Several structural changes in cotton (Gossypium hirsutum L.) leaves attendant on development under conditions of water deficit were examined. Cell size was less and cell wall thickness greater in the leaves of stressed plants than in leaves of well‐watered plants. A short review of the literature suggested that the lesser cell size is a fairly general observation and that it may contribute to plant resistance to moisture stress. A simple model is developed to investigate the influence of the reduction of cell size on cellular water relations. The predictions which can be drawn from simulations with this model are that smaller cells should maintain turgor to lower values of water potential than larger cells. Rather large changes in cell water relations are predicted for small changes in cell size. These effects are related principally to the changing proportion of cell water which resides in the cell wall and is external to the plasmalemma and the osmotic adjustment system. This prediction is in agreement with several observerations on the behavior of stress‐hardened plants and supports the hypothesis that plants or tissues with the smaller cell size will be more tolerant of low water potential.

Since it is now well demonstrated that Ca is required to maintain the structural and functional integrity of plant cell membranes (1,2,12,14,18), Ca was included at 0.2 mm in all experimental solutions. The pH of absorption solutions, initially 5.5, did not vary by more than 0.2 units during the experiments. The absorption period, which varied from 2 to 120 min, was terminated by one of two methods, depending on whether or not that fraction of labeled Cd associated with the tissue in readily exchangeable form was to be removed. When it was desirable to desorb the labile fraction the absorption period was followed by a desorption period in a solution containing 0.2 mm Ca(NO3)2 and unlabeled Cd, or other ions. When it was not desirable to remove this labile fraction, the samples were rinsed in water after the absorption period. All experimental solutions were of sufficient volume that depletion did not exceed 10% of the initial Cd content.Analyses of roots, as described by Epstein et al. (3), were carried out on a Beckman Low Beta II Counter. All points plotted in the figures represent the mean of duplicate analyses on two or more root samples.

Experiments were designed to test the hypothesis that the internal water relations of leaves are altered when cotton plants (Gossypium hirsutum L.‘Acala SJ‐2′) are conditioned by several cycles of water stress. Preliminary experiments suggested that plants so conditioned are less sensitive to water deficits and that the change might be partly explained by an accumulation of solutes or by structural alterations attendant on development under conditions of water stress. Leaves of preconditioned plants maintained turgor to lower values of water potential than did leaves of well‐watered plants. Accompanying this change was a lower osmotic potential at any given leaf water content in preconditioned plants. Tissue analysis of several osmotically active solutes indicated that soluble sugars and malate accumulate to about the same levels (dry‐weight basis) in both conditioned and unconditioned plants exposed to stress. These accumulations could not account for the turgor change. Analysis of the data on relative water content indicated that the leaves of conditioned plants had less water per unit dry weight than did leaves of controls. This change accounts for a substantial fraction of the difference between the osmotic potential of conditioned and control plants. The results of a simple model suggest that structural changes may play a significant role in explaining differences in the responses of conditioned and control plants to water stress.

The influence of water deficits on leaf elongation of several cultivars of upland rice (Oryza sativa L.) was studied. Mild water deficits resulted in marked reductions in leaf elongation rates. Previous conditioning, by exposing plants to one or more moderate water stresses, decreased the sensitivity to subsequent stress allowing elongation to continue to more negative water potential. This reduced sensitivity in conditioned plants was accompanied by an enhanced capacity for tugor maintenance resulting from osmotic adjustment but was not entirely explained by this adjustment. The relation between leaf elongation rate and turgor potential was also altered in conditioned plants, indicating that growthinfluencing characters in addition to tugor maintenance capacity were altered. Control and conditioned plants appeared to have similar turgor thresholds for leaf elongation but, above this threshold, control plants required less turgor for similar rates of elongation. Under slower drying conditions, despite reductions of water potential, bulk leaf turgor was unchanged as a result of osmotic adjustment. However, despite this turgor maintenance, leaf expansion rates were markedly reduced. Although the sensitivity of leaf expansion to water deficits was similar among the varieties studied, this response provides a sensitive basis for scoring for the incidence of stress and provides a means for rapid screening for drought resistance in larger varietal comparisons.

Some dynamic aspects of leaf elongation in rice were studied. Under both well watered and water-deficient conditions, leaf elongation rates were 15 to 30% greater during the day than during the night. Night temperatures below 27 C limited the rate of elongation at night but when night temperatures exceeded 27 C, night elongation rates exceeded rates during the day. The diurnal pattern of elongation was opposite to the pattern of bulk leaf turgor which was lower during the day than at night.Superimposed on the general diurnal pattern of leaf elongation were perturbations associated with the Ught/dark transitions. The rate of leaf elongation declined within minutes after illumination and remained low for 15 to 60 minutes, after which rapid rates ensued. Leaf elongation is one of the plant processes most sensitive to water deficits (16). This high sensitivity may provide an explanation for the effect of drought on crop yields (17). The role of water in leaf elongation is thought to be mediated by 4p2 which interacts with cell wall yield properties to determine the rate of expansion (21). In the absence of osmotic adjustment, 4p declines rapidly as water deficits develop and has been suggested to account for the high sensitivity of leaf expansion to water deficits (17). The turgor growth model appears to explain a large number of observations on cell and tissue expansion, although in perturbed or dynamic environments it is necessary that the parameters describing cell wall yield properties must be very responsive to changes of water status (1,12,13 In previous experiments we investigated the sensitivity of leaf elongation to soil drying in several upland rice cultivars (10). Total daily leaf elongation was very sensitive to the development of water deficits and the elongation of leaves of plants previously exposed to cycles of stress was less sensitive to subsequent deficits. The present experiments were undertaken to investigate the environmental influences on diurnal patterns of elongation and to further investigate the influence of water status on leaf elongation of rice. MATERIALS AND METHODSPlant Materials. Seeds of upland rice (Oryza sativa, L.) cv. 'Kinandang Patong' were soaked, planted, and grown in pots in a greenhouse (30-35 C day/20-25 C night) as previously described (9). Pots were transferred to a growth chamber programmed at 30 C day/25 C night 2-6 days before experimentation. Light (1,400-1,600 ,E m-2 s-1 at plant height) was supplied by a combination of fluorescent tubes and incandescent bulbs for 12 h during the day period. Control plants were irrigated twice daily and fertilized weekly . Conditioned plants were subjected to three cycles of stress during which predawn leaf rolling was observed and then irrigated normally for 2 days prior to experimentation. In some instances, plants were stressed by withholding irrigation for various amounts of time but were otherwise irrigated twice daily (well watered).Plants grown in the same greenhouse in an aeroponic culture system modified from that o...