Thesis (MScAgric)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: Low crop productivity in arid and semi-arid regions is a problem caused by water stress as well as associated high levels of soil and water salinity. An increased demand for salt tolerant crops is experienced in these regions. Amaranth is a glycophyte and C4 dicotyledonous crop, well adapted to arid and semi-arid regions. Previous studies on the physiological response of salt-stressed amaranths have indicated that this crop is salt tolerant. As vegetable, amaranths can be harvested by uprooting and by topping. The most common harvesting method is by topping, allowing repeated harvesting. When harvested by topping, the cutting height is an important parameter that may be manipulated to optimise growth rates. In this study, plants were exposed to different salt stress levels and harvesting procedures while yield and quality of Amaranthus tricolor were investigated. Nutrient solutions at four different electrical conducti vi ty (EC) levels were used to fertigate the plants. At high EC levels (4 mS cm-l and 8 mS cm-lj, the length and diameter of main stems, internode lengths, stem weights as well as root weights were reduced, especially with a longer growth period. However, the shoot: root ratio and leaf protein yields increased and flowering was delayed. The best leaf yield was obtained where plants were fertigated at an EC of 4 mS cm-l for 45 days. The cutting height did not affect leaf yield, growth rates and leaf protein yield in plants fertigated at EC levels of 1, 2 and 4 mS cm-l. At an EC of 8 mS cm-l, the growth rate recovered to a value similar to that of plants fertigated with an EC of 2 mS cmonly where plants were topped at 25%. With this less destructive cutting height (topped at 25%), leaf yields, growth rates and ·leaf calcium and protein yields at an EC of 8 mS cm" were superior to that of plants topped at 50%. In plants topped at 25%, the recovered growth rates at EC 8 mS cm-l was probably due to more photosynthetic active tissue left after cuttings, resulting in the accumulation of compatible solutes for osmotic adjustment. / AFRIKAANSE OPSOMMING: Die invloed van soutstremmings en oesprosedures op die groei, ontwikkeling en blaarkwaliteit van Amaranthus trico~or L. Lae produksie van gewasse in ariede en semi-ariede gebiede is 'n probleem wat veroorsaak word deur watertekorte asook geassosieerde hoë peile van grondverbrakking en soute in water. 'n Verhoogde vraag na soutverdraagsame gewasse word in hierdie gebiede ervaar. Amaranthus is 'n glikofiet en dikotiele C4 gewas wat goed in ariede en semi-ariede streke aangepas is. Vorige ondersoeke oor die fisiologiese reaksie van Amaranthus op soutstremmings het daarop gedui dat die gewas soutverdraagsaam is. As groentegewas word dit ge-oes deur dit uit te trek of deur dit gereeld te top. Waar dit getop word is die oes-tophoogte 'n belangrike parameter wat gemanipuleer kan word om die groeitempo te optimaliseer. In hierdie ondersoek is plante aan verskillende peile van soutstremmings en oesprosedures blootgestel terwyl die opbrengs en kwaliteit van Amaranthus tricolor ondersoek is. Voedingsoplossings is teen vier elektriese geleidingspeile (EC) gebruik om plante te voedsproei. Teen hoë EC peile (4 mS cm-l en 8 mS cm-l ), het lengtes en die deursnit van hoofstamme, internode lengtes, stam massas en wortelmassas afgeneem, veral met lang groeiperiodes. Die opbrengs het egter bogroei:wortel verhouding en toegeneem terwyl blomvorming blaarproteien vertraag is. Die beste blaar opbrengs is na 45 dae verkry waar plante teen 'n EC van 4 mS cm-l gevoedsproei is. Oes-tophoogte het nie blaar opbrengs, groeitempo of blaar protei en opbrengs beïnvloed met EC waardes in voedingsoplossings van 1, 2 en 4 mS cm-l nie. Met' n EC van 8 mS cm-l het die groeitempo herstel tot 'n vlak, gelykwaardig aan wat by 'n EC van 2 mS cm-l verkry is, slegs waar teen 25% tophoogtes ge-oes is. Met hierdie minder destruktiewe oesmetode (oes-tophoogte 25%), was blaar opbrengs, groei tempo en blaar kalsium en -proteien opbrengste by 'n EC van 8 mS cm-l betekenisvol beter as waar die oes-topdiepte 50% was. Plante wat met 25% topdieptes ge-oes is se herstel in groetempo teen EC 8 mS cm-1 was waarskynlik te danke aan meer fotosinteties aktiewe materiaal wat na oes op plante gelaat is. Dit kon tot die akkumulasie van oplosbare stowwe en osmotiese aanpassings aanleiding gegee het.

