This paper briefly reviews the current developments on the role of leaf hydration on leaf growth and photosynthesis and concludes that changes in growth and stomatal action are not always closely correlated with changes in leaf hydration. A case is developed for soil and root water relations affecting leaf growth and stomatal function, and a role for plant growth regulators is postulated. The influence of this changed perspective on the adaptation of plants to water deficits is discussed. In particular, the importance of the adaptation of roots to water deficits is highlighted and the need for more studies on the interaction between the shoot and the root is emphasized.

The influence of water deficits on the concentrations of major solutes in fully expanded sorghum leaves and fully expanded and partly expanded sunflower leaves was studied in order to assess their contribution to osmotic adjustment. The decreased osmotic potential at full turgor in fully expanded sorghum leaves at a moderate level of stress (predawn leaf water potential of -0.85 MPa) was fully accounted for by increases in sugars, potassium and chloride. The contributions of total inorganic ions and sugars (glucose and sucrose) were approximately equal. In fully expanded sunflower leaves stressed to a predawn leaf water potential of - 1.4 MPa, only half of the decrease in leaf osmotic potential was accounted for by changes in the concentrations of the solutes studied: increases in the concentrations of the inorganic ions, potassium, magnesium, calcium and nitrate, together with free amino acids were approximately equal to the decrease in leaf osmotic potential at full turgor, but the contributions of these solutes were offset by a decrease in the concentration of total carboxylic acids. Sugars did not contribute to the decrease in leaf osmotic potential at full turgor in fully expanded sunflower leaves. The major solutes responsible for the changes in leaf osmotic potential at full turgor in partly expanded sunflower leaves exposed to severe stress treatment (predawn leaf water potential of -2.3 MPa) were the inorganic anions, chloride and nitrate, and to a lesser extent carboxylic acids (principally aconitate). Free amino acids made a significant contribution (18%) to the decrease in leaf osmotic potential at full turgor, but there was a decrease in the level of soluble sugars.

Sunflower plants were grown in large volumes of soil and slowly water-stressed by withholding water. The tissue water relationships of leaves at various stages of stress and of leaves of equivalent well watered controls were studied by the pressure chamber technique. Plants were stressed either when leaf 17 was expanding or when it was fully expanded. When expanding leaves reached a moderate level of stress (predawn leaf water potential of -0.9 MPa), the osmotic potentials at full turgor and zero turgor were lower than the control values by 0.1 MPa and 0.2 MPa, respectively. When fully expanded leaves were stressed to a similar degree (predawn leaf water potential of - 1.1 MPa), the osmotic potentials at full turgor and zero turgor were lower than the control values by 0.2 MPa and 0.3 MPa, respectively. The development of more severe stress in the fully expanded leaves was not accompanied by any further osmotic adjustment. However, when the expanding leaves reached a predawn leaf water potential of -2.3 MPa, the values of leaf osmotic potential at full turgor and zero turgor were lower than the values for the well watered plants by 0.4 MPa and 0.6 MPa, respectively. In expanding leaves prestressed to a predawn leaf water potential of -2.3 MPa, the osmotic potential at full turgor was significantly less than the control values for at least 7 days after rewatering. Stress had no effect on the bulk modulus of elasticity. It is concluded that both expanding and fully expanded sunflower leaves show osmotic adjustment.

The soil and plant water status of irrigated and unirrigated sorghum [Sorghum bicolor (L.) Moench cv. TX610] and sunflower (Helianthus annuus L. cv. Hysun 30) crops were compared on several days from the late vegetative to the early grain-filling stages of development. Additionally, the stems of plants from the irrigated and unirrigated plots of both species were cut near their base; this caused the plants to quickly dry until the stomata closed. The leaf water potential and leaf osmotic potential were measured when the stomatal resistance reached 6 s cm-� to give the water potential for stomatal closure and to provide osmotic potentials at equal turgor. Carbohydrate and potassium levels of leaves were also monitored. The mean daily minimum leaf water potentials in the irrigated sorghum and sunflower did not decrease below - 1 7 MPa and - 2.0 MPa, respectively, but decreased to - 2.1 MPa in the unirrigated sorghum and -2.6 MPa in the unirrigated sunflower. The osmotic potential at stomatal closure in the rapidly dried plants decreased with increasing leaf water deficit in both sunflower and sorghum: in both species the osmotic potential decreased approximately 0.6 MPa for each megapascal decrease in leaf water potential. The results indicate that both sorghum and sunflower adjusted osmotically in response to water deficits and that adjustment occurred at a rate of at least 0.1 MPa per day. The lowering of osmotic potential persisted less than 9 days after the relief of stress in both sunflower and sorghum. The soluble sugar concentration increased linearly in both sunflower and sorghum with osmotic adjustment: the rate of increase of soluble sugars was significantly greater in sunflower than sorghum. No changes in potassium concentration were observed during osmotic adjustment. The water potential at which the stomata closed varied from - 1.5 to -2.6 MPa in sorghum and - 1.7 to -2.7 MPa in sunflower: the water potential that induced stomatal closure decreased as the osmotic potential decreased. Stomatal closure occurred at a mean turgor of -0-5 MPa in both species: systematic error in the measurement of osmotic potential on frozen and thawed leaf tissue is considered the reason for the low turgor potentials at stomatal closure. The adaxial stomatal closed before the abaxial stomata in the sorghum and unirrigated sunflower but, since the leaf water potential initially fell rapidly and then became stable before the adaxial stomata closed, both the adaxial and abaxial stomata closed at the same leaf water potential.

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