The 3D construction of leaves is coordinated with water use efficiency in conifers

Conifers dominate the canopies of many terrestrial biomes, holding a great ecological and economic importance globally. Current increases in temperature and aridity are resulting in conifer mortality and imposing high transpirational demands to global vegetation. Therefore, identifying leaf structural determinants of carbon acquisition and water use efficiency is essential in predicting physiological impacts due to environmental variation. Using synchrotron-generated microCT imaging, we extracted leaf volumetric anatomy and stomatal traits in 34 species across the conifers with a special focus on Pinus, the richest conifer genus. We show that intrinsic water use efficiency (WUEi) is driven by leaf vein volumetrics, with both traits scaling positively. The ratios of stomatal pore number per unit mesophyll or intercellular airspace volume emerged as powerful explanatory variables, accurately predicting both stomatal conductance and WUEi. Our results clarify how the three-dimensional organization of tissues within the leaf has a direct impact on plant water use and carbon uptake