Contribution of primary and stored photosynthates to photorespiration and respiration in the light and in the dark

Abstract

Using a radiogasometric method the rates of consumption of primary photosynthates (products of current photosynthesis) and stored photosynthates (products of previous photosynthesis, mainly sucrose and starch) in photorespiratory and respiratory decarboxylations of different C3 species have been determined. The species studied may be divided into two groups: cereals (wheat, rye, barley) with low rate of starch synthesis (less than 7% true photosynthesis) and "starch" plants (tobacco, potato, arabidopsis) where the rate of starch synthesis exceeds 30% the rate of true photosynthesis. In the light stored photosynthates are used not only in respiratory but also in photorespiratory decarboxylations. In leaves of cereals the total rate of photorespiratory and respiratory decarboxylation of stored photosynthates in the light was higher than in the dark while in "starch" plants, on the contrary, stored photosynthates were consumed with higher rate in the dark. In leaves of "starch" plants the rate of respiration in the dark was generally higher than in leaves of cereals. In all species tested light severely inhibited the consumption of stored photosynthates in respiration. In "starch" plants the total rate of respiration as sum of decarboxylations of primary and stored photosynthates was also suppressed by light while no definite light inhibition was found in cereals. Suppression of photorespiration by genetical modification of glycine decarboxylase in potato and by low temperature in barley resulted in an adequate increase of respiration. It is concluded that catabolism of starch but not of sucrose is blocked by light. The possible pathways of carbon transfer from sucrose to the photorespiratory cycle will be discussed.