Chloride as a macronutrient increases water‐use efficiency by anatomically driven reduced stomatal conductance and increased mesophyll diffusion to <scp>CO</scp>₂

Abstract

Chloride (Cl^- ) has been recently described as a beneficial macronutrient, playing specific roles in promoting plant growth and water-use efficiency (WUE). However, it is still unclear how Cl^- could be beneficial, especially in comparison with nitrate (NO₃^- ), an essential source of nitrogen that shares with Cl^- similar physical and osmotic properties, as well as common transport mechanisms. In tobacco plants, macronutrient levels of Cl^- specifically reduce stomatal conductance (g_s ) without a concomitant reduction in the net photosynthesis rate (A_N ). As stomata-mediated water loss through transpiration is inherent in the need of C₃ plants to capture CO₂ , simultaneous increase in photosynthesis and WUE is of great relevance to achieve a sustainable increase in C₃ crop productivity. Our results showed that Cl^- -mediated stimulation of larger leaf cells leads to a reduction in stomatal density, which in turn reduces g_s and water consumption. Conversely, Cl^- improves mesophyll diffusion conductance to CO₂ (g_m ) and photosynthetic performance due to a higher surface area of chloroplasts exposed to the intercellular airspace of mesophyll cells, possibly as a consequence of the stimulation of chloroplast biogenesis. A key finding of this study is the simultaneous improvement of A_N and WUE due to macronutrient Cl^- nutrition. This work identifies relevant and specific functions in which Cl^- participates as a beneficial macronutrient for higher plants, uncovering a sustainable approach to improve crop yield.