Empirical Study of the Effect of Elevated CO2 on Photosynthesis and Yield of Rice Under Nigerian Field Conditions

Elevated atmospheric CO2 concentrations expected under climate change scenarios offer potential photosynthesis stimulation and yield improvement for C3 crops like rice, but the actual magnitude of this CO2 fertilisation effect under the temperature and humidity conditions characteristic of Nigerian rice growing environments requires empirical characterisation. This study empirically investigated the effect of elevated CO2 (ambient plus 200 ppm) on photosynthesis rates, stomatal conductance, water use efficiency, and grain yield of two rice varieties (FARO 44 and FARO 60) under the temperature and humidity conditions of the Nigerian rice belt at an open-top chamber facility at the National Cereals Research Institute, Badeggi, Niger State. A split-plot design with CO2 concentration as the main plot and variety as the sub-plot was used over two seasons (2022 and 2023) with four replications. Gas exchange measurements, leaf area index, total dry matter, and grain yield were recorded. Results showed that elevated CO2 increased net photosynthesis by 23.6 percent and water use efficiency by 19.8 percent over ambient CO2 treatments. Grain yield increase was 14.3 percent under elevated CO2 for FARO 44 and 11.7 percent for FARO 60, with the yield response partially suppressed by concomitant temperature increases above the optimum. The study fills an empirical gap in characterising CO2 fertilisation under tropical Nigerian conditions and recommends that climate change models for Nigerian rice incorporate these CO2-temperature interaction findings for more accurate future yield projections.

Keywords: elevated CO2, rice photosynthesis, climate change, grain yield, Nigeria