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    The effect of mild water stress on vegetative growth in tomato (Lycopersicon esculentum Mill.) and Pyrus betulaefolia Bunge : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Horticultural Science at Massey University
    (Massey University, 1991) Saunders, Andrew Barrington
    The effects of mild water stress and physical root restriction on leaf parameters and assimilate partitioning have been studied in order to understand and refine orchard management techniques which utilise both of the above stress elements (e.g. regulated deficit irrigation (RDI)). Initially, a system for applying a controlled level of water stress was developed and plant responses within this system defined. The system involved using aeroponic tanks with water stress generated by cycling the misting pumps on and off (intermittent misting). Similar systems have been used by other workers to stimulate hormonal changes but no work has been reported detailing vegetative growth responses. The intermittent misting technique was compared to polyethylene glycol (PEG) generated water stress, using tomato (Lycopersicon esculentum Mill. cv. Virosa Fl) as a model plant. Water stress studies were then carried out on Pyrus betulaefolia (an important root-stock for the asian pear (nashi) fruit crop (Pyrus serotina)) using intermittent misting. These results were compared to those from a root restriction trial involving P. betulaefolia in a circulating hydroponic system. The performance of polyethylene glycol in the aeroponic system appeared to be better than in the various hydroponic systems which have been reported. Polyethylene glycol 4000 gave the best results (from PEG 1000, 4000 and 6000), in terms of in minimum level of phytotoxicity, up to a total nutrient solution water potential ('Pw) of around -6 bar. Under an intermittent misting regime, tomato plants were subjected to a range of misting pump off-times up to 1.55 hours, with a constant on-time of 1 minute (to saturate the root system). It was found that important plant parameters could be related in a negative logarithmic fashion to misting pump off-time (e.g. leaf Ww, p lant part dry weights, allometric k value, net photosynthetic · rate and stomatal conductance). For P. betulaefolia trees, exposed to misting pump off-times of up to two hours, plant parameters were also related to the negative logarithm of the misting pump off-time. This was despite the fact that the tomato seedlings grew approximately exponentially while P. betulaefolia plants grew in a more linear fashion. Hence it was concluded that intermittent misting was an ideal method for generating a controlled water stress (under which plant responses could be predicted) in both pure and applied experimental work. Under physical root restriction (with water stress minimized), no significant differences were found in several important parameters, including net photosynthetic rate. Also, in contrast to the water stress response, assimilate partitioning to the shoot system increased (increase in the allometric k value) . The relative increase in partitioning was greatest in the stem component, this being the plant part most severely affected by water stress.- Under both water stress and physical root restriction the allometric k value appeared to change rapidly, following application of the treatment, and then remain constant. This constancy was tested by using a previously unutilized plot involving the shoot/root ratio versus time linearized plant dry weight. The results of pl ant responses to both water stress and physical root restriction are discussed in relation to vegetative growth control measures in frui t crops such as RDI. Consideration is also given to the overall mechanisms behind observed growth responses under the two stress regimes.
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    The effect of water stress on water relations, carbon isotype discrimination, and shoot and root growth of sainfoin (Onobrychis viciifolia Scop.) and lucerne (Medicago sativa L.) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Department of Plant Science at Massey University
    (Massey University, 1994) Mir-Hosseini-Dehabadi, Seyed Reza
    Sainfoin (Onobrychis viciifolia Scop.) is a useful forage legume regarded as having drought resistant attributes. Also, it does not cause bloat in ruminants and is not sensitive to alfalfa weevil (Hypera postica. L). Although the physiological and morphological responses to water stress of lucerne (Medicago sativa L.) are well known the responses of sainfoin to water stress have not been fully studied. In this study the physiological and morphological responses of sainfoin to water stress were investigated, with lucerne used as a reference plant. The results of the indoor and outdoor studies showed sainfoin had useful characteristics for forage production in dry conditions. Relative to lucerne it had a lower yield, due to lower leaf area, lower stem number and poor regrowth. However, sainfoin responded to water stress at least as well as lucerne. Sainfoin had a higher root:shoot ratio and a lower specific leaf area ratio than lucerne, indicating a higher allocation of carbohydrate to the roots, and a lower leaf surface area for transpiration in