Canopy apparent photosynthesis, respiration and yield in wheat

Abstract

SUMMARYField experiments explored the relationship between canopy apparent photosynthesis (CAP) and grain yield and examined the effect of plant population density on CAP. Two cultivars of wheat were grown at five plant population densities in 1986 and 1987 in Taian, China. Measurements of CAP were made at intervals during the growth period by placing a large plastic-covered chamber over a 0·7 m2 segment of the canopy and measuring CO2 depletion with infra-red gas analysers for 1–2 min at c. 25 °C around noon when solar radiation was > 1100 umol/m2/s. Canopy respiration (CR) rates were measured by covering the whole chamber with a black screen in the daytime. Calculations of CAP and CR were made using the chamber volume, air temperature and changes in CO2 concentration over time and expressed on a land area basis.Maximum values of CAP and CR were 4–6 g and 2–3 g CO2/m2/h, respectively, at the anthesis stage, decreasing with age during grain filling. The decrease was more rapid at the high than at the low population density. The difference in CAP between plant densities before booting and after anthesis could be attributed to reductions in leaf area index (LAI) and photosynthetic rate.Grain yield also differed between plant population densities, the range being from 5250 to 7500 kg/ha in both years. Grain yield and 1000-grain weight were positively correlated with CAP during grain filling (r = 0·78 and 0·99, P < 0·01, 8 D.F.). Effects of plant density on CR were highly significant, but CR was not related to yield (r = 0·50).