Responses of flag-leaf photosynthetic pigments at late growth stage and rice yield components to elevated CO₂ under FACE system

To verify the effects of elevated atmospheric CO₂ concentration on photosynthetic capacity of rice leaves, the trend in photosynthetic pigments contents of flag-leaves at late growth stage and yield components of rice varieties of 'Songjing9' and 'Daohuaxiang2' were studied under the free-air CO₂ enrichment (FACE) system. Flag-leaf chlorophyll-a, chlorophyll-b and carotenoid contents were measured during booting and heading stages and yield components measured after harvest. The total chlorophyll content and the ratio of chlorophyll-a to chlorophyll-b were calculated and the correlations among yield components analyzed. The results showed that comparing to ABM (ambient CO₂) treatment, FACE treatment increased chlorophyll-a contents in flag-leaves of both rice varieties at booting to heading stage with respective peak increasing values of 28.46% and 19.58%, and decreased by 15.25% and 23.20% at 20 d after heading stage. FACE treatment decreased chlorophyll-b contents of flag-leaves of both rice varieties by 7.57% and 5.33% at 20 d after heading stage, then increased at respective rates of 4.19% and 9.46% at 30 d after heading stage. Carotenoid contents in flag-leaves of both rice varieties noticeably increased respectively by 9.47% and 13.55% at heading stage under FACE treatment and then decreased at 10 d after heading stage at rates of 13.54% and 16.67%. The trends in the effects of elevated CO₂ concentration on total chlorophyll content and chlorophyll-a to chlorophyll-b ratio of flag-leaf were similar. The trends increased from booting to heading stage and then decreased at 20 d after heading stage. Elevated CO₂ concentration was beneficial to yield components by increasing spike number per unit area, seed-setting rate and 1000-grain weight. The 1000-grain weight was increased by 8.6% and 4.5% of the two varieties, respectively. The results suggested that while elevated atmospheric CO₂ concentration enhanced photosynthetic pigment accumulation in flag-leaves of rice at booting and heading stages, it inhibited photosynthetic pigment accumulation after heading stage. Photosynthetic pigments of flag-leaves of different varieties of rice had different degrees of responses to elevated atmospheric CO₂ concentration. Increased 1000-grain weight was the key factor for yield increase of rice under elevated atmospheric CO₂ concentration.