Osmotic Adjustment and Solute Constituents in Leaves and Roots of Water-stressed Cherry (Prunus) Trees

Tissue osmotic potential and solute constituents were evaluated in leaves and roots of well-watered and water-stressed Prunus avium L. × pseudocerasus Lindl. ‘Colt’ and Prunus cerasus L. ‘Meteor’. Osmotic potential at full turgor decreased in response to water stress for leaves and roots of both cultivars. For ‘Colt’, a cultivar with an indeterminate growth habit, decreased by 0.56 MPa and 0.38 MPa for terminal expanding leaves and older expanded leaves, respectively. For ‘Meteor’, a cultivar with a determinate growth habit, decreased by ≈0.47 MPa in both terminal and older leaves. Root was alike for both cultivars and showed a similar decrease of 0.20 MPa in response to water stress. Roots had considerably higher than did leaves in both cultivars, irrespective of irrigation treatment. Soluble carbohydrates and potassium (K+) were the major solute constituents in both cultivars. Of the soluble carbohydrates, sorbitol was found in the greatest concentration and accounted for the bulk of water stress-induced solute accumulation in both cultivars. Regardless of the irrigation treatment, mature leaves of ‘Meteor’ consistently had lower (typically 0.4 MPa) than ‘Colt’. This variation in between Prunus cultivars suggests the potential for selection of cultivars with low and possibly superior drought resistance. The capacity for osmotic adjustment via solute accumulation has been reported for many woody plants (Hinckley et al., 1981; Jones et al., 1985; Tyree and Jarvis, 1982). Higher solute concentrations contribute to lower tissue maintenance of turgor potential, and improved tolerance of low tissue water potentials (Tyree and Jarvis, 1982). Low and the capacity for osmotic adjustment may also serve as useful criteria for selection and breeding of more drought-resistant species and cultivars (Tyree, 1976). Tissue and the capacity for osmotic adjustment can vary among organs within a plant. In apple, mature leaves showed seasonal osmotic adjustment while shoot tips did not (Lakso, 1983; Lakso et al., 1984). Also, mature leaves of woody plants typically have lower than do expanding leaves (Knipling, 1967; Lakso et al., 1984; Syvertsen et al., 1981; Tyree et al., 1978). This variation may be important in understanding the responses of leaf gas exchange and leaf area expansion to water stress (Jones et al., 1985). Osmotic adjustment also occurs in roots of woody plants in response to water stress (Kandiko et al., 1980; Osonubi and Davies, 1978; Parker and Pallardy, 1985, 1988). However, within a plant, roots typically have higher than do leaves when compared at full turgor (Kandiko et al., 1980; Parker and Pallardy, 1985, 1988). The capacity for osmotic adjustment and turgor maintenance in roots may influence root : shoot partitioning patterns, root growth, and leaf responses to water deficits through indirect effects of root-produced plant growth regulators (Turner, 1986). Soluble carbohydrates are often found to be important osmolytes, accumulating in response to water stress in herbaceous plants (Cutler and Rains, 1978; Ford and Wilson, 1981; Handa et al., 1983; Munns and Weir, 1981; Munns et al., 1979; Turner for publication 1 Nov. 1990. The cost of publishing this paper was in part by the payment of page charges. Under postal regulations, this refore must be hereby marked advertisement solely to indicate this address: North Carolina State Univ., Dept. of Horticultural Science, in Horticultural Crops Research and Extension Center, 2016 Fanning oad, Fletcher, NC 28732. Phone: (704) 684-3562. FAX: (704) 684et al., 1978). However, there has been little study of the solutes that contribute to and osmotic adjustment in woody plants. Alcohol sugars, or polyols, are important osmolytes in certain lichens, yeasts, algae, and fungi accumulating in response to osmotic stress (Bieleski, 1982). In one case, the polyol sorbitol accumulates in Plantago in response to salt stress (Ahmad et al., 1979). Sorbitol is the primary photosynthetic product and translocated carbohydrate in many woody rosaceous species (Bieleski, 1982), including Prunus, where substantial diurnal accumulation of sorbitol occurs (Rem et al., 1988). Jones et al., (1985) further hypothesized that sorbitol may be an important osmolyte contributing to osmotic adjustment in rosaceous fruit trees. Our objectives were the comparison of variations in and osmotic adjustment in response to water stress 1) between two Prunus cultivars, 2) between roots and leaves of different age, and 3) to determine the primary solutes that contribute to and osmotic adjustment in these plants. Materials and Methods Plant material and handling. Prunus avium × pseudocerasus ‘Colt’ and P. cerasus ‘Meteor’, 0.6 cm in caliper, were potted in 19-liter white plastic containers with a medium of 1 sphagnum peat moss : 1 vermiculite : 1 calcined clay (by volume) on 20 May 1987. Before initiation of treatments, plants were grown outside under natural conditions in Ithaca, N.Y., and were fertilized weekly with water soluble fertilizer (10N–10P2O5-10K2O) at an N concentration of 200 mg·liter. Plants were moved into the greenhouse where irrigation treatments commenced on 20 July 1987. Treatments. The experiment was a split-unit design (Cochran and Cox, 1957) consisting of a 2 × 2 factorial with two cultivars (Meteor and Colt) and two irrigation levels (control and water-stressed) at the whole-unit level with leaves and roots within a plant treated as subunits. There were five replications per treatment combination. Plants were irrigated each evening Abbreviations: osmotic potential; osmotic potential at full turgor. J. Amer. Soc. Hort. Sci. 116(4):684-688. 