Defoliation and Regrowth in the Graminaceous Plant: The Role of Current Assimilate

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Abstract
Infra-red gas analysis and a quantitative radiocarbon tracer technique were used to measure photosynthesis, and the export, distribution and utilization of current assimilate in the regrowth of leaf tissue and the growth of stem and root of partially-defoliated uniculm barley plants. After defoliation, which removed all leaf tissue above the ligule of leaf 3, the rate of photosynthesis of the remaining two older leaves fell to 90–95 per cent of that of control leaves, but they exported more of their assimilated carbon to meristems elsewhere in the plant during the first 48 h after the defoliation. The level of export from the two older leaves began to decline when new leaf tissue regrew from the shoot apex, and fell below that of the control leaves 4 days after defoliation. The two older leaves supplied the assimilate used in the regrowth of new leaf tissue immediately after defoliation: previously they had exported most of their assimilate to root. There was no evidence that ‘reserves’ were mobilized to meet the needs of regrowth at leaf meristems or, indeed, of the growth in stem and root; current photosynthesis supplied sufficient assimilate to account for all observed growth. In general, the plants responded to defoliation with a rapid and marked re-allocation of assimilate from root to leaf meristems, with the result that root growth was severely retarded but new leaf tissue grew at 70–100 per cent of the rate observed in control plants.

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