Carbon dioxide exchange rates and leaf nitrogen contents during ageing of the flag and penultimate leaves of five spring-wheat cultivars

Abstract

Postanthesis carbon dioxide exchange and transpiration rates of flag and penultimate leaves of five spring-wheat (Triticum aestivum L. emend. Thell.) cultivars were measured from complete flag-leaf expansion to senescence. Leaf nitrogen contents were determined from anthesis to maturity. Both the absolute level of and the time-related decline of the carbon dioxide exchange rate varied among the cultivars. The flag-leaf carbon dioxide exchange rate decreased steadily throughout for one cultivar and slowly for a varying period and then rapidly for most others. The penultimate-leaf carbon dioxide exchange rate decreased throughout with one cultivar but did not decline in the period from 1 to 3 weeks postanthesis in others. The transpiration rate peaked at or near anthesis for the flag leaves and then either declined or fluctuated around the peak value for 3 weeks. The penultimate-leaf transpiration rate increased to a second peak late in ontogeny for most genotypes. In general, the time course of the transpiration rate matched that of the carbon dioxide exchange rate, but the transpiration rate at a specific carbon dioxide exchange rate was lower for penultimate than for flag leaves. The carbon dioxide exchange rate was linearly related to leaf nitrogen content, with the same regression applying for both flag and penultimate leaves; regressions were similar for all genotypes. There were no marked deviations from the overall carbon dioxide exchange rate – nitrogen regression that could be indicative of a "sink" influence on the activity of photosynthetic enzymes.