Typescript, etc. / Digitized by Kansas State University Libraries

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LD2668 .T4 1942 M42 / Master of Science

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Call number: LD2668 .T4 1966 S761 / Master of Science

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The response of cassava (Manihot esculenta Crantz) to water stress was studied in two distinct sites (Tucson, Arizona and Santander de Quilichao, Colombia). A third experiment was conducted in Palmira, Colombia to evaluate the relationship between photosynthesis, relative humidity and yield of cassava cultivars HCol 1684. Plants stressed after 2 months of growth in Tucson reduced growth by reducing leaf formation, expansion and leaf area. Reductions in LAI due to stress imposed to MCol 1684 in Quilichao were attained by reducing leaf expansion rates and leaf formation in plants stressed after 3 months of age. Plants stressed later increased leaf fall and did not reduce leaf formation. Stress reduced dry matter production in all cultivars, especially when given to young plants. Stress also altered the patterns of dry matter partitioning in 3-month-old plants of MCol 1684, but not in those stressed at 6 months. Transpiration and diffusive conductances of MCol 1684 were reduced after 40 days of stress. These parameters were correlated to photosynthesis and leaf temperatures for non-stressed plants, and additionally with relative humidity in the plants stressed after 3 months of growth. Interestingly, leaf temperatures were lower in stressed plants of MCol 1684, which suggests that stressed cassava plants can avoid excessive heating caused by stomatal closure simply changing leaf orientation and increasing reflectance. Leaf water potential was slightly reduced by stress in Tucson except for MVen 218. Stressed plants of MCol 1684 in Quilichao showed lower values of (L) than those of non-stressed plants after 30-40 days of treatment, suggesting an adaptation of stressed plants acquired during the stress period. The effects of air humidity on stomatal functioning of MCol 1684 seems to be strong as suggested by the dependence of transpiration, conductances and photosynthesis on relative humidity.

