The role of idioblasts in leaf water relations of tropical Rhododendron

Although much is known about the anatomy of idioblasts, the relationship of leaf idioblasts to leaf physiology is poorly known. Our goal was to understand the relationship between idioblast abundance and leaf water relations. Tropical epiphytic and shrub Rhododendron species were the model system.

Leaf succulence and idioblast metrics of 61 plants representing 17 species were compared with stomatal and water relation metrics. Correlation, ANOVA, and regression were used to understand which water relation traits were best aligned with the proportion of the leaf occupied by idioblasts for epiphytic and shrub species.

Idioblast volume per leaf area varied from 1.4-9.5 mm³·cm^-2 among accessions, and an index of stomatal area per leaf area varied from 0.08-3.3. Succulence, stomatal, and water relation metrics varied significantly among species. Total idioblast volume was negatively correlated with leaf succulence metrics, and positively correlated with relative water content at the turgor loss point. Idioblasts were better related to capacitance in thin leaves. All idioblast metrics were significantly greater in epiphytic species.

Leaf idioblasts can have a significant effect on leaf-lamina water relations. Idioblasts likely function as a water buffering system for thin leaves in tropical Rhododendron species. The association of idioblast prominence with leaf water buffering is greatest for epiphytic species with thin leaves. Thus, the proliferation of leaf idioblasts may have been an important innovation for Rhododendron species diversification into tropical epiphytic habitats.