Effects of CO2Enrichment at Different Irradiances on Growth and Yield of Wheat

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Abstract
Spring wheat plants were grown in a cage with a glass roof until three days after anthesis and then subjected to treatments in constant environment rooms with any one of all combinations of four irradiances and two concentrations of carbon dioxide. The photoperiod was 16 h and day/night temperatures 19°C/14°C. Growth and yield of grain were saturated at the two brightest irradiances. Carbon dioxide enrichment from 350 to 1200 mm3 dm−3 increased shoot dry weight and grain yield at final harvest at all irradiances, by averages of 10.5 (not significant) and 23.5 (significant) percent respectively. However, increasing the irradiance from 150 to 613 μE m−2 s−1 caused much larger yield increases (approximately 3-fold). Increased grain production by increased light was caused by both increases in dry weight per grain and by increases in grain number per spikelet. The increase caused by CO2 enrichment was mainly because of increased dry weight per grain. Increase in ear dry weight caused by CO2 enrichment took place between 30 and 60 d after anthesis. The increase in shoot dry weight took place immediately after exposure to increased CO2 from 3 to 15 d after anthesis. Net photosynthesis by flag leaves on the main shoots was almost doubled 16 d after anthesis by the CO2 enrichment even though stomatal resistance was also doubled. However, this increase was not reflected by a proportional increase in yield, probably because increased mutual shading by bigger stems and late tillers reduced total assimilation and because of increased respiration by the shoots. The increase in photosynthesis was not due to a decrease in photorespiration but to an increase in gross photosynthesis.

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