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Xeromorphism is a set of structural traits facilitating plant functioning under the conditions of water deficit. While leaf xeromorphic traits have been widely described in a variety of species, much less is known about the ability of plants to acclimate to drought by plastically modifying their leaf structure. We hypothesized that such modification would recapitulate the xeromophic syndrome.

We conducted a greenhouse experiment in which a genetically and phenotypically diverse population of barley (Hordeum vulgare L.) cultivars and hybrid lines were subjected to controlled drought followed by re watering. Leaves produced after the onset of drought were sampled for determination of lamina length and 18 anatomical traits of adaxial epidermis and central transverse lamina sections.

Across all studied genotypes, mean leaf length, width, and dimensions of all internal structures were reduced by drought, whereas the density of veins, stomata, and trichomes increased. However, since anatomical traits were commonly correlated with leaf length and following drought the leaves became shorter, we statistically controlled for the leaf size-related variation. Although leaf shortening explained much anatomical response to drought, several traits exhibited additional, leaf-length independent modification. For example, vein and stomatal density increased and xylem area and vessel diameter decreased to a greater extent than expected from leaf size reduction.

These results confirm the occurrence of modest facultative xeromorphism in barley but also emphasize the need to use allometric analysis to uncouple the true plasticity of traits from changes attributable to altered plant and organ size.