CARBOHYDRATE STARVATION IS A MAJOR DETERMINANT OF THE LOSS OF GREENING CAPACITY IN COTYLEDONS OF DARK-GROWN SUGAR-BEET SEEDLINGS

The transition of cotyledons from heterotrophy to autotrophy is a crit ical step of seedling establishment. We have studied the greening capa city of sugar beet (Beta vulgaris L. cv. Vega) cotyledons in relation to carbohydrate and energy metabolism during dark growth. During early growth, sugar beet cotyledons behaved mainly as a lipid-mobilizing an d gluconeogenic tissue providing substrates to the seedling. Reserve m obilization was followed by a maximum of the adenine nucleotide pool o n day 6 in strict correlation with the maximum of greening capacity. T his was immediately followed by the onset of a typical situation of ca rbohydrate starvation characterized by substrate limitation of respira tion, a decrease in the adenine nucleotide pool and, as shown by the r espiratory quotient and the loss of proteins, a probable utilization o f cellular proteins and lipids to sustain respiration. The conversion from etioplast to chloroplast, as determined by the rate of chlorophyl l synthesis, was less and less efficient as carbohydrate starvation co ntinued, finally leading to incomplete and heterogeneous greening on d ay 12. The relationship of the loss of greening capacity with carbohyd rate starvation is discussed.