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This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.

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Physiological and agronomic responses of two pearl millet (Pennisetum americanum L. K. Schum) parents and their hybrid were evaluated under different water levels on a Brazito sandy loam soil at Tucson, Arizona in 1983 and 1984. A line source sprinkler irrigation gradient was used to create the treatments. Soil moisture and physiological parameters were measured under field conditions at weekly intervals. Growth was analyzed every other week, while yield and yield components were evaluated at the end of the experiment. Harvest Index, drought tolerance index, and water use efficiency (WUE) were calculated. Percent crude protein of seeds was analyzed, and photosynthesis was measured in 1984 only. Regression analysis was used to compare the performance of the cultivars for stability. Water stress significantly affected all parameters measured, and the stress effects were more pronounced in 1983. There was no significant difference between entries at each water level for most parameters. Plant height, dry matter, and total leaf area decreased as watering level decreased. The leaf area of the hybrid was significantly higher at high water level in 1983. Firing ratio was increased significantly by stress in both seasons and was significantly lower for the male at low water level in 1983. Yield and yield components were reduced significantly by stress in both seasons, which was reflected in the reduced harvest index. Relative yield of dry matter was higher for the male in both seasons. The hybrid had the highest drought tolerance index in 1983, while the female had the highest drought tolerance index in 1984. WUE of dry matter increased with stress, while WUE of grain yield decreased with stress in both seasons. The significant reduction in transpiration was 57, 56 and 66% in 1983 and 45, 53, and 52% in 1984 for female, male, and hybrid respectively. Leaf diffusive resistance and leaf temperature were increased by water stress in both seasons. Stability regression analysis provided no adequate differentiation among cultivars. The percent crude protein (weight basis) was increased significantly by stress by 38, 43, and 28% in 1983 and by 47, 33, 46% for female, male, and hybrid, respectively.

