Pigment Accumulation, Dark Respiration and Photosynthesis during the Greening of Sunflower Cotyledons

Abstract

Summary Sunflower seedlings ( Helianthus annuus L.) were grown in darkness or irradiated with continuous white light (WL) of medium intensity (100 micromole quanta per square meter per second). During the first 6 days after sowing, seedling development was accompanied by a significant loss in dry mass. In cotyledons of etiolated seedlings the carotenoid lutein is the dominant pigment that causes the yellow colour of the organs. In WL, a large increase in the rate of carotenoid accumulation occurred. Synthesis of chlorophylls a and b was induced rapidly upon exposure to WL. In etiolated cotyledons the rate of oxygen uptake (dark respiration) increased until day 3 after sowing, and thereafter declined. In WL, a stimulation of dark respiration was measured. The rate of photosynthetic oxygen evolution of lightgrown cotyledons was positively correlated with the chlorophyll content of the organ (r = 0.96). It follows that chlorophyll biosynthesis is one of the limiting factors that determines the rate of photosynthesis during de-etiolation of the seedling. However, oxygen evolution was significantly lower than the rate of dark respiration. Our results indicate that the green, de-etiolated cotyledons of the sunflower seedling are still heterotrophic organs.