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There is a great deal of work currently being done in the field of desiccation tolerance. Generally workers studying desiccation-tolerant plant tissues have concentrated on the mechanisms of desiccation tolerance without concomitant studies on why most plants cannot survive desiccation. The present study considers both a desiccation-tolerant plant as well as a range of desiccation-sensitive plants. The work incorporates physiological, biophysical, biochemical and ultrastructural studies in an attempt to get a holistic picture of vegetative material as it dries and then rehydrates. The plant species used in this study are: Craterostigma nanum, a so-called resurrection plant; Garcinia livingstonei, a drought-tolerant small tree; Isoglossa woodii, an understorey shrub which shows a remarkable ability to recover from wilting; Pisum sativum seedlings, which have a very high water content at full turgor; and finally, Adiantum raddianum, the maiden hair fern, which wilts very quickly and does not recover from wilting. The desiccation-tolerant plant, C. nanum, had an unusual pressure-volume (PV) curve which indicated that while large volume changes were taking place there was little concomitant change in pressure or water potential. The unusual nature of this PV curve made it difficult to assess the relative water content (RWC) at which turgor was lost. The desiccation-sensitive plants exhibited standard curvi-linear PV curves. The amount of nonfreezable water in the five species was studied and found to show no correlation with the ability to withstand dehydration or with the lethal water content. There were no differences in the melting enthalpy of tissue water between the tolerant and most of the sensitive plants. Isoglossa woodii had a lower melting enthalpy than the tolerant and the other sensitive species. Survival studies showed that the desiccation-sensitive plants all had similar lethal RWCs. The tolerant plant survived dehydration to as low as 1% RWC, recovering on rehydration within 24 hours. Membrane leakage studies showed that the sensitive plants all exhibited membrane damage at different absolute water contents, but very similar RWCs and water potentials. The increase in leakage corresponded to the lethal RWC for all the sensitive species. The desiccation-tolerant plant recovered from dehydration to very low water contents and did not show an increase in membrane leakage if prior rehydration had taken place. Without prior rehydration this tolerant plant exhibited an increase in leakage at similar RWCs and water potentials to that of the sensitive species. There did not appear to be much difference in the RWC at which damage to membranes occurred whether the material was dried rapidly or slowly. Respiration and chlorophyll fluorescence were studied to determine what effect drying and rehydration have on the electron transport· processes of the leaf. The chlorophyll fluorescence studies gave an indication of damage to the photosynthetic apparatus. Both qualitative changes as well as quantitative changes in fluorescence parameters were assessed. Characteristics like quantum efficiency (Fv/Fm)remained fairly constant for a wide range of RWCs until a critical RWC was reached where there was a sharp decline in Fv/Fm. Upon rehydration, C. nanum recovered to pre-stress levels, I. woodii showed no recovery and no further damage on rehydration, whilst the other species exhibited even more damage on rehydration than they had on dehydration. Respiration remained fairly constant or increased slightly during drying until a critical RWC was reached at which it suddenly declined. The RWC at which this decline occurred ranged from 15% and 20% in P. sativum and C. nanum respectively, to 50% for G. livingstonei. On rehydration respiration exceeded the levels measured in dehydrated material for the sensitive species. Unsuccessful attempts were made to fix material anhydrously for ultrastructural studies so standard fIxation was used. The ultrastructural studies revealed that changes had occurred in the ultrastructure of leaves of the sensitive species dried to 30% RWC particularly in A. raddianum and P. sativum. Drying to 5% RWC revealed extensive ultrastructural degradation which was worsened on rehydration in the sensitive species. The tolerant species showed ultrastructural changes on drying but these were not as severe as occurred in the sensitive species. The cell walls of the tolerant species folded in on drying. This folding was possibly responsible for the unusual PV curves found in this species. At 5% RWC the cells were closely packed and very irregular in shape. The cell contents were clearly resolved and evenly spread throughout the cell. The large central vacuole appeared to have subdivided into a number of smaller vacuoles. On rehydration the cells regained their shape and the cell contents had moved towards the periphery as the large central vacuole was reformed. Beading of membranes, which was common in the sensitive species, was not found in the tolerant species suggesting that membrane damage was not as severe in the tolerant species. Western Blot analysis of the proteins present during drying was performed to determine whether a class of desiccation-induced proteins, called dehydrins, were present. These proteins have been suggested to play a protective role in desiccation-tolerant tissue. It was found that C. nanum did, in fact, possess dehydrins, but so did P. sativum. The other three sensitive species did not show any appreciable levels of dehydrin proteins. The presence of dehydrins alone is, therefore, not sufficient to confer desiccation tolerance. While physiologically the damage occurring in the sensitive plants was similar to that of the tolerant plant, at an ultrastructural level the damage appeared less in the tolerant plant. On rehydration from low RWCs damage appeared to become exacerbated in the sensitive plants. This was in contrast to the tolerant plant where damage was apparently repaired. There appears, therefore, to be a combination of protection and repair mechanisms responsible for the ability of C. nanum to tolerate desiccation. The lethal RWC of the sensitive species was higher than that at which protective mechanisms, such as water replacement, might come into play. So it is not just the possible ability to replace tightly bound water that set the tolerant plant aside. It must also have mechanisms to tolerate damage at the higher RWCs which were damaging and lethal to the sensitive plants. The lethal damage to sensitive species appeared to be related to a critical volume, thus it is concluded that the tolerant plant had the ability to tolerate or avoid this mechanical damage during drying as well as the ability to remain viable in the dry state. It is hypothesised that the ability of the walls to fold in and the unusual nature of the PV curve may provide some answers to the enigma of desiccation tolerance. / Thesis (Ph.D.)-University of Natal, 1995.

