Contributions of soluble carbohydrates to the osmotic adjustment in the C4 grass Setaria sphacelata: a comparison between rapidly and slowly imposed water stress.

Photosynthetic carbohydrate content in Setaria sphacelata var. splendida under rapidly and slowly induced water deficit and its contribution to osmotic adjustment were studied. In short-term stress experiments, a decrease in the total content of sucrose (Su) and starch (St) was observed in leaf discs submitted to stress. An increase in the ratio between free hexoses and sucrose was found in stressed leaves, but no significant differences were found in the amount of free hexoses nor in the ratio between soluble and insoluble sugars. In long-term stress experiments, a higher amount of soluble sugars and a lower amount of starch were found in stressed leaves, when compared to the control. The ratios of free hexoses to sucrose and of soluble to insoluble sugars were also higher in stressed leaves. The contribution of the accumulation of soluble sugars to osmotic adjustment was absent in rapidly stressed leaves and was of minor importance in slowly stressed leaves.