Thesis (MTech (Horticulture))--Cape Peninsula University of Technology, 2017 / Transpiration serves in leaf cooling, maintaining turgor pressure, promoting xylem transport of nutrient solutes from roots to shoots and delivering mobile soil nutrients to root surfaces. Soil availability of nitrogen can modulate transpiration rates, consequently powering nutrient delivery to the root surfaces (‗mass-flow'). Although such knowledge on N-regulation of transpiration is available, it remains unknown, however, whether it is NO3- or NH4+ that regulates transpiration. Given that both nitrogen forms co-occur in soils, it is not known how they interact at varying ratios in modulating stomatal behaviour. To test the functional role of NO3- and NH4+ in regulating water fluxes for mass-flow nutrient acquisition, P. vulgaris L. plants were grown with NO3- or NH4+ placed at one of four distances behind a nylon mesh, which prevented direct root access to nitrogen, whilst control plants intercepted the nitrogen source (Chapter 3). Day- and night-time stomatal conductance and transpiration, measured using Infra-Red Gas Analyser (IRGA) declined in NO3- fed plants with the increased distance behind a nylon mesh, with maximum water fluxes at the closest distance (ca. 0 mm), demonstrating a regulatory role of NO3- on stomata closure. An opposite trend was displayed by NH4+ -fed plants, which indicated the incapacity of NH4+ to down-regulate water fluxes and ammoniacal syndrome at high concentrations. To test how different [NO3-] and [NH4+] regulate day- and night-time stomatal conductance and transpiration (Chapter 4), P. vulgaris was fed with six concentrations (0, 0.25, 0.5, 1, 2, 4 and 8 mM) of each nitrogen form. A biphasic trend emerged, as postulated in previous studies (Wilkinson et al., 2007; Matimati et al., 2013), characterized by an increase in stomatal conductance and transpiration as [NO3-] increased, attaining a maximum before declining with higher [NO3-]. Plants displayed 2-fold higher photosynthetic rates, 2.2-fold higher stomatal conductance and 2.3-fold higher transpiration rates at 4 mM than at 0.25 mM of [NO3-]. The lowest [NO3-] up-regulated night-time stomatal conductance and transpiration, indicating that NO3- -fed plants opened their stomata at night-time, but reduced night-time water loss at higher [NO3-]. NH4+-fed plants had the incapacity to regulate day- and night-time water fluxes, but rather displayed wilting and stress known as ‗ammoniacal syndrome'. Thus, under NO3- deprived soil conditions P. vulgaris may be opportunistic in their water uptake, transpiring more when water is available in order to draw nutrients through ‗mass-flow'. This thesis explored and confirmed the functional role of NO3- in regulating day- and night-time water fluxes as a mechanism for increasing ‗mass-flow' acquisition of N and possibly other nutrients, signalling a down-regulation of day-time and night-time water fluxes when [NO3-] is replete (Chapter 3 & 4). Where both NO3- and NH4+ are present in soils, it is the [NO3-] and not [NH4+] that regulated stomatal conductance and transpiration. Since organic nitrogen forms such as amino acids also occur in soils, there is a need for further work on their role in stomatal behaviour. Using amino acids laced with 15N isotopes as a nitrogen source can allow their acquisition and role on stomatal behaviour to be discovered. Current trends in research are focussed around developing real-time in-situ sensing of soil nitrogen status to promote enhanced nitrogen and water use efficiency in agricultural systems. This thesis provides the vital literature on stomatal regulation by [NO3-].

No description available.

The actual evapotranspiration rate for wheat at the Campus Agriculture Center (CAC) and alfalfa at the Maricopa Agricultural Center (MAC) were measured using the Bowen ratio technique for near real-time irrigation scheduling. The Bowen ratio method underestimated evapotranspiration when compared with AZMET and Penman data. There were problems with the hygrometer and the net radiometer which might have caused this underestimation. The height-fetch ratio requirement was not met, and this may have affected the data. Irrigation scheduling programs and the technique to schedule irrigation in a near real-time were developed. Irrigation programs were not field tested because of the problem with field data. Simulated data was successfully used to demonstrate its application. It was found that irrigation could be scheduled in a near real-time with the programs provided reliable data and proper data acquisition systems are used.

