The Influence of Water Deficit on the Factors Controlling the Daily Pattern of Growth ofPhaseolusTrifoliates

Phaseolus seedlings were grown in liquid culture under controlled temperature and irradiance and measurements were made of daily variation in growth of the first trifoliate leaves. Leaf growth rate was significantly enhanced within a few hours of the start of the light period. Over a similar time, a small decrease in leaf turgor and an increase in cell wall plasticity were recorded. Slowly declining growth rates as the light period progressed may have been caused by decreases in turgor during this time. When water availability to the leaves was restricted by growing the plants for several days in nutrient solution maintained at a low temperature (12°C), the daily pattern of growth of the trifoliates was changed quite markedly. Dark-growth rates were slightly enhanced, while light-growth rates were significantly reduced when compared to growth rates of plants well-supplied with water (roots at 20°C). Relative ‘plateau’ growth rates of plants well-supplied (ww) with water or suffering a restricted supply (ws) in the light (L) and in the dark (D) were as follows: ww L > ws D > ww D > ws L. In the light, turgors of the two groups of plants were similar, suggesting that the reduced growth rate of the cooled plants resulted from a change in cell wall structure and/or properties. Immediately before the lights were switched on, plants grown with a restricted water supply showed relatively high turgors in the trifoliates and these were presumably responsible for the enhanced growth rates at this time. Restriction of water availability may have slightly increased the plasticity of cell walls and decreased the yield threshold for growth. The control of leaf growth in plants developing water deficit is discussed.