Spatial stability of photosynthetic and optical traits in common bean leaves under moderate drought revealed by...

Leaves are structurally and functionally heterogeneous organs, and their photosynthetic performance often varies across the lamina. Environmental stress, particularly drought, is often assumed to increase this heterogeneity by disrupting stomatal behaviour, mesophyll function, and pigment content. However, the extent to which moderate water deficit alters the spatial organisation of gas exchange, photochemistry, and spectral traits within a single leaf remains unclear. This study investigated within-leaf variability in Phaseolus vulgaris under well-watered and moderate drought conditions by combining gas exchange with chlorophyll fluorescence and multispectral imaging. The aim was to determine whether drought amplifies or reduces intrinsic spatial patterns and to assess whether imaging-derived parameters reflect micro-scale variation in stomatal conductance.

Control leaves showed subtle but consistent spatial gradients, including slightly lower photosynthetic activity, photochemical efficiency, and pigment-related indices at the apical region compared with basal and middle segments. Stomatal traits also varied modestly along the lamina, reflecting developmental polarity. Drought substantially reduced net photosynthesis, stomatal conductance, relative electron transport rate, and vegetation indices, yet generally reduced positional differences across measured traits. Leaf relative water content declined without significant positional differences, indicating coherent hydration under stress. Chlorophyll fluorescence and multispectral imaging demonstrated strong reductions in trait magnitude under drought but only weak spatial divergence, with widespread increases in non-photochemical quenching and pigment-related indices. Canonical discriminant analysis clearly separated leaf regions in control plants but showed reduced discrimination under drought, confirming a loss of spatial differentiation. Partial least squares regression revealed strong predictive links between imaging traits and stomatal conductance under control conditions but poor prediction under drought, indicating reduced predictive performance once stomatal conductance becomes strongly constrained.

Moderate drought did not intensify within-leaf heterogeneity. Instead, it promoted a higher degree of spatially coherent down-regulation of photosynthesis, photochemistry, and optical properties. These findings suggest that common bean maintains coordinated stomatal, hydraulic, and photoprotective responses across the lamina under moderate water deficit, preserving functional integrity despite reduced assimilation. The results underscore the importance of accounting for spatial stability when interpreting drought responses and highlight the utility of imaging-based phenotyping for assessing coordinated leaf-level acclimation.