Physiology of yield determination of rice under elevated carbon dioxide at high temperatures in a subhumid tropical climate

A substantial portion of rice in South Asia is grown in tropical and sub-tropical climates under relatively high temperatures (i.e. >30 °C). Increases in atmospheric carbon dioxide (Ca) concentration have been shown to increase total biomass and grain yield of C3 crops including rice. However, doubts have been expressed whether the expected yield increases in response to increased Ca could be sustained under high temperature regimes. Therefore, the main objective of the present study was to quantify the response of rice to elevated Ca at high temperatures (i.e. >30 °C) in a sub-humid tropical environment in terms of radiation interception, radiation use efficiency (RUE) and biomass partitioning to grains. It was also investigated whether RUE of rice growing under elevated Ca decreases during the post-heading period, possibly due to a reduction of leaf nitrogen concentration.Rice was grown over two seasons in a sub-humid tropical climate in Sri Lanka at elevated (ca. 567 ± 28 μmol mol−1) and ambient (ca. 363 ± 16 μmol mol−1) Ca in open top chambers with open field plots to estimate chamber effects. Ca within chambers was maintained around target concentrations by a computer-based real time data acquisition and control system. Radiation interception was measured continuously by tube solarimeters. Seasonal fraction of incoming radiation intercepted did not change with CO2 enrichment. Rice under elevated Ca showed significantly greater (20% and 11% in the two seasons) RUE relative to ambient Ca. RUE under elevated Ca did not show a reduction during the post-heading period. Consequently, the total biomass at harvest was 23–37% greater under elevated Ca. Number of grains initiated and percentage of grains filled were significantly greater under elevated Ca resulting in final seed yields being 24% and 39% greater than the ambient. During grain filling, the fraction of biomass partitioned to grains under elevated Ca did not exceed that under ambient Ca. Based on the above results, it is concluded that rice yields respond positively to increasing Ca even at the higher range of growing temperatures. Greater RUE and greater initiation of grains are the primary causes of this yield stimulation at the crop level.