Photosynthetic acclimation to elevated CO2 combined with partial rootzone drying results in improved water use efficiency, drought tolerance and leaf carbon balance of grapevines (Vitis labrusca)

We considered the interactive effects of elevated CO2 concentration [CO2] and reduced water availability (Partial Rootzone Drying - PRD and drought) on a variety of important physiological and growth traits in grapevine (Vitis labrusca). The following questions were addressed: (i) Will there be a down-regulation of photosynthesis at elevated [CO2] and what role do leaf nitrogen (N) concentration and specific leaf weight (SLW) have in this process? (ii) What are the effects of PRD under elevated [CO2]? (iii) Can elevated [CO2] delay the negative effects of drought in grapevines? and (iv) What is the impact of leaf respiration in the light (Rlight) and in the dark (Rdark) on leaf carbon balance (LCB) of grapevines? Three water management treatments (WMT) were applied [full-irrigated - both sides of the rootzone were irrigated to saturation; PRD - only one side of the rootzone was irrigated to saturation; and non-irrigated - irrigation was suspended on both sides of the rootzone] in each of two 1.4 m2 growth chambers, each one with different [CO2] (400 ppm or 800 ppm). We found that: (i) net photosynthetic rate (Anet) is down-regulated under elevated [CO2], which may be caused by reductions in Rubisco content and/or total activity and by a reduction in the efficiency of the photochemical apparatus due to N dilution within leaves with higher SLW; (ii) Under elevated [CO2], Rubisco carboxylation rate is increased when PRD is applied, leading to increased Anet, thereby leading to greater water use efficiency (WUE) and intrinsic WUE (iWUE). However, these responses were not linked to photochemical or stomatal effects. (iii) Elevated [CO2] delayed drought effects on both Anet and Rubisco activity for four days, by reducing stomatal conductance, transpiration and stomatal density; (iv) Leaf respiration responses depend on drought level; and (v) Conditions that reduce N concentration, such as elevated [CO2], also reduce Rdark and Rlight. In addition, elevated [CO2] intensifies light inhibition of respiration and reduces Rubisco oxygenation, as well as improves LCB and contributes for mitigating deleterious effects of drought on LCB in grapevines.