Light-mediated Kleaf induction and contribution of both the PIP1s and PIP2s aquaporins in five tree species: walnut (Juglans regia) case study

Understanding the response of leaf hydraulic conductance (K-leaf) to light is a challenge in elucidating plant-water relationships. Recent data have shown that the effect of light on K-leaf is not systematically related to aquaporin regulation, leading to conflicting conclusions. Here we investigated the relationship between light, K-leaf, and aquaporin transcript levels in five tree species (Juglans regia L., Fagus sylvatica L., Quercus robur L., Salix alba L. and Populus tremula L.) grown in the same environmental conditions, but differing in their K-leaf responses to light. Moreover, the K-leaf was measured by two independent methods (high-pressure flow metre (HPFM) and evaporative flux method (EFM)) in the most (J. regia) and least (S. alba) responsive species and the transcript levels of aquaporins were analyzed in perfused and unperfused leaves. Here, we found that the light-induced K-leaf value was closely related to stronger expression of both the PIP1 and PIP2 aquaporin genes in walnut (J. regia), but to stimulation of PIP1 aquaporins alone in F. sylvatica and Q. robur. In walnut, all newly identified aquaporins were found to be upregulated in the light and downregulated in the dark, further supporting the relationship between the light-mediated induction of K-leaf and aquaporin expression in walnut. We also demonstrated that the K-leaf response to light was quality-dependent, K-leaf being 60% lower in the absence of blue light. This decrease in K-leaf was correlated with strong downregulation of three PIP2 aquaporins and of all the PIP1 aquaporins tested. These data support a relationship between light-mediated K-leaf regulation and the abundance of aquaporin transcripts in the walnut tree.