Paper Reviewed
Sreeharsha, R.V., Mudalkar, S., Sengupta, D., Unnikrishnan, D.K. and Reddy, A.R. 2019. Mitigation of drought-induced oxidative damage by enhanced carbon assimilation and an efficient antioxidative metabolism under high CO2 environment in pigeonpea (Cajanus cajan L.). Photosynthesis Research 139: 425-439.

Writing as background for their work, Sreeharsha et al. (2019) note that pigeonpea (Cajanus cajan) is “the main protein source for more than a billion people in the developing world and a cash crop that supports the livelihoods of millions of resource-poor farmers globally.” Given such agricultural and economic prominence, they thus write that it is “highly relevant to understand the impact of climate change on this promising food crop.”

As their contribution to the topic, the five Indian researchers grew pigeonpea for two growing seasons at the University of Hyderabad (Hyderabad, Telangana State, India) in open-top chambers under ambient (390 ppm) or elevated (600 ppm) CO2. Then, at the end of the vegetative phase of the plant in each growing season (60 days), Sreeharsha et al. instituted drought stress in half of the plants in each CO2 treatment by withholding water for nine days, followed by a rewatering to normal levels immediately thereafter. During and after the drought stress period the researchers conducted a series of measurements to determine various physiological responses of the crops under the two CO2 treatments.

Results indicated that under well-watered conditions, elevated CO2 increased plant photosynthesis by 64%, nodule number by 38% and nodule mass by 64%. Photosynthesis and nodule mass were also significantly higher under elevated CO2 during drought stress and the recovery period thereafter. In particular, Sreeharsha et al. report that “elevated CO2 plants showed 35% higher photosynthesis rates than ambient CO2 grown plants at severe drought stress” (9 days after stress initiation), adding that “elevated CO2 grown plants recovered their photosynthesis rates to 100% of their well-watered counterparts, whereas ambient CO2 grown plants recovered to 85%. Such findings, in the words of the authors, reveal that “the enhanced carbon supply under elevated CO2 favored better growth in pigeonpea under drought stress associated with effective sink capacity in the form of nodules mediating better nitrogen assimilation.”

Another important finding reported by Sreeharsha et al. was noted in differences among antioxidant levels in the plants growing in elevated CO2 under both well-watered and water-stressed conditions. Significant antioxidant upregulation under drought at elevated CO2 revealed a CO2-induced protective mechanism that helped guard the plants against oxidative damage caused by reactive oxygen species.

Commenting on these and other of their findings, Sreeharsha et al. say their results demonstrate “a coordinated and integrated mechanism in pigeonpea to cope with drought stress by taking advantage of elevated CO2,” whereby that “enhanced antioxidative systems ultimately [provide] stress tolerance in elevated CO2 grown pigeonpea which [gains] comparative advantage in growth and metabolism over ambient grown plants.” And that is great news for the billions of persons in the developing world who rely on this key cash crop!