I compared the interrelationships among traits affecting interspecific variation in the growth of seedlings of 22 hardwood tree species subjected to chronic intermittent drought. Leaf traits were the most responsive to contrasting water regimes; canopy architecture, hydraulic conductivity and leaf phenology were less affected. Relative growth rate (RGR) under drought, however, was most closely related to variation in canopy duration: species in leaf longer had greater RGR. Secondarily, maintaining higher stomatal conductance (g) under drought further enhanced RGR. Higher g is associated with a higher photosynthetic rate (A), but the relationship between A and g varied with internal CO$ sb2$ concentration (c$ sb{ rm i}$), foliar nitrogen and specific leaf mass. Species with weaker homeostatic control over c$ rm sb{i}$ had higher water use efficiency. While photosynthetic characteristics were related primarily to species' shade tolerance, water use efficiency was most closely related to habitat moisture preference.

No description available.

Thesis (PhD(Agric) (Agronomy))--University of Stellenbosch, 2009. / ENGLISH ABSTRACT: The use of capacitance water sensors for the scheduling of irrigation for hydroponic tomato and cucumber crops grown in coir was investigated in a series of laboratory and glasshouse experiments in the Free State province of South Africa. Laboratory experiments in a climate controlled chamber were conducted to accurately calibrate ECH2O capacitance sensors, models EC-10 and EC-20, in coir with an improved calibration procedure. Water content predictions by the coir-specific calibration and manufacturer’s calibration equations were compared to actual water content measured from mass loss of the coir sample. The manufacturer’s calibration equation indicated a poor accuracy of prediction, which mostly underestimated the volumetric water content, compared to the near perfect prediction of the coir-specific calibration of individual sensors. A rapid calibration procedure for EC-10 and EC-20 sensors was proposed to reduce the calibration time of the sensors and promote their commercial use for irrigation management in coir. The accuracy of prediction by the rapid calibration procedure for the plant available water content range was high for both EC-10 and EC-20 sensors and allowed for the compensation for variation between sensors. Glasshouse studies aimed to characterise the water retention and ability of coir to supply water to greenhouse tomato and cucumber crops through the continuous monitoring of medium water content in small and large growing bags with the EC-10 and EC-20 capacitance sensors during a drying cycle, compared to well-watered plants. Stages of crop water stress were identified and, based only on the plant’s response to the drying cycle, it was suggested that water depletion can be allowed to the point of mild water stress for both greenhouse tomato and cucumber crops, which can be detected by soil water sensors. In a second series of glasshouse experiments, the identified stages of crop water stress were used to determine and apply depletion levels in coir and compare this irrigation strategy to a well-watered treatment for greenhouse cucumber and tomato plants, with regard to the water balance components, yield and water use efficiency for different bag sizes. Results indicated that irrigation was successfully managed to the pre-determined water depletion levels for cucumber and tomato plants in coir, through the use of in situ calibrated capacitance sensors. For both crops the depletion of water varied between bag sizes, indicating that various bag sizes require different irrigation management strategies. Scheduling to the highest pre-determined by 124 L m-2 in the small and 240 L m-2 in the large bags for cucumbers and 427 L m-2 in the small and 487 L m-2 in the large bags for tomato plants, compared to the well-watered treatments. Yields achieved by the greenhouse tomato plants in the large growing bags and cucumber plants in the small and large bags were maintained or improved when scheduled to the highest depletion level (approximately 60% available water content) compared to the well-watered treatment. The combination of reduced irrigation and improved or maintained yields resulted in improved water use efficiencies (based on irrigation and transpiration) for the highest depletion level compared to the well-watered treatments. In all glasshouse experiments the well-watered treatment resulted in luxury water use by the plants. Finally, a study was conducted in order to compare crop water stress of greenhouse cucumber and tomato plants under luxury water supply and cyclic water deficit conditions. The comparison was based on the transpiration ratio and yield, while the use of capacitance sensors was evaluated for irrigation scheduling in coir for both crops. Transpiration data indicated that cucumber and tomato plants subjected to luxury water supply experience water stress earlier than plants subjected to cyclic water deficit conditions, irrespective of bag size. Results also indicated that irrigation scheduling according to water depletion levels in small bags is not yet recommended for greenhouse tomato and cucumber plants grown in coir, until further research is conducted. Scheduling to water depletion levels in large bags is, however, justified by the improved or maintained yields of the greenhouse cucumber and tomato plants. The estimated depletion levels for large bags beyond which yield are reduced was at 85% for tomatoes and 70% for cucumbers. In conclusion, the results clearly indicated that the use of capacitance sensors in large growing bags improves irrigation management of hydroponic