Photosynthesis and assimilate distribution in lolium multiflorum lam. Following differential tiller defoliation

Abstract

The rate of net CO2 exchange and the distribution of 14C-assimilates was studied in single plants of L. multi/lorum. Defoliation of tillers, while leaving the main shoot intact, diverted some of the 14C assimilated by leaf 10 of the main shoot to the regrowing tillers. The level of this diversion continued to increase throughout a 9-day experiment when regrowth was trimmed daily, but reached a peak after 2 days when the regrowth was allowed to remain on the plant. The diversion of assimilate to defoliated tillers was largely at the expense of the main shoot when the stress was mild but also at the expense of the roots for the more severe treatments. For a leaf in a phase of declining net photosynthesis the first effect of tiller defoliation on the gas-exchange properties of main shoot leaves was a reduction of gas-phase resistance. Continued regrowth trimming maintained this low gas-phase resistance and also prevented the increase of residual resistance with age which was evident in control plants. For a leaf which had not quite reached full expansion when the tiller defoliation-regrowth trimming procedure started, the major source of the higher net photosynthesis rate after 10 days (relative to the controls) was a low gas-phase resistance. These experiments confirm that although established tillers are usually independent of the main shoot for assimilate, when under stress they again become dependent on the main shoot. This support was sustained if the stress was sustained, and the enhanced demand for assimilate from the main shoot delayed the normal decline of photosynthesis rate with age in main-shoot leaves. © 1973 ASEG.