1991. either to container through-flow (control) or with only sufficient water to restore a bulk soil water potential of – 1.2 MPa (waterstressed), based on a soil moisture release curve determined using a pressure chamber and ceramic plate (Soilmoisture Equipment Corp., Santa Barbara, Calif.). Imposition of water stress in this manner can result in variation in soil water content within a container at certain times. However, this method makes it possible to subject several plants to similar levels of water stress for extended periods. These irrigation treatments were imposed for 30 days. Plant water relations. Leaf and root samples were collected before dawn to minimize variation in solute accumulation during the light period. Leaves were collected at four-node intervals along the current season’s growth and at the shoot terminal. Because ‘Colt’ is a strongly indeterminate grower and did not set a terminal bud during the experiment, terminal leaves of ‘Colt’ were only partially expanded. In contrast, ‘Meteor’ plants had set terminal buds before sampling and terminal leaves were nearly mature (i.e., fully expanded). Roots were excised at a point where the root diameter was 5 mm and included the portion of the root system distal to the excision. Excised tissue was hydrated by recutting under water and holding for 2 h, covered with plastic, in the dark, with the cut end submerged. This method was sufficient to fully rehydrate tissue, i.e., result in a water potential of 0 MPa. was determined on expressed sap from fully hydrated tissue after freezing and thawing. Osmolality of expressed sap was determined using a vapor pressure osmometer (Wescor model 5100C, Logan, Utah). The of the expressed sap was then calculated for 20C, based on the van’t Hoff relation as given by Nobel (1983): The of expressed sap represents a mixture of cell contents and can yield values of slightly higher (more dilute) than would measurement of symplastic due to the dilution of symplastic solutes by apoplastic water. However, previous research has shown that measurements of of expressed sap were well correlated with measurements of determined using pressure-volume methodology for leaves of apple (Lakso et al., 1984), over a range of from 0 to –2.5 MPa, and for leaves of various ages and roots of both well-watered and water-stressed cherry trees (Ranney, 1989). Solute analysis. Analysis for soluble carbohydrates and potassium was performed on terminal leaves, mature leaves (16th node), and roots after 30 days of irrigation treatments. Carbohydrates were extracted and then analyzed using high-performance liquid chromatography following Boersig and Negro (1985), modified by using a BioRad carbohydrate column (model HPX87C; BioRad, Richmond, Calif.) at 85C. Potassium was analyzed by inductively coupled argon plasma emission spectroscopy. Osmotic contribution of solutes. The contribution of individual solutes to the osmotic potential of the expressed sap was calculated based on the relative dry weight (RDW) at saturation [dry weight/(saturated weight – dry weight)] determined for each sample, the solute concentration on a tissue dry-weight basis, the molecular weight of each solute, and the van’t Hoff relation. Calculated at 20C this gives: where C is the solute concentration and MW is the weight of a given solute. molecular J. Amer. Soc. Hort. Sci. 116(4):684-688. 1991. Results Water stress induced decreases in for leaves and roots of both cultivars (Fig. 1). For ‘Colt’, terminal leaves had higher than older leaves when well-watered. When stressed, all leaves of ‘Colt’ adjusted osmotically, with the terminal leaves showing the greatest decrease in (0.56 MPa) while expanded leaves typically decreased by 0.38 MPa. This differential response resulted in all of the leaves having similar when stressed. The roots of ‘Colt’ had substantially higher than leaves, with a of –0.56 and –0.74 MPa for control and stressed plants, respectively. There was no difference in among any of the leaves of well-irrigated ‘Meteor’ plants that had a mean leaf of –2.02 MPa (Fig. 1). Leaves from node 12 (from the base) up to and including the terminal leaves showed similar decreases in of 0.47 MPa in response to water stress. Leaves at node 8 decreased slightly (0.17 MPa) and leaves at node 4 showed no significant osmotic adjustment. As with ‘Colt’, roots of ‘Meteor’ had higher than did leaves with a of –0.57 MPa and –0.80 MPa for control and stressed plants, respectively. Comparison among cultivars showed that mature leaves of ‘Meteor’ consistently had lower (typically 0.4 MPa) than ‘Colt’ for plants under the same irrigation treatment except for node 4 where stress leaves of both ‘Meteor’ and ‘Colt’ had similar Roots showed very similar levels of Roots showed very similar levels of for both cultivars. Analysis of tissue solutes showed that soluble carbohydrates and potassium were prominent solutes for ‘Colt’ and ‘Meteor’ (Table 1). Of the soluble carbohydrates, sorbitol was found in the greatest concentration regardless of cultivar, tissue, or irrigation treatment. Total soluble carbohydrates increased in response to the stress in leaves and roots of both cultivars. However, these increases in total soluble carbohydrates resulted primarily from increases in sorbitol alone. Concentrations of potassium, on a tissue dry-weight basis, did not change in response to the stress in leaves and actually decreased in roots of ‘Colt’ (Table 1). For ‘Meteor’, tissue dry-weight concentrations of potassium decreased in leaves and roots in response to stress. The RDW at saturation increased significantly for terminal leaves and roots of both cultivars (Fig. 2). As a result, stressinduced changes in solute concentration, expressed as