cucumbers and tomatoes in coir by eliminating over-irrigation and improving water use efficiency. More research is needed before a conclusion can be made regarding irrigation scheduling with capacitance sensors in small growing bags. / AFRIKAANSE OPSOMMING: Die gebruik van kapasitansie water sensors vir besproeiingskedulering van tamatie en komkommer plante wat hidroponies in kokosveen gegroei is, is ondersoek in ‘n reeks laboratorium en glashuis eksperimente in die Vrystaat provinsie van Suid Afrika. Laboratorium eksperimente is uitgevoer in ‘n klimaat beheerde kas om ECH2O kapasitansie sensors, modelle EC-10 en EC-20, akkuraat te kalibreer vir kokosveen deur ’n verbeterde kalibrasie prosedure. Waterinhoud voorspellings deur die kokosveen spesifieke kalibrasie en die vervaardiger se kalibrasie vergelykings is vergelyk met die werklike waterinhoud wat gemeet is deur die kokosveen monster se massaverlies te monitor. Akkuraatheid van voorspelling deur die vervaardiger se kalibrasie vergelykings was swak en het meestal die volumetriese waterinhoud onderskat in vergelyking met die byna perfekte voorspelling deur die kokosveen spesifieke kalibrasie van individuele sensors. ’n Vinnige kalibrasie prosedure vir die EC-10 en EC-20 sensors is voorgestel om die kalibrasie tyd te verkort en die kommersiële gebruik van die sensors vir besproeiingsbestuur in kokosveen aan te moedig. Die akkuraatheid van voorspelling deur die vinnige kalibrasie prosedure, binne die grense van plant beskikbare waterinhoud, was hoog vir beide EC-10 en EC-20 sensors, terwyl die prosedure ook voorsiening maak vir variasie tussen sensors. Glashuis studies is uitgevoer om die water retensie en vermoë van kokosveen om water te voorsien aan tamatie en komkommer gewasse in kweekhuise, te karakteriseer. Dit is bereik deur die mediumwaterinhoud van klein en groot plantsakke deurlopend te monitor met behulp van die EC-10 en EC-20 kapasitansie sensors gedurende ’n uitdroging siklus, en dit te vergelyk met ’n waterryke behandeling vir elke gewas waarvolgens die plante agt keer per dag besproei is. Fases van gewas waterstremming is geïdentifiseer en, volgens die reaksie van die plant tot die drogingsiklus, is dit voorgestel dat wateronttrekking toegelaat kan word tot die punt van matige waterstremming wat aangewys kan word deur kapasitansie water sensors vir beide kweekhuis tamatie en komkommer gewasse. In ’n tweede reeks glashuis eksperimente is die geïdentifiseerde fases van gewas waterstremming gebruik om onttrekkingsvlakke vir kokosveen te bepaal en toe te pas as besproeiingskeduleringstrategie vir kweekhuis komkommer en tamatie plante. Toegepaste vlakke is vir elke gewas vergelyk met ’n waterryke behandeling ten opsigte van die waterbalans komponente, opbrengs en watergebruiksdoeltreffendheid in verskillede sakgroottes. Resultate het aangedui dat besproeiing suksesvol bestuur is tot die voorafbepaalde wateronttrekkingsvlakke vir komkommer entamatie plante in kokosveen, deur gebruik te maak van in situ gekalibreerde kapasitansie sensors. Die onttrekking van water deur beide gewasse het verskil tussen klein en groot sakke, wat aangedui het dat verskillende sakgroottes verskillende besproeiingsbestuur strategieë vereis. Skedulering tot die hoogste voorafbepaalde onttrekkingsvlak het, in vergelyking met die waterryke behandelings, besproeiing verminder met 124 L m-2 in die klein en 240 L m-2 in die groot sakke vir komkommers, en 427 L m-2 in die klein en 487 L m-2 in die groot sakke vir tamatie plante. Opbrengste van kweekhuis tamatie plante in die groot plantsakke en komkommer plante in die klein en groot sakke is gehandhaaf of verbeter deur skedulering tot die hoogste onttrekkingsvlak (ongeveer 60% van beskikbare water inhoud), in vergelyking met die waterryke behandeling. Die kombinasie van verminderde besproeiing en verbeterde of gehandhaafde opbrengste het gelei tot verbeterde watergebruiksdoeltreffendheid (besproeiing en transpirasie) vir die hoogste onttrekkingsvlak, in vergelyking met die waterryke behandelings. In al die glashuis eksperimente het die waterryke behandeling gelei tot oorvloedige watergebruik deur plante. ’n Finale studie is uitgevoer om gewas waterstremming van kweekhuis komkommer en tamatie plante wat onderwerp is aan oorvloedige watervoorsiening deur agt keer per dag te besproei en sikliese watertekorttoestande, te vergelyk. Die vergelyking is gebaseer op die transpirasie verhouding en opbrengs, terwyl die gebruik van kapasitansie sensors vir besproeiingskedulering in kokosveen vir beide gewasse geëvalueer is. Transpirasie data het aangedui dat komkommer en tamatie plante wat onderwerp is aan oorvloedige watervoorsiening vroeër waterstremming ervaar as plante wat onderwerp is aan sikliese watertekorttoestande, ongeag van die sakgrootte. Resultate het aangedui dat besproeiingskedulering volgens wateronttrekkingsvlakke vir klein sakke nog nie aanbeveel kan word vir kweekhuis tamatie en komkommer plante alvorens verdere navorsing gedoen is nie. Skedulering tot wateronttrekkingsvlakke vir groot sakke word egter geregverdig deur die verbeterde of gehandhaafde opbrengste van kweekhuis komkommers en tamaties. Die beraamde laagste onttrekkingsvlakke vir groot sakke wat nie opbrengs betekenisvol sal beïnvloed nie is 85% vir tamaties en 70% vir komkommers. Ten slotte dui die resultate duidelik daarop dat die gebruik van kapasitansie sensors in groot plantsakke besproeiingsbestuur van hidroponiese komkommers en tamaties in kokosveen verbeter deur oorbesproeiing uit te skakel en die watergebruiksdoeltreffendheid te verbeter. Meer navorsing is nodig alvorens ’n gevolgtrekking gemaak kan word ten opsigte van besproeiingskedulering met kapasitansie sensors in klein plantsakke.

