RESUMO
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.
Assuntos
Dióxido de Carbono , Solo , Dióxido de Carbono/metabolismo , Fotossíntese , Folhas de Planta/metabolismo , Temperatura , Água/metabolismoRESUMO
Tropical pastures play a significant role in the global carbon cycle and are crucial for world livestock production. Despite its importance, there is a paucity of field studies that clarify how tropical pasture species will be affected by environmental changes predicted for tropical regions. Using a temperature-free air-controlled enhancement (T-FACE) system, we increased canopy temperature (+2 °C over ambient) and evaluated the effects of warming under two soil moisture conditions in a factorial design over the physiology, forage production, and forage quality of a tropical forage legume, Stylosanthes capitata. Under well-watered conditions, warming increased the PSII efficiency, net photosynthesis, and aboveground biomass accumulation, but reduced forage quality and digestibility by decreasing crude protein content and increasing lignin content. Non-irrigated conditions under ambient temperature reduced leaf water status presumably promoting the reduction in net photosynthesis, forage production, and forage quality and digestibility. Under the combination of canopy warming and non-irrigated conditions, warming mitigated the effects of reduced soil moisture on leaf photosynthesis and biomass production, but a significant interaction reduced forage quality and digestibility more than under isolated treatments of warming or non-irrigated conditions. We found a potential physiological acclimation of the tropical forage species to moderate warming when grown under rainfed or well-watered conditions. However, this acclimation was achieved due to a trade-off that reduced forage nutritional value and digestibility that may impact future animal feeding, livestock production, and would contribute to methane emissions.
Assuntos
Fabaceae , Solo , Aclimatação , Animais , Dióxido de Carbono , Fotossíntese , Folhas de Planta , ÁguaRESUMO
Global warming is predicted to cause more intense extreme events such as heat waves, flooding and severe droughts, producing significant effects on agriculture. In tropics, climate change will severely impact livestock production affecting water availability, forage quality and food for cattle. We investigated the isolated and combined effects of soil water deficit (wS) and + 2°C increase in canopy temperature (eT) on leaf gas exchange, chlorophyll fluorescence, carbohydrate content, forage quality and in vitro dry matter digestibility (IVDMD) of a field-grown C4 tropical forage grass Panicum maximum Jacq. using a temperature-free air-controlled enhancement (T-FACE) system. The wS and eT treatments showed no effects on photosystem II photochemistry. However, wS under ambient temperature decreased net photosynthesis rate (A), stomatal conductance (gs ) and maximum rate of carboxylation of Rubisco (Vcmax ), leading to a reduced starch content in leaves. A 16% reduction in leaf dry mass (LDM) and reduction in forage quality by increasing fibers, reducing crude protein (CP) and decreasing the IVDMD was also observed by effect of wS. Warming under adequate soil moisture (eT) significantly increased LDM by 25% but reduced the forage quality, increasing the lignin content and reducing starch, CP and digestibility. The combined wSeT treatment reduced A, gs , Vcmax and the forage quality. When compared to control, the lignin content in leaves increased by 43, 28 and 17% in wS, eT and wSeT, respectively, causing a significant reduction in IVDMD. We concluded that despite physiological mechanisms to acclimate to warming, both warming and water deficit will impair the quality and digestibility of C4 tropical pastures.