Barley and its environment. 4. Sources of assimilate for the grain.
Rates of photosynthesis of leaves and ears were measured after anthesis in a field of barley cv. Proctor using a simple leaf chamber with polypropylene walls and an infra-red gas analyser mounted on a trolley. Photosynthesis-light curves for different organs were established by taking advantage of changes in irradiance or by shading. The maximum photosynthesis rate of the flag leaf decreased from 2.7 g CO2/m2 leaf h on 21 June to 0.9 g CO2/m2 leaf h 5 weeks later; other green organs lost their ability to assimilate within a similar period. Photosynthesis of the whole canopy was estimated from the photosynthesis-light curves of individual organs, the vertical distribution of radiant energy in the canopy, and the vertical distribution of leaf area. From 28 June to 26 July, estimates of weekly net photosynthesis derived in this way agreed with micrometeorological measurements of CO2 uptake by the canopy and the agreement extended to the diurnal course of photosynthesis on individual days. In the middle of July the flag leaf sheath and the ear itself were the main sources of new assimilate for filling the grain, but by the end of the month photosynthesis by the canopy had almost ceased. The grain continued to fill, nevetherless, and achieve a final weight of 477 g dry matter/m2 ground. Only 70% of this total weight could be accounted for by photosynthesis of the canopy after anthesis (57% from laminae and sheaths and 13% from the ear). The remaining 30% of the total weight could be accounted for by translocation from the stem which decreased in weight by 250 g dry matter/m2 ground after 12 July. The contribution of ear photosynthesis to grain dry weight was smaller than previous studies suggested.