variations in osmoticadjustment and correlation to physiological traits. OA level in leaves increased during drought stress in all genotypes ( Fig. 4 ). The OA was significantly greater in ColxCr14, ColxCr190, and SAGIPT41 than ColxCr679, ‘Penncross’, and
. In an earlier review article, Munns (1988) pointed out that growth reduction under drought stress may also contribute to osmoticadjustment, with a 30% reduction in a relative growth rate of leaves resulting in a reduction in ψ S by 0.1 to 0.2 MPa
stresses such as high and low temperatures, drought, and salinity ( Liu et al., 2017 ). Osmoticadjustment is a mechanism for the maintenance of water potential in plant cells during periods of heat and cold stress ( Zaher-Ara et al., 2016 ). Compatible
Expansion of green-white and red fruit in control (watered) and water-stressed greenhouse-grown strawberry (Fragaria ×ananassa Duch. `Brighton') plants was monitored with pressure transducers. Expansion of green-white fruit in control plants was rapid, showing little diurnal variation; whereas in water-stressed plants, fruit expansion occurred only during dark periods and shrinkage during the day. Red fruit were mature and failed to show net expansion. The apoplastic water potential (ψaw), measured with in situ psychrometers in control plants was always higher in leaves than in green-white fruit. In stressed plants, ψaw of leaves was higher than that of green-white fruit only in the dark, corresponding to the period when these fruit expanded. To determine the ability of fruit to osmotically adjust, fruit were removed from control and water-stressed plants, and hydrated for 12 hours; then, solute potential at full turgor (ψs100) was measured. Water-stressed green-white fruit showed osmotic adjustment with a ψs100 that was 0.28 MPa lower than that of control fruit. Mature leaves of water-stressed plants showed a similar level of osmotic adjustment, whereas water stress did not have a significant effect on the ψs100 of red fruit. Fruit also were severed to permit rapid dehydration, and fruit solute potential (ψs) was plotted against relative water content [RWC = (fresh mass - dry mass ÷ fully turgid mass - dry mass) × 100]. Water-stressed, green-white fruit had a lower ψs for a given RWC than control fruit, further confirming the occurrence of osmotic adjustment in the stressed fruit tissue. The lack of a linear relationship between turgor pressure and RWC prevented the calculation of cell elasticity or volumetric elastic modulus. Osmotic adjustment resulted in about a 2.5-fold increase in glucose and sucrose levels in water-stressed green-white fruit. Although green-white fruit on water-stressed plants showed osmotic adjustment, it was not sufficient to maintain fruit expansion during the day.
contributor to OA in creeping bentgrass. Fig. 4. Osmoticadjustment of creeping bentgrass under well-watered (W) and drought stress without (D) and with glycine betaine (D + GB) or spermidine (Spd; D + Spd) treatments. Drought stress was imposed on 12 June
Growth-chamber studies were conducted to examine the ability of seven vegetable crops-`Blue Lake' bean (Phaseolus vulgaris L.), `Detroit Dark Red' beet (Beta vulgaris L.), `Burgundy' okra (Abelmoschus esculentus (Moench), `Little Marvel' pea (Pisum sativum L.), `California Wonder' bell pepper (Capsicum annuum L.), `New Zealand' spinach (Spinacia oleracea L.), and `Beefsteak' tomato (Lycopersicon esculentum Mill.)–to adjust osmotically in response to water-deficit stress. Water stress was imposed by withholding water for 3 days, and the adjustment of leaf and root osmotic potentials upon relief of the stress and rehydration were monitored with thermocouple psychrometers. Despite similar reductions in leaf water potential and stomata1 conductance among the species studied, crop-specific differences were observed in leaf and root osmotic adjustment. Leaf osmotic adjustment was observed for bean, pepper, and tomato following water-deficit stress. Root osmotic adjustment was significant in bean, okra, pea, and tomato. Furthermore, differences in leaf and root osmotic adjustment were also observed among five tomato cultivars. Leaf osmotic adjustment was not associated with the maintenance of leaf growth following water-deficit stress, since leaf expansion of water-stressed bean and pepper, two species capable of osmotic adjustment, was similar to that of spinach, which exhibited no leaf osmotic adjustment.
Seedlings of ‘Halford’ peach [Prunus persica (L.) Batsch] maintained in a growth chamber and exposed to leaf water deficits as low as – 38 bars exhibited slight osmotic adjustment before turgor potential reached zero. This adjustment was inadequate to maintain high turgor potential, which decreased until reaching zero at a total leaf water potential of ca. —20 bars. Stomatal conductance was linearly related to total leaf water potential and independent of the average turgor potential of the leaf.
