Future warming will change the chemical composition and leaf blade structure of tropical C3 and C4 forage species depending on soil moisture levels

Abstract

Temperature and soil moisture strongly affect the nutritional value and digestibility of forage plants through changes in leaf chemical composition or the proportion of leaf blade tissues. In this study, we aimed to evaluate leaf blade anatomical modifications of two tropical forage species, Stylosanthes capitata (C3) and Megathyrsus maximus (C4) under warmed conditions (+2 °C) at well-watered and rainfed conditions and investigate the interactions between leaf anatomical alterations, leaf chemical composition, and leaf digestibility. Experiments were conducted under field conditions using a Temperature-free air-controlled enhancement (T-FACE) system. We observed that plants under elevated temperature produced leaves with smaller stomata and thinner mesophyll tissue and reduced total leaf thickness, potentially impacting gas exchange. On the other hand, reduced soil moisture increased stomatal density and thickness of the adaxial epidermis. In both species, leaf fibrous fractions concentration increased under warmed and non-irrigated conditions, while crude protein concentration and digestibility decreased. However, leaf digestibility was associated with leaf chemical composition rather than the proportion of different leaf blade tissues. We concluded that although both species developed leaf anatomical modifications to acclimate under future warming conditions, leaf nutritional value and digestibility will be reduced, potentially impacting future livestock production and methane emissions by ruminants.