COS 105-5 - Evolutionary adaptation of leaf traits in Cycads revealed decoupling strategies to adapt to contrasting climate conditions.

Abstract: Although there is strong evidence of a universal constraining in leaf physical and chemical trait syndromes reflecting a spectrum in functioning, we still remain ignorant about the evolutionary processes involved in the diversification of foliar organs across plants. Moreover, leaf traits can vary in correlation with other factors such as ancestry or changes in growth habits, confounding the effect of evolutionary adaptations in response to environmental conditions. To address this limitation, we studied the trait variation of leaves in Cycadales, an ancient group of closely related plants that have maintained the same growth habit for millions of years. We collected data on foliar area, leaf mass area, thickness, leaf density, toughness, and flavonoid concentration as well as climatic information from 80 Cycads representing the entire global distribution of the order. Our results indicated that leaf traits in Cycadales followed a strong tradeoff between LMA, leaf area, and leaf density, but this tradeoff was independent from leaf thickness. Moreover, hinokiflavone content, a compound associated with herbivore protection, aligned positively with leaf moisture content. In contrast, amentoflavone, a compound associated with protection against UV radiation and light stress, aligned positively with leaf thickness. The physical and chemical traits were associated with precipitation and temperature variables. Amentoflavone content and thickness related negatively to the contrast in temperatures between seasons with respect to mean daily temperatures, whereas hinokiflavone and foliar areas, were negatively associated with the annual temperature range. Our results suggest that the climatic adaptation of leaves in this ancient group required a decoupling between traits associated with tissue investment and leaf thickness. Adaptation to more humid, less seasonal areas likely required increased chemical protection against herbivores, whereas species in areas with more seasonal climates were selected for thicker leaves better protected against excessive radiation.