Growth, Osmotic Adjustment, and Cell-Wall Mechanics of Expanding Grape Leaves during Water Deficits Hans R. Schultz and Mark A. Matthews* ABSTRACT The responses of growth, components of leaf waterpotential, and mechanical properties of leaves to ontogeny and soil water deficits were investigated in wine grape Vitis vinifera L. Vines were cultured in soil in controlledenvironments, and water deficits were imposed by withholding water. Osmotic adjustment of several hundred kilo- pascals maintained turgot at or above that of controls, but growth was completely inhibited. The bulk modulus of elasticity increased almost twofold, from 8.1 MPa for a rapidly expanding leaf to 14.7 MPa for a similar leaf in which growth had ceasedafter withholding water for several days, and continued to increase to 20.3 MPa 2 d after growth had ceased.Stress-strain relationships of isolated leaf strips were determined with the Instron analyzer. Total extension (elastic and plastic components) under 20- or 30-g loads was linearly related to the rate of leaf expansion determined prior to sampling. Total extension of well-watered leaves exhibited a diurnal pattern, with a minimum extensibility near midnight and high extensibility in the light. Plastic extensibility was estimated as the difference in ex- tension in successive load cycles.Plastic extension was linearly related to the rate of leaf expansion. The results indicate that the extensibility (as determined by Instron analysis) of grape leaf tissue decreases con- comitant withthe rate of expansion as leaves age. Similarly,plastic and elastic extensibility decrease when growing leaves are exposed to water deficits. The results are consistent with the hypothesis that de- creased cell-wallextensibility contributes to the inhibition of leaf ex- pansion caused by water deficits. L EAF GROWTH is one of the most sensitive of plant processes to water deficits andis frequently inhib- ited in field crops(Hsiao, 1973). Water deficits that in- hibit leaf expansion developin a range of time scales from minutes to months, but most often over several days. The sensitivity of leaf growth to water deficits can changediurnally and seasonally, but, in general, our understanding of the nature of the inhibition and of adap- tive mechanisms is limited. To a large extent, growthis cell enlargement,with water absorption andirreversible (plastic) cell-wall ex- pansion as critical components of the process. In theory (Lockhart, 1965), cell expansion is achieved when loos- ening of the cell wall causes it to yield under the stress of the internal pressure (turgor, 0o)- Water absorption the expanding cell is determined mainly by the water potential gradient andthe hydraulic conductivity between the cell and the external medium (A~b). The high sensitivity of expansive growth to rapid var- iations in water potential (~) suggests that the initial ef- fect of reduced ~b on expansion is physical andis mediated by Oo (Green et al., 1971; Acevedo et al., 1971). This implies that plants that can adapt to low leaf water po- tential (~) and sustain growth do so by maintaining the H.R. Schultz, c/o M.A. Matthews; and M.A. Matthews, Dep. of Viticulture and Enology, Univ. of California, Davis, CA 95616- 8749. This research was supported in part by USDA-CRGO Grant no. 85-CRCR-l-1648 to M.A. Matthews and by a Fulbright schol- arship to H.R.Schultz. Received 18 Feb. 1992. *Corresponding author. Published in CropSci. 33:287-294 (1993). necessary ~p for growthand A~b for water uptake. One such adaptive mechanism is osmoticadjustment,in which a low source water potential is compensated by a low tissue solute potential (~bs) (Meyer and Boyer, 1972). Osmotic adjustment has been observed in mature and growing leaves of several species (Hsiao et al., 1976, Matsuda and Riazi, 1981; Michelina and Boyer, 1982), but in some of these studies leaf growth was inhibited with no apparentloss of ~bpin the growing zone(Matsuda and Riazi, 1981; Michelinaand Boyer, 1982; Matthews et al., 1984). This implies that factors other than ~bp limited growth. In grape leaves, rapid changes in the turgor-growth relationship occurred following step changes in turgor (Shackel et al., 1987). These obser- vations indicate that the turgor-growth relationship is itself environmentallydependent. This study was con- ducted to investigate the propensity for osmotic adjust- mentand growth maintenance at low water potential in grape leaves. MATERIALS AND METHODS Growth Conditions Grapevine plants (cv. White Riesling) were grown from dor- mant footings (ViticultureFieldStation, University of Cali- fornia,Davis) in 8-L potscontaining a soil/peat/perlite mixture (1:3:3) in a controlled environment (30/20 - 2 °C, 50/90 10% relative humidity, 13-h photoperiod with 550 to 1000 /~mol photons m -2 s-1 PPFD from cool-white fluorescent bulbs). One week after bud break (2-3 wk after planting), plants were thinned to one or two shoots and trained to vertical stakes. All plants were watered daily to saturation, and excess water was permitted to drain. Plantswere fertilized with half-strength Hoagland solution (Hoagland andArnon, 1950) two timesper week.At a shoot length of 70 - 10 cm (=20leaves > 5 mm lamina length,= 10 leaves fully expanded), water deficits were imposed by withholding water from some plants. In some ex- periments, the soil was covered with aluminum foil to prevent soil evaporation andretard soil drying. Well-watered plants usedfor measurements at the end of the dry-down periodswere termed agecontrols, in contrastto plants before dry down. Leaf Growth Growth was measured as increases in leaf length with a hand-held micrometer. Measurements were performed once every 24 h at the beginning of the photoperiod, unless stated otherwise. Leaves with the highest growth rates (7 and 8 nodes from the shootapex, Schultz and Matthews, 1988) were used unless statedotherwise. Measurements of Water Potential and Solute Potential Leafwater andsolute potentials of both growing andnon- growing leaves weredetermined by isopiestic thermocouple psychrometry (Boyer and Knipling, 1965), using sample chambers coated with melted and resolidified petrolatum (Boyer, 1966), andwere corrected for the heat of respiration (Barrs, Abbreviations: PPFD,photosynthetic flux density; RWC, rela- tive water content; e, bulk modulus of elasticity; ~b, water poten- tial; tpp, turgor; ~bs, solute potential; A~b, hydraulic conductivity betweenthe cell and the external medium. 287 Published March, 1993