Aust. J. Plant Physiol., 1979, 6, 379-89 Solute AccumuIation in the Apex and Leaves of Wheat during Water Stress Rana ~zknns~', C. J. ~rady~ and E. W. R. ~ a r l o w ~ A School of Biological Sciences, Macquarie University, North Ryde, N.S.W. 21 13. Plant Physiology Unit, CSIRO, Macquarie University, North Ryde, N.S.W. 2113. Present address: Agronomy Department, University of Western Australia, Nedlands, W.A. 6009. Abstract Accumulation of several low-molecular-weight solutes was measured in the developing floral apex, in an enclosed, elongating leaf, and in an expanded leaf of wheat plants during a 13-day period of water stress. In the apices and enclosed leaves, osmotic potential fell from - 1.2 to -4.0 MPa. The main contribution to the decline in osmotic potential during the first 3 days of stress was from an increase in the content of ethanol-soluble carbohydrate. Later, increases in the concentrations of both carbohydrates and amino acids made major contributions. Of the amino acids, the largest increases were in asparagine and proline. The enclosed tissues lost little water, although the water- to-dry matter ratio declined as a result of imported solutes. The ethanol-insoluble nitrogen content of apices remained high, and growth of apices and enclosed leaves recommenced when plants were watered after 13 days. In exposed leaves, increases in carbohydrate and amino acid contents were comparatively small, and the content of ethanol-insoluble nitrogen decreased by 50%. These leaves dehydrated within 6 days, and failed to recover when the plants were rewatered. Introduction Various tissues within a plant respond differently to a water deficit. Gates (1968) and Ludlow and Ng (1974) have shown that water stress can cause the accelerated death of older leaves and at the same time merely suspend ageing in younger leaves. We have recently shown that wheat apices survived water potentials of -6 MPa during a period of stress which killed the expanded leaves (Barlow et al. 1977). The present paper explores possible reasons for this differential response. The ability of a tissue to prevent decreases in water content despite a decrease in water potential should favour survival during a water stress, as many metabolic processes are unaffected by low water potential per se, at least as low as - 2.0 MPa (Greenway et al. 1972). Furthermore, avoidance of a loss of turgor would be of adaptive value as a decrease in turgor, without concurrent changes in cell wall exten- sibility, is responsible for the reduction or suspension of expansion growth during a water stress (see reviews by Begg and Turner 1976; Hsiao et al. 1976). The term osmotic adjustment describes the situation in which the osmotic potehtial falls due to an increase in the solute content per cell rather than to a decrease in cell water content (Hsiao et al. 1976). Maintenance of turgor by decreases in osmotic potential under conditions of decreasing water potential has been measured in expanded leaves of several higher plants, e.g. Hsiao et al. (1976); Morgan (1977) and Jones and Turner (1978). However, the water relations and the metabolic aspects of the response of meristematic and elongating tissues have received little attention.