Six-year-old trees were repeatedly conditioned by withholding irrigation until Ψpd (predawn) thresholds of either –0.9 (“mild”) or –1.4 MPa (“moderate”) were attained. After conditioning, trees were exposed to severe drought (Ψpd –2.0 MPa) and then to 10 days of well-watered conditions. Throughout the investigation, osmotic potential (Ψπ), leaf RWC, transpiration, and total water potential (Ψx) were measured. Water stress was quantified by integrating Ψx. Conditioning caused a significant, but modest, degree of osmotic adjustment (0.08 to 0.28 MPa), which persisted after a brief relief from stress and transpiration rates were reduced 35% to 50%. Osmotic adjustment was not significantly enhanced by more than one stress exposure or conditioning beyond the mild threshold of stress. During severe drought, the moderate group maintained less negative Ψx and lower transpiration rates (38%). After prolonged stress relief, Ψx was similar among all treatments and daily transpiration rates and Ψx gradually recovered. Thuja occidentalis appears to rely on increased stomatal resistance more than osmotic adjustment to tolerate drought stress.
Greenhouse grown 2-year-old potted `Jonathan' apple trees (Malus domestica Borkh.) were subjected to various levels of water stress in February. Midday leaf water potential (ψW), leaf osmotic potential (ψS), soluble sugars, and starch contents of mature leaves were measured throughout the development of water stress to determine whether active osmotic adjustment could be detected and whether carbohydrates were involved. Active adjustments of 0.6 MPa were observed 3 and 5 days, respectively, after water stress was initiated. Leaf turgor potential (ψP) could not be maintained through the osmotic adjustment when ψW dropped below -1.6 MPa. Sorbitol, glucose, and fructose concentrations increased while sucrose and starch levels decreased significantly as water stress developed, strongly suggesting that sugar alcohol and monosaccharide are the most important osmotica for adjustment. Sorbitol was a primary carbohydrate in the cell sap and accounted for > 50% of total osmotic adjustment. The partitioning of newly fixed W-labeled photosynthates in mature leaves was not affected by water stress immediately after the 30-min 14CO2 treatment. All the W-labeled carbohydrates decreased in the labeled leaves very rapidly after 14CO2 labeling. The decrease in 14C-sorbitol was greater than the decrease in other carbohydrates under both well-watered and stressed conditions. After 24 hours of water stress, however, the percentage of 14C-sorbitol increased while the percentages of sucrose, starch, glucose, and fructose decreased significantly with increasing levels of stress. The ratio of 14C-sorbitol in leaves with ψW = -3.5 MPa to leaves with ψW = -0.5 MPa was significantly higher than that of 14C-sucrose, 14C-glucose, W-fructose, or 14C-starch.
Citrus rootstocks have well-known effects on tree size, crop load, fruit size, and various fruit quality factors. Fruit from trees budded on invigorating rootstocks are generally larger with lower soluble solids concentration (SSC) and titratable acidity compared to fruit from trees budded on less invigorating rootstocks. Although it is unclear how rootstocks exert their influence on juice quality of Citrus L. species, plant water relations are thought to play a central role. In addition, the larger fruit size associated with invigorating rootstocks and the inverse relationship between SSC and fruit size implies that fruit borne on trees on invigorating rootstocks have lower SSC due to dilution effects in larger fruit. To determine how rootstock type affects sugar accumulation in fruit of Citrus species, controlled water-deficit stress was applied to mature `Valencia' sweet orange [C. sinensis (L.) Osb.] trees on Carrizo citrange [C. sinensis × Poncirus trifoliata (L.) Raf.] or rough lemon (C. jambhiri Lush.) rootstocks. Withholding water from the root zone of citrus trees during stage II of fruit development decreased midday stem water potential and increased the concentrations of primary osmotica, fructose and glucose. Sucrose concentration was not affected, suggesting that sucrose hydrolysis took place. Increased concentrations of sugars and SSC in fruit from moderately water-stressed trees occurred independently of fruit size and juice content. Thus, passive dehydration of juice sacs, and concentration of soluble solids, was not the primary cause of differences in sugar accumulation. Controlled water-deficit stress caused active osmotic adjustment in fruit of `Valencia' sweet orange. However, when water-deficit stress was applied later in fruit development (e.g., stage III) there was no increase in sugars or SSC. The evidence presented supports the hypothesis that differential sugar accumulation of citrus fruit from trees on rootstocks of contrasting vigor and, hence, plant water relations, is caused by differences in tree water status and the enhancement of sucrose hydrolysis into component hexose sugars resulting in osmotic adjustment. Therefore, inherent rootstock differences affecting plant water relations are proposed as a primary cause of differences in sugar accumulation and SSC among citrus rootstocks.