Effect of Water Stress on Rate of Photosynthesis, Transpiration, and Chloroplast Ultrastructure in Willow Leaves

Abstract

Formation of crystals, supposed to be Fraction I-protein, in the chloroplast stroma has been reported frequently in the literature (1). In our previous study with willow (Salix sp. ‘aquatica gigantea’) (2), under the strong light regime large crystal formations were observed with increasing water stress. In weak light these crystal structures could be seen in the well-watered controls, but seemed to disappear as water stress became severe. Since CO₂ assimilation at low p(CO₂) has been thought to follow the kinetics of RuP₂-carboxylase-oxygenase and to be determined by the RuP₂ saturated rate of the enzyme (3), an experiment was designed to determine whether crystal inclusions observed in the chloroplasts are correlated with photosynthesis during water stress. The characteristics of CO₂ exchange and transpiration at 20°C and at a quantum flux density of 550–700 μE m^–2 s^–1 were examined in willow leaves (Salix sp. ‘aquatica gigantea’) at several concentrations of intercellular CO₂.