Mycorrhizal (VAM) and phosphorus (P)-supplemented nonmycorrhizal neem plants (non-VAM) of comparable size and tissue nutrition were subjected to a slowly developing drought. VAM and non-VAM plants responded to drought similarly. However, mycorrhiza compensated for low P supply, allowing VAM plants to have comparable growth, tissue P, and other physiological parameters as non-VAM plants, which received higher P supply. Drought decreased growth, transpiration (E), photosynthetic rate (A), stomatal conductance (gs ), and plant water status. Osmotic adjustment did not occur, but the relatively low osmotic potential of this species helped maintain turgor during drought. Plant water relations and A of stressed plants fully recovered in 24 hours after rehydration, while gs and E partially recovered. Instantaneous water use efficiency (A/E) increased during drought and recovery, except for a decrease at peak stress due to very low A. Carbon isotope discrimination (D) values of mature leaves remained constant regardless of mycorrhiza or drought. However, D decreased in expanding leaves that developed during a drought period, indicating an increased long-term water use efficiency of these leaves.
Plants of F. chiloensis cv. BSP14 (FC) and F. virginiana cv. NCC85-13V (FV) were stressed until wilting, then watered for 2 days prior to measurement. Diurnal measurements of leaf conductance and water relations were conducted. Leaf conductance of stressed FC plants was generally lower, than that of controls at most times, but there wee no difference between the two in FV. Leaf conductance and transpiration rates had not fully recovered to pre-stress levels within this recovery period, Leaf wafer potential declined from predawn to midday, more in stressed than control plants of both species. Leaf osmotic potential averaged 0.4 and 0.2 MPa lower in stressed than control FC and FV plants, respectively, Greater differences occurred at midday than predawn. Leaf pressure potential of stressed plants was higher predawn than midday, 1.4 vs. 0.7 MPa, in FC; it was not different for FV at most times. The difference in water relations between these two species may be explained by a greater residual effect from the osmotic adjustment in FC es compared to FV that occurred during prior water deficit stress.
To understand the relationship between preharvest water stress and postharvest weight loss, carrot cultivars Eagle and Paramount were grown in muck soil in 6-L pots (eight carrots per pot) in a greenhouse at the Univ. of British Columbia. The plants were watered to field capacity every second day for 4 months before receiving 100, 75, 50, and 25% field capacity water stress treatments, henceforth referred to as low, medium, high, and severe water stress, respectively. Postharvest weight loss of carrots was monitored at 13°C and 32% relative humidity. Carrot weight loss increased with duration of storage in all treatments. It was low in the low-water-stressed and high in severely water-stressed carrots for both cultivars. Root crown diameter, weight, water, and osmotic potential decreased, and specific surface area and relative solute leakage increased with increasing preharvest water stress. Water potential followed by relative solute leakage were the variables that affected weight loss the most. The results show that carrots adjust to water stress by lowering water and osmotic potential. Preharvest water stress lowers membrane integrity of carrot roots making them lose more moisture during storage.
Root hydraulic conductivity (Lp) and osmotic potential (π) were measured in young, drought-stressed and non-stressed peach (Prunus persica), Olive (Olea europea), Citrumelo (Citrus paradisi x Poncirus trifoliata) and Pistachio (Pistachia integerrima) plants. Drought stress reduced Lp 2.5 to 4.2-fold, depending on species, but π was reduced only in expanded citrumelo leaves and unexpanded olive leaves by 0.34 and 1.4 MPa, respectively. A simulation model of plant water uptake and leaf water relations was constructed to quantify the offsetting effects of reduced Lp and osmotic adjustment (OA) on turgor maintenance. For olive data, a 2.5-fold reduction of Lp caused a linear decrease in turgor pressure difference between stressed and non-stressed plants, such that the effect of OA was totally offset at a leaf water potential (stressed) of ≈ -3.0 MPa. For citrumelo, because the degree of OA was lower, the water potential at which the effects of OA and reduced Lp were offsetting with respect to turgor maintenance was ≈ -0.6 MPa. The analysis suggests that some level of stomatal closure would be necessary to extend the water potential range over which stressed plants maintain higher turgor than non-stressed plants for citrumelo. Conversely, no degree of stomatal closure would be required of stressed olive plants to maintain higher turgor than non-stressed counterparts over a physiologically meaningful range of leaf water potential.
In order to evaluate and compare adaptability to dry sites, plant water relations and leaf gas exchange were compared in response to water stress among six birch species: monarch birch (Betula maximowicziana), river birch (B. nigra), paper birch (B. papyrifera), European birch (B. pendula), `Whitespire' Japanese birch (B. platyphylla var. japonica `Whitespire'), and gray birch (B. pendula). After 28 days without irrigation, Japanese birch maintained significantly higher stomatal conductance (gs) and net photosynthesis (Pn) than did any of the other species, despite having one of the lowest mid-day water potentials. Evaluation of tissue water relations, using pressure-volume methodology, showed no evidence of osmotic adjustment for any of these species in response to water stress. However, there was substantial variation among species in the water potential at the turgor loss point; varying from a high of -1.34 MPa for river birch to a low of -1.78 MPa for Japanese birch. Rates of Pn and gs under mild stress (mean predawn leaf water potential of -0.61 MPa) were negatively correlated with leaf osmotic potential at full turgor and the leaf water potential at the turgor loss point.
