Paper Reviewed
Kumar, A., Nayak, A.K., Sah, R.P., Sanghamitra, P. and Das, B.S. 2017. Effects of elevated CO2 concentration on water productivity and antioxidant enzyme activities of rice (Oryza sativa L.) under water deficit stress. Field Crops Research 212: 61-72.

Introducing their study, Kumar et al. (2017) note that rice is the principal staple food of India, contributing greatly to the country's food security. One of the most significant threats to that security is water stress, which threat may increase in the future due to climate change, potentially causing drastic reductions in rice yields. However, projections of rice yield loss due to moisture stress might well be overcome by the yield-enhancing benefits that are expected to accrue due to atmospheric CO2 enrichment, which latter phenomenon has been shown to reduce plant moisture requirements while increasing both plant growth and yield.

In an effort to elucidate the interactive effects of these two competing forces (water stress and atmospheric CO2 enrichment), Kumar et al. embarked on a two-year experiment where they grew rice (Oryza sativa, cv Naveen) in open-top chambers under two moisture regimes and three CO2 concentrations. The study was performed at the Central Rice Research Institute in Cuttack, India over two consecutive growing seasons. Plant moisture regimes included (1) well-watered, where the water depth of the soil was maintained at 3 ± 2 cm, or (2) water-deficit, where surface irrigation was only applied when the soil water potential at 15 cm reached -60 kPa. Atmospheric CO2 concentrations were maintained at either (1) ambient (400 ppm), (2) mid-elevation (550 ppm), or (3) high-elevation (700 ppm) during daylight hours only. Multiple growth- and water-related parameters were measured at different stages of plant development, as well as the concentration of several antioxidant enzymes. And what did these several measurements reveal?

Atmospheric CO2 enrichment (to both 550 and 700 ppm), in the words of the authors, "exhibited a positive response on plant growth, grain yield and [water use efficiency] of rice as compared to ambient CO2." In fact, with respect to grain yield, elevated levels of CO2 increased this parameter by 15-18% under well-watered conditions and by a larger 39-43% under water-deficit conditions. In terms of water use, under elevated CO2 there was a decline of 11-14% and 5% in irrigation water input under well-watered and water-deficit conditions, respectively. What is more, the water use efficiency (ratio of grain yield to total water input) of rice in the two CO2-enriched chambers increased by 30-35% under well-watered conditions and by approximately 48% under water deficit conditions, relative to that observed in the ambient CO2 chambers.

The five researchers also report that higher levels of CO2 significantly altered leaf tissue parameters (e.g., relative water content, leaf water potential and electrolyte leakage) under moisture-stress, so as to help mitigate the negative impacts of water-deficit. In addition, they found that the concentrations of certain antioxidant metabolites were reduced in plants growing under elevated CO2 in the moisture-stress treatment. This latter observation further supports the notion that elevated CO2 helps mitigate water stress in rice -- the CO2-induced mitigation of the water stress reduces the production of harmful reactive oxygen species, which subsequently reduces the need for plants to produce antioxidant enzymes to counter them.

Taken together, the above findings signal good news for the future food security of India. Rising atmospheric CO2 concentrations will increase rice yields under normal watering conditions, needing less water to do so, while under water-deficit conditions it will help to mitigate (and potentially overcome) yield losses.