Orientador: Rafael Silva Oliveira / Texto em português e inglês / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Biologia / Made available in DSpace on 2018-08-25T08:28:23Z (GMT). No. of bitstreams: 1 Teodoro_GrazielleSales_D.pdf: 9847099 bytes, checksum: dea93bd8d08793dfcbbfed314d895252 (MD5) Previous issue date: 2014 / Resumo: Eventos de secas extremas têm ocorrido com frequência nos últimos anos em regiões tropicais. Plantas em montanhas tropicais são particularmente sensíveis a mudanças no clima e pouco se sabe sobre a resistência e resiliência de comunidades vegetais ricas em espécies, como os campos rupestres, a condições climáticas extremas. Nosso objetivo foi avaliar as respostas fenológicas e ecofisiológicas e o crescimento de seis espécies abundantes de campo rupestre sob condições de seca extrema, sendo duas rosetas (Vellozia nivea e Vellozia aff. variabilis) e quatro arbustivas (Campomanesia pubecens, Eremanthus seidelii, Mimosa clausenii e Vernonia warmingiana). Para simular eventos de seca extrema, realizamos um experimento de exclusão de chuva, constituído por 12 parcelas (4x4 metros), sendo quatro controle e oito de exclusão, em uma area de campo rupestre no Parque Nacional da Serra da Canastra. Avaliamos o desempenho ecofisiológico das plantas submetidas a condições climáticas contrastantes mediante um conjunto de atributos morfofisiológicos, incluindo: trocas gasosas, composição isotópica do carbono (?13CCELL) e oxigênio (?18OCELL) da celulose foliar (medidas integradoras da assimilação de carbono e condutância estomática), curvas de vulnerabilidade à cavitação, curvas pressão-volume, carboidratos não-estruturais (CNE), crescimento e fenologia foliar. As espécies estudadas mostraram-se bastante resistente à seca, pois após 17 meses de exclusão de chuva não encontramos diferenças de crescimento e mortalidade entre populações nas parcelas controle e experimentais. As espécies apresentaram um contínuo de estratégias de uso de água, variando de espécies isohídricas (E. seidellii) à anisohídricas (C. pubecens). O uso da abordagem isotópica dupla (?13CCELL e ?18OCELL) foi válida para avaliar e predizer as respostas fotossintéticas e estomáticas à seca apenas para as espécies isohídricas. Além disso, a composição isotópica da celulose da folha pode ser influenciada pela mobilização de reservas de CNE nas raízes. As duas espécies congenéricas de Velloziaceae apresentaram estratégias constrastantes para lidar com a seca: uma espécie é tolerante à dessecação (TD) (V. nivea) e a Vellozia aff. variabilis é não-TD. A espécie TD apresentou maiores taxas de trocas gasosas durante a estação chuvosa e durante a seca prolongada entrou em estado de "dormência", exibindo um comportamento oportunista de uso de água. Já a espécie não-DT apresentou uma estratégia conservadora de uso de água. Durante a seca, observamos um aumento no estoque de CNE% nas raízes na espécie TD, o que pode representar um mecanismo chave para sobrevivência dessa espécie durante os ciclos de dessecação e rehidratação. As três espécies arbustivas (C. pubecens, E. seidellii, V. warmingiana) apresentaram sistemas hidráulicos contrastantes, com diferentes pontos de perda de turgor (?TLP) e vulnerabilidade à cavitação (estimada mediante o P50 - potencial hídrico que corresponde à 50% de perda de condutividade). As espécies mais resistentes à cavitação (maior P50) apresentam menores reservas de CNE nas raízes, sugerindo um trade-off entre atributos hidráulicos e de estoque de carbono. Nossos resultados trazem importantes contribuições para o entendimento de funcionamento de ecossistemas sazonais limitados por água e ilustram a diversidade de estratégias hidráulicas que conferem resiliência à seca / Abstract: In the past years, extreme droughts have been frequently recorded in several tropical regions. Plants in tropical mountains are particularly sensitive to changes in climate little is known about the plant physiological adjustments and responses to changes in rainfall patterns. Our objective was to evaluate the phenological and ecophysiological responses of six dominant species with contrasting life forms in campos rupestres under an extreme drought condition. We studied two rosettes species (Vellozia nivea and Vellozia aff. variabilis) and four shrubs (Campomanesia pubecens, Eremanthus seidelii, Mimosa clausenii e Vernonia warmingiana). We implemented a rainwater exclusion experiment with 12 plots (4x4 meters), in which eight was considered drought treatment and four control treatment. We monitored several aspects of the plant ecophysiology, such as gas exchange using instantaneous measurements and stable isotope as integrators of leaf functioning, pressure-volume curves, vulnerability curves, non-structural carbohydrates (NSC) in roots, growth, biomass and leaf phenology. Species showed a continuum of water use strategies, varying from anysohydric (C. pubecens) to completely isohydric (E. seidelii) species. The relationship between ?18OCELL and gsMAX was negative and significant only for isohydric species. The use of dual isotopic approach also was valid only for isohydric species. Across species, we found a strong positive relationship between ?18OCELL and NSC% for individuals under drought treatment, suggesting that the use of NSC% reserves in water stress conditions can affect leaf isotope composition. Our results suggest that in communities dominated to species with a great diversity of hydraulic strategies, such as the campos rupestres, ?18OCELL should be used with caution to infer physiological responses. We evaluated a congeneric pair of Velloziaceae species with contrasting strategies to cope with the erratic water availability in campos rupestres. One species is desiccation tolerant (DT) and the other is drought resistance (non-DT). The DT species showed high gsMAX and low WUEi during the rainny season, showing a profligate water use. By contrast, the non-DT showed a conservative water use throughout the year. The DT species also increased the %NSC storage in roots during the prolonged drought, presumably a key mechanism that allow survival to desiccation in this species. The three shrub species (C. pubecens, E. seidelii and M. clausenii) showed different P50 (water potential that corresponds 50% of conductivity loss) and ?TLP (water potential at turgor loss point). The biomass, in general, was little affected by our drought treatment. This three species showed a trade-off between the vulnerability to cavitation and NSC storage, in which, species more resistant to cavitation (more negative P50) showed lower NSC% storage in roots. Our results illustrate the diversity of strategies that plants might present to deal with drought and bring important contributions to understand the functioning of water-limited environments / Doutorado / Biologia Vegetal / Doutora em Biologia Vegetal