Opportunities for improving intrinsic water use efficiency in C4 plants under climate change.

C4 photosynthesis is inherently efficient, saturating at low intercellular CO2 (Ci) and operating under low stomatal conductance (gs), resulting in high intrinsic water use efficiency (iWUE = assimilation rates, (A)/ stomatal conductance (gs)). While iWUE is generally higher in C4 than in C3 plants, future climate scenarios, marked by increasing atmospheric CO2, temperatures, and aridity, present ongoing pressures on crop productivity in C4-dominated environments. This review explores the physiological, anatomical, and environmental factors controlling iWUE in C4 plants, with a focus on balancing productivity and resilience under climate change. Using process models of photosynthesis and stomatal behaviour, we assess how iWUE varies with photosynthetic limitations, highlight the central role of stomata in controlling iWUE, and predict the physiological adjustments needed to optimize carbon gain per unit water loss. Recent evidence points to leaf width as a useful trait influencing leaf energy balance, temperature, and transpiration. We propose integrated physiological and breeding strategies to optimize crop iWUE and yield under climate stress, supported by emerging sensing and selection technologies.