No description available.

Both red (Trifolium pratense L.) and white (Trifolium repens L.) clover seed yields can be highly variable and low in western Oregon. The objectives of this study were to: i) determine crop water requirements and supplemental irrigation timing, and ii) quantify the effects of soil and water status on inflorescence production, seed yield and seed yield components for red and white clover seed crops. In each species, five supplemental irrigation treatments were applied in 1990 and 1991 to first and second year seed crops grown on a Woodburn silt loam (fine-silty, mixed, mesic Aquultic Argixeroll) near Corvallis, OR. Non-irrigated controls were also maintained. In red clover, increased plant water stress reduced the duration of the season-long bud and flower production, stem length, potential floral capacity (PFC), and seed yield (SY). Root rot index (RRI) increased with increasing levels of plant water stress, indicating that supplemental water applications reduced second-year root rot severity. The reduction in SY from increasing plant water stress was primarily caused by a decrease in floral fertility, and less conclusively by reductions in inflorescence number per unit area. One irrigation to fill the soil active profile during peak flowering provided adequate water to maintain efficient seed production. In white clover, SY was maximum in 1990 when water application was delayed until 68% of the available soil-water was used by the crop which maintained an even flush of flowers and restricted vegetative growth. In 1991, all irrigation treatments yielded the same or less than the non-watered control. This was due to the excessive vegetative growth from stolons that had grown between the planted rows the previous and present crop year. In both years excessive amounts of irrigation water favored profuse vegetative growth and reduced SY. Inflorescence density was increased by constraining soil-water in 1990 and was the yield component that most affected SY both years. Crop water stress index (CWSI) was a useful indicator of plant stress status and can be used to schedule irrigations in red and white clovers grown for seed under typical climatic conditions of western Oregon. / Graduation date: 1993

The application of the crop water stress index (CWSI) method to irrigation scheduling is enhanced by knowledge of the relationship between CWSI and soil water potential (SWP) and how this relationship is affected by soil texture. A study using the same cultivar of cotton on three different soils was conducted in southern Arizona over a single growing season. Detailed data were collected of CWSI and soil moisture content for several treatments that scheduled irrigations at threshold CWSI values. CWSI was correlated with soil water potential values calculated from pressure plate determined moisture release curves. Spatial variability of soil characteristics necessitated use of average rather than plot specific moisture release curves. Analysis showed a linear CWSI-SWP relationship that varied greatly with soil depth and study site. The study concluded that soil profile average SWP alone does not normalize the CWSI between sites with different soil textures.