PIGMENT ACCUMULATION, DARK RESPIRATION AND PHOTOSYNTHESIS DURING THE GREENING OF SUNFLOWER COTYLEDONS

Sunflower seedlings (Helianthus annuus L.) were grown in darkness or i rradiated with continuous white light (WL) of medium intensity (100 mi cromole quanta per square meter per second). During the first 6 days a fter sowing, seedling development was accompanied by a significant los s in dry mass. In cotyledons of etiolated seedlings the carotenoid lut ein is the dominant pigment that causes the yellow colour of the organ s. In WL, a large increase in the rate of carotenoid accumulation occu rred. Synthesis of chlorophylls a and b was induced rapidly upon expos ure to WL. In etiolated cotyledons the rate of oxygen uptake (dark res piration) increased until day 3 after sowing, and thereafter declined. In WL, a stimulation of dark respiration was measured. The rate of ph otosynthetic oxygen evolution of light-grown cotyledons was positively correlated with the chlorophyll content of the organ (r = 0.96). It f ollows that chlorophyll biosynthesis is one of the limiting factors th at determines the rate of photosynthesis during de-etiolation of the s eedling. However, oxygen evolution was significantly lower than the ra te of dark respiration. Our results indicate that the green, de-etiola ted cotyledons of the sunflower seedling are still heterotrophic organ s.