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

Abstract

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 (C a) concentration have been shown to increase total biomass and grain yield of C 3 crops including rice. However, doubts have been expressed whether the expected yield increases in response to increased C a could be sustained under high temperature regimes. Therefore, the main objective of the present study was to quantify the response of rice to elevated C a 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 C a 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) C a in open top chambers with open field plots to estimate chamber effects. C a 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 CO 2 enrichment. Rice under elevated C a showed significantly greater (20% and 11% in the two seasons) RUE relative to ambient C a. RUE under elevated C a did not show a reduction during the post-heading period. Consequently, the total biomass at harvest was 23–37% greater under elevated C a. Number of grains initiated and percentage of grains filled were significantly greater under elevated C a 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 C a did not exceed that under ambient C a. Based on the above results, it is concluded that rice yields respond positively to increasing C a 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.