Assessing the potential functions of nocturnal stomatal conductance in C₃ and C₄ plants

Nocturnal stomatal conductance contributes to water loss at night without carbon gain in C₃ or C₄ plants because photosynthesis does not occur in the dark. The functional relevance of nocturnal conductance thus remains an unresolved conundrum. Here, we review and re-analyse previously published datasets on nocturnal conductance (g_n ) globally (176 species) to synthesize our current understanding on its potential biological function and to identify remaining research gaps. We found that g_n was positively correlated with relative growth rate, which is compatible with the postulate that circadian-driven nocturnal conductance enhances predawn stomatal conductance, thereby priming stomata for photosynthesis in early daylight. The variation in g_n across plant species and functional types was not consistent with the hypotheses that the main function of g_n is to: remove excess CO_2, which might limit growth; enhance oxygen delivery to the functional sapwood; enhance nutrient supply; or that g_n is due to stomatal leakiness. We suggest further study regarding the potential of g_n to be an important functional and ecological trait influencing competitive outcomes and we outline a research programme to achieve that objective.