Interactive Effects of Elevated [CO2], Water Deficit and Thermal Stress on Peanut Physiology and Productivity

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2018-05-17

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Abstract

To the extent rising atmospheric [CO2] will impact crop production or alter the carbon and water dynamics is a topic of debate. Uncertainties on predictions are mostly based on inadequate designs and unreliable simulation models, which have failed to incorporate the effect of combined stresses on agroecosystems, particularly from arid and semiarid regions, the major drivers of inter-annual variabilities at global scale. This project focuses on the individual and interactive effects of long term elevated [CO2], periodic water stress, and random intermittent heat waves on peanut agroecosystem productivity and the molecular mechanisms used by crops to cope with these abiotic stresses. To address these questions, peanut (Arachis hypogaea L.) runner type cultivar C76-16 grown under both ambient (400 ppm) and elevated (650ppm) [CO2] was exposed to three induced water deficit episodes during critical reproductive stages (R3, R5, R7) and evaluated at the plant-soil level using combined physiological, structural and transcriptomic approaches. The open path canopy evapotranspiration and assimilation (CETA) technology was used to simulate and monitor the environmental parameters in the field. Unlike the free atmospheric CO2 enrichment (FACE) technology, which is expensive, with CO2 fumigation only during daytime, the CETA technology maintains a continuous CO2 enrichment throughout the entire growing season. Peanut through microbial symbiotic associations capabilities, represents a unique model to capture the complexity of the plant-microbe-soil-environment system. Here we show that agroecosystems exposed to elevated [CO2] (250 ppm over the ambient background) increased net ecosystem productivity (NEP) and respiration with a significant reduction during periods of low soil water availability, indicating that the degree to which system C sink capacity could offset the increase in atmospheric [CO2] might be reduced as well. This could also be attributed to the enhanced soil emissions in EC systems, and could be mostly due to the increase in soil metabolic activity and the shift in microbial composition towards fungi populations, especially arbuscular mycorrhizae, which are known to improve plant water and nutrient acquisition. At the leaf level, no photosynthetic acclimation to elevated [CO2] was observed. However, at the system level, although higher C assimilation was also observed towards maturity, the differential response between the treatments was reduced, showing evidence of acclimation. The increase in NEP was coupled with an increased system water use, resulting in significantly reduced water use efficiency. These results challenge the assumption that lower transpiration and system water use are expected in enriched [CO2] system and question previous projections regarding peanut production sustainability with the intensification of drought in future climates. Interestingly the transcriptomic analysis revealed that maintenance of high net assimilation (Anet) in EC systems prior to any water deficit at early reproductive stage (R2) is achieved mostly by downregulation of the photorespiratory pathway (C2). However, different enzymes of the C2 pathway or other photosynthetic related pathways such as the Calvin cycle and electron transport chain gain more relevance in later developmental stages.

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Keywords

CO2, drought, peanut, climate change, photosynthetic acclimation, soil microbial community, transcriptomic, physiology,

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