sainfoin than for lucerne. Water stress decreased the yield of lucerne proportionally more than sainfoin mostly due to the greater reduction in the above ground dry weight of lucerne. The indoor study of root characteristics of sainfoin and lucerne in 1m tall tubes showed that in terms of root development sainfoin responded to water stress better than lucerne. Although sainfoin had equal root mass and root length to lucerne, the root distribution of sainfoin at below 0.6 m depths was greater than for lucerne. As water stress developed sainfoin roots grew below 0.6 m earlier than lucerne roots. Sainfoin had a higher root osmotic adjustment than lucerne and also maintained higher (less negative) leaf water potential than lucerne. The stomatal resistances (Rs) of sainfoin and lucerne were equal, but Rs was not distributed equally between adaxial and abaxial leaf surfaces. The Rs of the adaxial leaf surface of sainfoin was lower and more sensitive to water stress than the Rs of the abaxial leaf surface. The different Rs of the adaxial and abaxial leaf surfaces of sainfoin was partly due to the different stomatal frequencies of the respective surfaces. Comparison of sainfoin cultivars in a climate room showed that the water use efficiencies (WUE) of Remont, Fakir, Cotswold-Common, and Eski, were similar. Remont was more sensitive to water stress than the other three cultivars, and Eski produced a greater root length and mass than other cultivars. The growth of Eski was initially slower than that of the Remont in both the indoor and the outdoor studies. However, lucerne grew faster than all the sainfoin cultivars. Over three harvests in the field the yields of Eski and Remont were similar but lucerne out yielded both sainfoin cultivars. Sainfoin produced a greater proportion of its yield earlier than lucerne, whereas lucerne distributed its yield throughout the whole season, indicating that sainfoin is adapted to regions with precipitation in only winter and spring. The results of the carbon isotope discrimination (Δ) analysis for the indoor and outdoor studies showed Δ had a negative correlation with WUE, leaf water potential, osmotic potential, and stomatal resistance, but had a positive correlation with relative water content, turgor potential, transpiration rate, and photosynthetic rate. These correlations demonstrated the usefulness of this technique for evaluating the responses of plants to water stress. The stressed plants always had lower Δ than the control plants showing the higher WUE of stressed plants. The Δ of roots was higher than the Δ of the leaves suggesting that the growth of leaves occurred in conditions that were an average drier than for the growth of roots. This was supported by the lower (more negative) water potential of leaves than roots. The Δ of the roots below 0.6 m depth was higher than the Δ of roots above 0.1 m depth suggesting the roots above 0.1m grew under higher water stress than the roots below 0.6m depth. Over three harvests in the field the Δ of Eski and lucerne were similar and the Δ of Remont was higher than for Eski and lucerne. In conclusion, sainfoin was found to have several useful attributes for growth and survival in dry regions. Of the sainfoin cultivars examined Eski was the best adapted to water stress. Relative to lucerne, sainfoin yielded less, but had a similar water use efficiency, a shorter season of growth, a greater root: shoot ratio, deeper roots and better maintenance of leaf water potential under water stress.
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    Aspects of water deficit and vegetative growth in selected pasture and forage grasses : a thesis presented in partial fulfilment of the requirement of the degree of Doctor of Philosophy at Massey University, Palmerston North, New Zealand
    (Massey University, 1979) Chu, Alexander Cheong Ping
    In this study, the sensitivity of leaf extension to plant water status, the ability of the plant to recover after different periods of water deficit and the plant's reaction to atmospheric pre-conditioning were examined using different pasture and forage grasses. The sensitivity of leaf extension to plant water status was studied in 3 separate experiments using sudax (SX-6, a forage sorghum hybrid, Sorghum bicolor (L) Moench x S. Sudanese (piper) Staff), prairie grass (Bromus catharticus Vahl. cv. Grasslands Matua) and 2 cultivars of perennial ryegrass (Lolium perenne L. c.v. Grasslands Nui and Grasslands Ruanui). The sudax experiment was conducted in the field, whereas the prairie grass and ryegrass experiments were conducted in the Climate Laboratory, D.S.I.R. The day/night temperatures used in the prairie grass experiment was 22.5°/l2.5°C. For the ryegrass, 2 contrasting temperature regimes were used; these were high (H), 27.5°/12.5°C, and low (L) 17.5°/12.5°C day/night temperatures. It was found that leaf extension was very sensitive to small changes in leaf water potential during the initial stages of desiccation, but the response became less sensitive with increasing levels of desiccation. However, the relationship between leaf water potential and leaf extension rate was not unique. It varied according to the environmental conditions. The true relationship between leaf water potential and leaf extension rate can only