Leaf water status controls day-time but not daily rates of leaf expansion in salt-treated barley.

Barley plants were grown in pots that would fit inside a pressure chamber, so that their shoots could be kept fully turgid by applying pressure in the chamber to bring the xylem sap of the shoot to the point of bleeding. Pressurisation increased the growth rate of NaCl-treated plants in the light period but not in the dark. The promotive effect on growth was greatest in the light period of the first day of pressurisation, but disappeared during the first night. Pressurisation promoted growth the next day during the light period, but on the second night the elongation rate was significantly lower than that of unpressurised NaCl-treated plants. This pattern of high day-time and low night-time growth then continued indefinitely. The lower night-time growth counteracted the higher day-time growth, with the result that total growth over 24 h was the same as in NaCl-treated plants that were not pressurised. Levels of total reserve carbohydrates were unaffected by pressurisation, indicating that the slower growth of the pres-surised plants during the night was not due to depletion of assimilates. These results are interpreted in the context of hormonal signals controlling growth on a 24-h basis, such that any short-term stimulation of growth arising from unusually high water status during the light period is counterbalanced by slower growth during the night.