Cannabis sativa genotypes with larger leaf areas have higher potential to adjust stomatal size and density in response to water deficit: The effect on stomatal conductance and physiological stomatal behaviour

Abstract

Effective stomatal control is fundamental to successful plant responses to water deficit stress. Control of stomatal conductance (G_s) can be exerted through modification of stomatal morphology (size and density) in newly developing leaves, or physiological adjustment of stomatal pore aperture. We investigated the potential coordination of stomatal morphological and physiological responses to water deficit in three varieties of hemp (Cannabis sativa L.) grown under field conditions. The three hemp varieties had contrasting leaf areas under well-watered irrigation (control): Earlina 8FC < Fedora 17 < Fibror 79. Stomatal size, density and the speed of physiological adjustment of G_s were not significantly different under full irrigation. Under water deficit conditions the leaf areas of the two varieties with the largest leaves decreased to match those of Earlina 8FC. This reduction in leaf area, alongside an increase in stomatal initiation, resulted in higher densities of smaller stomata. This effect was most evident in Fibror 79 that showed the largest reduction in leaf area, increase in stomatal density and decrease in stomatal size. This corresponded to the most rapid physiological adjustment of G_s to fluctuations in photosynthetic photon flux density of the hemp varieties occurring in Fibror 79 when subject to water deficit. The coincidence of the fastest velocity of G_s adjustment with the highest densities of small stomata may support interpretations of a functional advantage of high densities of small stomata in the physiological regulation of G_s to fluctuating conditions. The larger leaf area of Fibror 79 appeared to be associated with higher capacity to respond to water deficit through modification of stomatal morphology and physiological behaviour. This result indicates that phenotyping of crop species and genotypes to identify traits conducive to water deficit tolerance through effective stomatal control should consider the foliar plasticity of genotypes to water deficit, and the potential implications for stomatal morphological and physiological control of transpirative water loss and photosynthetic CO₂-uptake.