Growth Pattern, Carbon Dioxide Exchange and Dry Weight Distribution in Wheat Growing Under Differing Photosynthetic Environments

Abstract

Wheat (cv. WW15) was grown as a crop stand in different CO₂ concentrations (ambient, ambient plus 200 ± 20 vpm CO₂, ambient minus 150 ± 20 vpm CO₂) from germination to maturity in naturally lit growth cabinets under winter or summer light conditions, at 21°C by day and 16°C at night. Ambient CO₂ concentration during the daylight hours averaged 280-300 vpm. CO₂ level had little effect on phenology of the mainshoot; most of the growth response was through tillering. From data on flag leaves in the winter light experiment, there was no indication of any positive or negative feedback on growth acting through maximum leaf net photosynthesis rate. Leaf area index was increased by CO₂ at low light and the related self-shading acted as a negative feedback partially countering the effect due to an enhanced rate of CO₂ uptake per unit leaf area. Dark respiratory CO₂ loss represented a greater proportion of CO₂ uptake in the light for the CO₂-depleted crop than for the control crop. But the reciprocal effect was not evident for the enriched crop. Contrary to classical ideas on growth responses to variation of colimiting factors, the growth response to CO₂ enrichment was relatively greater under the low radiation than the high radiation regime. The grain was the tissue most flexible in its responsiveness to changes in assimilation under the conditions of the summer experiment. For this crop, for which the grain yield of the control was very high (0.97 kgm^-2), response of yield to CO₂ enrichment corresponded to 0.25% per vpm.