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 C-3 or C-4 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 CO2, 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.