Water relation parameters were calculated from analysis of 92 pressure-volume isotherms of leaves of two olive varieties, `Leccino' and `Frantoio', measured after 4 weeks of salinity stress and 4 weeks of subsequent relief either in hydroponics or soil culture. `Frantoio' was more salt-tolerant than `Leccino', but no major differences in water relation parameters emerged between the two varieties. Increasing salinity from 0 to 200 mM NaCl decreased predawn leaf water potential from –0.5 MPa to –1.3 MPa, relative water content (RWC) from 97.6% to 89%, and leaf osmotic potential (Ψπ) from –2.0 to –3.5 MPa. Relative water content at turgor loss point (RWCtlp) was decreased from 89% to 85% (soil culture) and from 86% to 80% (hydroponic culture) in 0 to 200 mM CaCl-treated plants, respectively; a lower RWCtlp was also retained during the relief from salinity. Active osmotic adjustments induced by salinity was the result of accumulation of both inorganic ions and compatible solutes (e.g., mannitol). Maintenance of lower Ψπ and RWCtlp during relief indicated that salinized plants were better adapted to withstand further stress and that this potential might be exploited to harden olive plants to be used in arid or saline environments.
Diurnal and seasonal water relations and ecophysiological variables (soil humidity, transpiration, evapotranspiration, stomatal resistance, morphological changes, production), matched with some microclimatological variables, were studied in a hot pepper (Capsicum frutescens) experimental plot. Two treatments of plants with plastic mulches were assigned, black and blank-opaque, to compare them with plants without a mulch, established at the Experimental Station of CIBNOR in La Paz Baja California Sur, Mexico. Plants with blank-opaque plastic mulch showed the highest values of flower number, fruit production, leaf area, and canopy-projected area. Also, the biggest evapotranspiration rates were recorded from January to April for plants under the blank-opaque plastic mulch. Soil water content appeared to be a primary determinant factor for production. Soils under the blank-opaque plastic mulch had the biggest water content along the experiment. Plants without any plastic mulch had the lowest availability of soil water, rendered the lowest fruit production, and registered the highest evapotranspiration rates. May and June were the months with the highest air temperature during the experiment. Plants with black plastic mulch had intermediate records among the other two groups. When plants were allowed to face a drought stress, they responded through an osmotic adjustment for maintaining a low water potential, and thus supporting a partial turgor pressure. This adjustment was evident to be coupled with a stomatal regulation in order to minimize the loss of water through the transpiration process. Some drought tolerance strategies as a leaf size reduction were more evident in plants without a mulch.
Freeman maples (Acer × freemanii E. Murray) are marketed as stress-resistant alternatives to red maples (Acer rubrum L.), but few data from direct comparisons of these species are available. As a first step in comparing the stress resistance of red maple and Freeman maple, responses to drought were studied in Acer × freemanii `Autumn Fantasy', `Celebration', and `Marmo'. Plants grown from rooted cuttings were treated by withholding irrigation through four drought cycles of increasing severity that were separated by irrigation to container capacity. Drought reduced shoot dry mass, root dry mass, and height growth by 64%, 43%, and 79%, respectively, over all cultivars. Predawn leaf water potential was reduced by 1.16 MPa over all cultivars, and stomatal conductance data indicated water use was more conservative over all root-zone moisture contents after repeated cycles of drought. Specific mass of drought-stressed leaves increased by 25% for `Autumn Fantasy', and microscopy to determine leaf thickness and cellular anatomy is ongoing. `Autumn Fantasy' also had the lowest ratio of leaf surface area to xylem diameter, and `Autumn Fantasy' and `Celebration' had higher ratios of root to shoot mass than `Marmo'. Pressure-volume curve analysis revealed osmotic potential of drought-stressed plants at full turgor was 0.24 MPa more negative than controls, and droughted plants had a greater apoplastic water percentage than controls. Although osmotic adjustment during drought was similar among cultivars, differences in specific mass of leaves and in ratios of transpiring and conducting tissues suggest cultivars of Freeman maple vary in resistance to drought in the landscape.
Abbreviations: FC, Fragaria chiloensis Duch. cv. `BSP14'; FV, Fragaria virginiana Duch. cv. `NCC85-13V'; RWC, relative water content; TSC, total soluble carbohydrates; ψ water potential; ψ π osmotic potential; ψ π 100 osmotic potential at full
Information on fruit water relations is scant for apple trees, especially under deficit irrigation. Here we discuss plant and fruit responses to deficit irrigation. Three-year-old potted `Braeburn' trees were studied in a glasshouse. The treatments were: well-watered control (C), early deficit (D1), and late deficit (D2). The latter two were, respectively, water stressed during 61–183 and 109–183 days after full bloom (DAFB). The final harvest was at 183 DAFB. Photosynthesis, stomatal conductance, and trunk circumference were lower in D1 and D2 than in C. Leaf area and shoot growth was reduced only in D1. Root length remained the same for all treatments. Fruit were smaller in D1 than in C; however, fruit growth was less sensitive to deficit irrigation than was vegetative growth. Fruit growth in D2 was the same as in C. Fruit concentrations of K+, fructose, sorbitol, total sugars, and titratable acidity were higher in D1 than in D2 and C. Total soluble solids were higher in D1 and D2 than in C. Although fruit water potential was lower in D1 than in C, a concomitant lowering of osmotic potential in D1 fruit led to maintenance of turgor potential, indicating osmotic adjustment. This could have been effected, at least partially, through accumulation of K+ and soluble sugars. Water relations of D2 fruit were not affected by deficit irrigation, although leaf water potential was lower than in C. Fruit water relations and fruit growth are therefore less sensitive to deficit irrigation than are those of vegetative parts.