The response of three Eucalyptus spp. clones (GC550, GU210 and TAG14) to water availability was assessed in terms of growth, plant water status, leaf gas exchange, whole plant hydraulic characteristics (both root and shoot), stem xylem vulnerability. Furthermore, to experimentally assess the influence of hydraulic conductance on leaf physiology and plant growth, specimens of two of the clones were subjected to long-term root chilling. Prior to harvesting data were collected on the diurnal variation in leaf water potential (ΨL), transpiration rate (E), stomatal conductance (gs) and net CO2 assimilation rate (A). Main stem xylem vulnerability was assessed using ultrasonic acoustic emissions (UAE). Vulnerability of the main stem was assessed as the leaf water potential corresponding to the maximum rate of acoustic emissions (ΨL, EPHmax), and as the critical water potential triggering cavitation events, calculated as the mean of the water potentials of data points lying between 5 and 10% of the total accumulated emissions (ΨCAV,cUAE,%). Hydraulic conductance was measured on roots and shoots using the high-pressure flow meter (HPFM). Root data were expressed per unit root dry mass (Kr/trdw) and per unit leaf areas (Kr/LA), shoot data expressed per unit shoot dry mass (Ks/tsdw) and per unit leaf area (Ks/LA), and whole plant conductance was expressed per unit leaf area (KP/LA). Soil-to-leaf hydraulic conductance was also assessed as the inverse of the slope of the relationship between leaf water potential and transpiration rate (the evaporative flux, EF, method). A field study was undertaken on three month old TAGl4 and GU210 plants. Diurnal values of leaf water potential ΨL, E and gs were consistently higher in TAG14 than GU210, but A did not differ among the clones. Main stem xylem vulnerability (ΨCAV, cUAE,%) was higher in TAG14 than GU210. In both clones midday ΨL fell below ΨcAv,cUAE,%, suggesting lack of stomatal control of xylem cavitation. Kr/LA was higher in TAGl4 than GU210, whereas, Ks/LA and Ks/tsdw was higher in GU210 than TAGI4. A greater proportion of hydraulic resistances resided in the roots, particularly in GU210. Kp/LA was higher in TAGl4 than GU210 clone, although the significance was marginal (P=0.089). However, all the physiological measurements, were consistent with the concept of higher hydraulic conductances in TAGl4 leading to lower leaf level water stress. Above ground biomass was higher in TAG14 than GU210, in agreement with this concept, although this clone was more vulnerable than GU210. Material grown for 14 months in 25 l pots clones showed no differences in ΨSoil between the high and low watering supply, indicating that even the 'high' supply was inadequate to prevent water stress. In accordance with this, diurnal values of ΨL, gs, E and A did not differ significantly between treatments and clones. Early stomatal closure was apparent, maintaining ΨL constant during the middle of the day. Stem xylem vulnerability, assessed as both ΨL,EPHrnax and ΨCAV,cUAE,% showed that the main stem of GC550 was more vulnerable than other two clones, and that low watered plants were more resistant to xylem cavitation than those receiving high water. Midday ΨL fell below the vulnerability values assessed by both measures across treatments and clones, suggesting lack of stomatal control preventing stem xylem cavitation. There was no relationship between stem xylem cavitation and the shoot hydraulic conductances. Root pressures did not differ between either treatment or clones. Kr/LA was marginally higher in high watered plants, and Ks/LA and Ks/tsdw were higher in low watered plants, possibly by adjustment of leaf hydraulic architecture, and there were no clonal differences. Kr/LA was much lower than Ks/LA. Kp/LA did not differ between the watering treatment, but there was a clonal effect. Growth in dry mass was higher in high watered than low watered plants, but there were no differences among clones. As KP/LA was not affected by watering treatment there was no relationship between KP/LA and growth in total biomass. In plants grown for 21 months in 85 l pots low water treatment decreased midday ΨL, gs, E and A relative to high watered plants. Interclonal differences occurred at midday. Stem xylem vulnerability assessed as ΨCAV,cUAE,% and as ΨL,EPHrnax show similar trends as in the 14 months saplings, clonal differences being significant in ΨL, EPHmax. There was a 1:1 relationship between minimum leaf water potential and ΨL, EPHmax, suggesting that the water potential developed was limited by stem vulnerability. This implies stomatal control to reduce transpiration rates to prevent extensive cavitation occurring. These plants did not develop positive root pressures, indicating that recovery from xylem cavitations occurred through some other process. Kr/LA was higher in high watered plants than those receiving low water, and clonal differences were observed in Kr/trdw. There was no treatment effect in KS/LA and KS/tsdw, but a clonal effect was apparent. KP/LA was significantly different between treatment, and was reduced by low water in two clones, and increased by this in TAGI4. Reduced water availability reduced biomass production, with a greater effect on roots than shoots, such that low watering reduced root:shoot ratios. There was a weak but significant relationship between whole plant hydraulic conductance and maximum stomatal conductance, and between plant conductance and total biomass produced; these data are consistent with the concept of some hydraulic limitation to growth. Root chilling (achieved through chilling the soil) of two of the clones was used to experimentally manipulate hydraulic conductance to test the hydraulic limitation hypothesis. Short-term root chilling decreased both Kr/LA and KP/LA in both clones, but had marginal effects on leaf gas exchange. With long-term chilling the decrease in Kr/LA was observed only in GU210, with TAGl4 showing some adjustment to the treatment. As the roots constitute the major hydraulic resistance, KP/LA largely reflected those of the roots. Long-term root chilling significantly affected leaf physiological characteristics, despite the lack of effect on hydraulic conductance in TAGI4. Long term chilling decreased the whole plant dry mass, but the effect was smaller in TAGI4, and this clone also showed morphological adjustment, in that root growth was less adversely affected than shoot growth. The data from GU210 support the hydraulic limitation hypothesis; because of the morphological and physiological adjustment to long-term root chilling in TAGI4, the data are unsuitable to directly assess the hypothesis. / Thesis (Ph.D.)-University of Natal, Durban, 2002.

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-].

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