Growth at elevated CO2 delays the adverse effects of drought stress on leaf photosynthesis of the C4 sugarcane

SummarySugarcane (Saccharum officinarum L. cv. CP72-2086) was grown in sunlit greenhouses at daytime [CO2] of 360 (ambient) and 720 (elevated) μmol mol−1. Drought stress was imposed for 13 d when plants were 4 months old, and various photosynthetic parameters and levels of nonstructural carbohydrates were determined for uppermost fully expanded leaves of well-watered (control) and drought stress plants. Control plants at elevated [CO2] were 34% and 25% lower in leaf stomatal conductance (gs) and transpiration rate (E) and 35% greater in leaf water-use efficiency (WUE) than their counterparts at ambient [CO2]. Leaf CO2 exchange rate (CER) and activities of Rubisco, NADP-malate dehydrogenase, NADP-malic enzyme and pyruvate Pi dikinase were marginally affected by elevated [CO2], but were reduced by drought, whereas activity of PEP carboxylase was reduced by elevated [CO2], but not by drought. At severe drought developed at day 12, leaf gs and WUE of ambient-[CO2] stress plants declined to 5% and 7%, while elevated-[CO2] stress plants still maintained gs and WUE at 20% and 74% of their controls. In control plants, elevated [CO2] did not enhance the midday levels of starch, sucrose, or reducing sugars. For both ambient- and elevated-[CO2] stress plants, severe drought did not affect the midday level of sucrose but substantially reduced that of starch. Nighttime starch decomposition in control plants was 55% for ambient [CO2] and 59% for elevated [CO2], but was negligible for stress plants of both [CO2] treatments. For both ambient-[CO2] control and stress plants, midday sucrose level at day 12 was similar to the predawn value at day 13. In contrast, sucrose levels of elevated-[CO2] control and stress plants at predawn of day 13 were 61–65% of the midday values of day 12. Levels of reducing sugars were much greater for both ambient- and elevated-[CO2] stress plants, implying an adaptation to drought stress. Sugarcane grown at elevated [CO2] had lower leaf gs and E and greater leaf WUE, which helped to delay the adverse effects of drought and, thus, allowed the stress plants to continue photosynthesis for at least an extra day during episodic drought cycles.