The Functional Significance of Leaf Shape Lies in Water Relations Over Thermoregulation: A Case Study With an Arid Legume Herb

Leaf shape displays remarkable diversity, with its evolution hypothesized to reflect adaptive ecophysiological functions. Theoretical models propose that variation in leaf shape-particularly through modifications in effective leaf width (we)-primarily influences thermoregulation and hydraulic efficiency. However, comprehensive empirical tests of these hypotheses are lacking. Oxytropis diversifolia E. Peter (Fabaceae) has natural variation in leaf shape (1 leaflet, 1-3 leaflets, and 3 leaflets) and exhibits clinal variation, making it an ideal candidate to test those functional relationships. Here, we quantified leaf morphometrics across populations, logged in situ leaf temperature and gas exchange, and examined leaf anatomy associated with water balance. We confirmed that the production of more leaflets did reduce we. While leaves with reduced we could stay cooler during the day, the extent of leaf-to-air temperature difference was typically small (often within 1°C), suggesting a limited biological impact. Crucially, we identified a key anatomical trade-off in water relations: reduced we yielded beneficial lower chlorenchyma-to-midrib ratios and higher vein density, but at the cost of smaller vascular dimensions. This trade-off likely underpins the observed, context-dependent superior gas exchange of the intermediate phenotype. We propose that the functional significance of leaf shape lies in water relations over thermoregulation, with balancing selection on the anatomical trade-off providing a plausible mechanism maintaining the polymorphism.