Diurnal Production and Utilization of Photosynthate in Nodulated White Clover

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Abstract
Gordon, A. J., Mitchell, D. F., Ryle, G. J. A. and Powell, C. E. 1987. Diurnal production and utilization of photosynthate in nodulated white clover.—J. exp. Bot. 38: 84–98. A steady-state 14C-labelling technique was used to examine the diurnal carbon fixation, storage and export characteristics of white clover leaves. Approximately 70% of fixed carbon was exported to other organs during the photoperiod. The remaining carbon was stored mainly as starch (80% at the end of the photoperiod) with smaller amounts of sucrose, hexoses and charged compounds. Carbon export from the leaf at night was provided by remobilization of starch. During the photoperiod it was estimated that c.60% of carbon exported from the leaf was directed towards the nodulated root; 45% to nodules and 15% to roots. The 40% directed towards the shoot was supplemented by a further 11% of carbon (in the form of amides) re-exported from the nodules. During the photoperiod, all organs of the plant accumulated carbohydrate which was available for use during darkness, in conjunction with a diminished supply of exported carbon from leaves. Nodules exhibited a striking pattern of carbohydrate storage and depletion. The levels of sucrose and starch in the nodules at the end of the photoperiod were sufficient to maintain N2 fixation for 8–9 h of the 12 h dark period. We propose that continued import from leaves provided the additional sucrose necessary to support undiminished nodule function throughout the entire dark period.