The Dynamics of Carbon Supply from Leaves of Barley Plants Grown in Long or Short Days

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Abstract
The role of the mature leaf in supplying carbon for growth in other parts of the plant was examined using a steady-rate 14CO2 labelling technique. The pattern of events occurring in the leaf during one complete 24 h cycle was compared in plants grown in, and adapted to long and short photoperiods. The rates of leaf photosynthesis, night respiration and daytime loss of carbon from the growing regions of the plant Were similar in long or short photoperiods. As a percentage of the total carbon fixed during the photoperiod, total respiration was c. 50% for short day plants but only 25% for long day plants. Thirty to forty per cent of the carbon fixed during the photoperiod was retained in the leaf for export during darkness—the rest was exported immediately. In leaves of short day plants sucrose and starch were the main form of the stored carbon. By the end of the dark period these compounds had been almost completely depleted. In leaves of long day plants there were much larger basal levels of sucrose and starch, upon which the diurnal variations were superimposed. These leaves also accumulated fructosans. The delay in starch remobilization previously found in leaves of short day plants was also evident in leaves of long day plants even though large concentrations of sucrose and fructosans were present This suggests the presence of distinct pools of sucrose in the leaf.