Effect of long-term heat stress on structure and function of epidermal tissues in needles of treeline conifer seedlings.

Tree seedlings at the treeline face various environmental extremes; in particular, their needles can heat up close to thermal limits. Unlike mature-tree needles, treeline conifer seedlings resist heat-induced water loss better, even though their cuticle lacks an outer layer. To determine whether these differences are due to ontogenetic factors or acclimative responses to higher heat loads, conifer seedlings were exposed to a controlled in situ long-term heat treatment. By comparing the rate of water loss through the cuticle (minimum diffusive conductance, gmin), cuticle thickness and chemical micro-composition of control versus heated seedlings of deciduous Larix decidua Mill. and evergreen Picea abies (L.), we intended to assess the acclimative plasticity of these traits. We also aimed to identify specific heat-induced changes in cuticle structure and function. The controlled, in situ, long-term heat treatment lasted for 6 weeks with air temperature set 10-15 K above the ambient air (controls) with a set maximum leaf temperature of 42 °C. This heat treatment significantly reduced gmin in L. decidua but it was not significant in P. abies. In both species, exposure to heat induced an increase in cuticle thickness, as well as provoking species-specific microchemical responses. Lignification of the middle lamella in L. decidua revealed a heat-acclimative potential that likely contributed to reduced cuticular water loss. In P. abies, despite an obvious increase in cuticle thickness, no significant acclimative change was seen in gmin. Flavonoids (kaempferol) were detected throughout the cuticle and outer cell wall of both species, but their contribution to reduced gmin is still unresolved. Our results demonstrate that cuticle and cell wall changes in conifer seedlings are species-specific and trait-dependent, and they highlight the role of epidermal chemistry in shaping resilience to climate warming at the alpine treeline.