Correlation of Activity and Amount of Ribulose 1,5-bisphosphate Carboxylase with Chloroplast Stroma Crystals in Water-Stressed Willow Leaves

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Abstract
Changes in the activity and amount of ribulose 1,5-bisphosphate(RuBP) carboxylase (E.C. 4.1.1.39) were studied in well-watered plants of Salix ‘aquatica gigantea’ and in similar plants during three different water stress treatments and after rewatering. The chloroplast ultrastructure of these plants was examined by electron microscopy. The amounts of crystallized protein in the chloroplast stroma were assessed according to the area of crystal structure seen in the thin sections. RuBP carboxylase activity decreased with decreasing leaf water potentials but recovered upon rewatering, except when leaves had been exposed to severe water stress. The percentage of total chloroplast area made up of crystal inclusions decreased with decreasing leaf water potentials. After rewatering, the crystals either disappeared or the amount decreased markedly. Both RuBP carboxylase activity and the area of crystal inclusions increased initially with increased extractable RuBP carboxylase protein but decreased with further increases above 6700–7000 μg RuBP carboxylase protein mg−1 chlorophyll. In well-watered and water-stressed plants the activity of RuBP carboxylase, based on amount of chlorophyll, increased with an increasing amount of crystal inclusions in the chloroplast stroma. In rewatered plants no such correlation was observed, and the low percentage of crystal inclusions in the chloroplast area was independent of RuBP carboxylase activity.