be established when the leaf water potential at the site of measurement can be related unequivocally to the leaf water potential at the site of elongation. The rates of recovery in leaf extension, leaf emergence, tiller number, green leaf number, leaf area and dry weight per plant were followed after different water deficit treatments in one experiment with prairie grass and in another experiment with 2 cultivars of perennial ryegrass under 2 contrasting temperature regimes. The environmental conditions for these experiments were the same as those used in the leaf extension experiments. In prairie grass, upon relief of water deficit, the previously desiccated plants showed an "accelerated" rate of leaf extension up to 20% higher than those of the well-watered control plants of the same physiological age. The "accelerated" rate lasted for over 28 days after rewatering during which time 4 to 5 new leaves emerged. However no such "accelerated" rates were observed in the ryegrasses. The "accelerated" response following rewatering in prairie grass would be consistent with a differential sensitivity of cell division and cell elongation to water deficit. The desiccation treatment was more severe in the ryegrass experiment where both cell division and cell elongation could be suppressed, and this could account for the absence of a similar response in the ryegrasses. Under well-watered conditions, the mean leaf emergence rate was 4.1 days per leaf for the prairie grass. The corresponding mean leaf emergence rates for Nui and Ruanui were 5.7 and 6.3 days/leaf under the H and 6.6 and 6.6 days/leaf under the L temperature regimes respectively. Within the grass species, post-desiccation leaf emergence rates between the previously desiccated and the well-watered plants were similar. During desiccation, tiller number was the least sensitive parameter to water deficit, followed by dry weight and leaf number. Leaf area was the parameter most sensitive to desiccation. Amongst the dry weight components, lamina, component was the most sensitive followed by the root component with the sheath component the least sensitive to desiccation. The pattern of recovery from desiccation was examined to see if positive or negative carryover effects occurred. To enable valid comparison of desiccated and control plants physiological age was adjusted by removing a number of "drought" days from the chronological age. It was found that when the desiccation was mild e.g., in the prairie grass experiment, reductions in plant dry weights were proportional to the number of "drought" days. On the other hand, under a more severe level of desiccation, e.g., as in the ryegrass experiments, using the same method of adjustment, it was found that the dry weights of the previously desiccated plants were substantially lower than those of the well-watered control plants of the same physiological age. The reduction in dry weight was more pronounced under the H than under the L temperature regime. After rewatering, in both prairie grass and ryegrass, the relative rates of increase of leaf area were higher in the previously desiccated plants than the well-watered control plants. In contrast to this, the relative rates of increase in dry weight, tiller number and leaf number in the previously desiccated plants were either similar to, or slightly less than those of the well-watered control plants. Although the pattern of recovery in the leaf extension rates was different between the two experiments, this had no apparent positive or negative carryover effects on the relative rates of recovery in the growth parameters measured (e.g., tiller number, green leaf number, leaf area and dry weight per plant). The reaction of prairie grass to desiccation following either a "dry" or a "wet" atmospheric pre-conditioning was compared with those plants that were grown continuously under either the "dry" or the "wet" vapour pressure environments. Plants with a previously "dry" history were able to grow longer into the desiccation period than those with the "wet" history as well as those under the continuous "wet" or "dry" conditions. This apparent "adaptation" was due to a more efficient rate of water use per unit leaf area by the "hardened" plants. But the mechanism that enabled these plants to use water more efficiently was not known. Nui had been reported to outyield Ruanui under the summer and autumn conditions in New Zealand. Because of the importance of perennial ryegrass to New Zealand, a comparison of these 2 cultivars was also made in this study. Between the two cultivars of ryegrass, Nui had a higher leaf extension rate (+20%) under the H temperature regime, it also had a heavier mean tiller dry weight (+28%), a larger mean area per leaf (+24%), but a lower tiller number (-24%) and a lower green leaf number (-18%) per plant than Ruanui. All the other paramters measured, including total leaf area and total dry weight per plant, top to root ratio, specific leaf area, leaf area ratio, stomatal resistance and transpiration rates were similar between the two ryegrass cultivars. Some of the possible reasons for the lack of difference on a per plant basis are discussed. The possibility of using leaf extension rate to predict plant dry weight changes in water deficit studies is also discussed.