuilles à la sécheresse, le potentiel hydrique des feuilles au point de perte de turgescence (ptlp), est validée et utilisée, permettant de quantifier pour la première fois un tel trait de tolérance à la sécheresse dans une forêt amazonienne à l'échelle de la communauté. Ce jeu de données permet l'exploration des déterminants de la tolérance à la sécheresse des feuilles, à travers les espèces d'arbres, les tailles des individus, les stades de succession, les expositions à la lumière, ainsi que les lianes. La variabilité de ptlp observée suggère une large diversité de réponses à la sécheresse au sein des communautés de plantes amazoniennes. Ceci est confirmé par le suivi direct du flux de sève au cours d'une saison sèche sur divers arbres de canopée. Enfin, je discute les implications de ces résultats pour le développement des futurs modèles de végétation. / A great part of uncertainties in our current understanding and projections of the carbon cycle lies in the vegetation compartment. The problem of biodiversity representation in vegetation models has long been an impediment to a detailed understanding of ecosystem processes. The high biodiversity of tropical forests, their disproportionate role in global biogeochemical cycles, together with their vulnerability to direct and indirect anthropogenic perturbations, amplify the relevance of this research challenge. In particular, the predicted increase in drought intensity and frequency in the tropics may impact forest structure and composition, as already observed in natural and artificial experiments. This thesis explores how new advances in modelling and ecophysiology should help improve our understanding of these processes in the future. In the first chapter, I describe an individual-based and spatially-explicit forest growth simulator, TROLL, that integrates recent advances in plant physiology. Processes are linked to species-specific functional traits parameterized for an Amazonian tropical rainforest. This model is used to simulate a forest regeneration, which is validated against observations in French Guiana. Model sensitivity is assessed for a number of key global parameters. Finally, we test the influence of varying the species richness and composition on ecosystem properties. Tropical forest response to drought is not well understood, and this hampers attempts to model these processes. In chapters 2 to 5 I aimed at documenting drought-tolerance and its diversity in an Amazonian forest. A rapid method of determination of a leaf drought tolerance trait, the leaf water potential at turgor loss point (ptlp), was validated and applied to a range of plant species. We established the first community-wide assessment of drought tolerance in an Amazonian forest. These results inform on the drivers and determinants of leaf drought tolerance, across tree species and lianas, tree size, successional stages, light exposition, and seasons. Variability in ptlp among species indicates the potential for a range of species responses to drought within Amazonian forest communities. This is further confirmed by direct monitoring of whole-plant water use on diverse canopy trees during a marked dry season. Finally, I discuss the implications of these results to increase the dialogue between the vegetation modeling community and ecology, to enhance model's predictive ability, and to inform policy choices.

Force est de constater que les dépérissements forestiers augmentent. Ces observations vont de pairs avec l’accroissement des événements climatiques extrêmes. Aussi dans ce contexte, il est nécessaire d’identifier denouveaux caractères de resistance à la sécheresse. La résistance à la cavitation est actuellement le meilleurmarqueur de la survie d’une espèce à la sécheresse.Cette thèse avait deux objectifs : (i) comprendre le mécanisme de propagation de la cavitation dans le xylèmechez les gymnospermes. (ii) Quantifier la variation phénotypique in situ de ce caractère chez Pinus pinaster ainsique (iii) quantifier la variation génétique, et sa plasticité phénotypique.La démarché a été la suivante (i) une étude interspécifique de la résistance à la cavitation a été couplé à desmesures micro-anatomiques. (ii) Pour le volet intraspécifique, nous avons phénotypé 6 populations dans deuxtest de populations-descendances, ainsi qu’en population naturelles in situ.La propagation de l’embolie chez les Pinaceae et les ex-Taxodiaceae pourrait être due au passage du germe d’air(rupture capillaire) à travers des nanopores dans le torus. En effet, la pression de rupture d’un ménisque air-sèveest corrélée à l’entrée de l’air dans le xylème (P12). Alors que la variation interspécifique est grande, la résistanceà la cavitation varie faiblement au sein d’une espèce. Ainsi les populations provenant de climat contrasté neprésentent pas ou peu de différence génétique (en test de provenance) ou en populations naturelles in situ. Cecaractère présente une plasticité phénotypique mais faible comparée à celle de la croissance en hauteur parexemple. La comparaison entre la variation génétique entre populations et la variation des marqueurs neutresentre ces mêmes populations montrent que la variation de ce caractère semble réduite par l’architecturegénétique sous-jacente. La resistance à la cavitation est vraisemblablement un trait canalisé. / Several review reported global forest die-back that are caused, directly or indirectly, by extreme climatic events(like heat waves or prolonged drought). In this context, there is an urgent need to identify new traits to tracedrought tolerance. Resistance to cavitation is one of the best proxy for survival during extreme drought.The aim of this work was (i) to understand how spreads cavitation in the vascular pathway of gymnosperms (ii)to quantify the phenotypic variation of resistance to cavitation for Pinus pinaster species, (iii) to determine theamount of the genetic variation and phenotypic plasticity available for this trait.A micro-anatomy study was coupled to measurement of resistance to cavitation for various species to foundwhere air-seeding occurs in the bordered pit. To quantify the variability of resistance to cavitation, wephenotyped 506 genotypes using to replicated provenance-progeny trials and on natural in situ populations.The spread of embolism for Pinaceae and ex-Taxodiaceae could be due to minute pore in tori, which are remainsof secondary plasmodesmata. We found that the pressure needed to break a water/air meniscus in these minutepores is correlated with the xylem air entry (P12). Despite the great variability of resistance to cavitation betweenspecies, we found low variability within species. Most of the variability is within population, rather than betweenpopulations. The phenotypic plasticity of resistance to cavitation is low compare to growth traits. Comparisonbetween QST and FST shows that populations exhibit less variation compare to what it is expected under geneticdrift. The variation of resistance to cavitation seems to be narrowed by the genetic architecture, which is the signof canalisation.

No description available.