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 ( w e)—primarily influences thermoregulation and hydraulic efficiency. However, comprehensive empirical tests of both hypotheses are lacking. Oxytropis diversifolia (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 found that the production of more leaflets did reduce w e. Leaves with reduced w e could stay cooler during the day, but the extent of leaf-to-air temperature difference was typically small (often within 1℃); to some extents, the intermediate 1–3-leaflet phenotype exhibited superior gas exchange. Leaf anatomy showed a benefit-cost: reduced w e correlated with a decreased mesophyll-to-mid-vein ratio and increased vein density, whereas vascular dimensions were also significantly decreased. We propose that the functional significance of leaf shape lies in water relations over thermoregulation. The trade-off in water balance could probably lead to balancing selection maintaining leaf-shape variation.