Water relations, gas exchange, leaf anatomy and thermographic analysis of two forage species under warming and drought stress at field conditions

Abstract

The Industrial Revolution was a landmark in the development of new technologies and of access to new sources of energy. However, from that period was initiated an intense carbon transfer to the atmosphere through the burning of fossil fuels, causing an increase in atmospheric CO2 concentration ([CO2]). According to the Intergovernmental Panel on Climate Change, the increase of [CO2], and the emissions of other greenhouse gases, will cause an average increase in surface temperature from 2 °C to 5 °C until 2100. According to the climate scenario RCP6, it is expected that the development of new technologies, international policies and emissions between 650-750 GtCO2/year will maintain an average increase of 2 °C in the average temperature of the Earth by 2100. With the increase in temperature, it is expected that extreme weather events will become more frequent, leading to increased water stress situations and intensity, frequency and duration of heat waves. Water stress and high temperatures are the main environmental factors that limit the plant survival and productivity of cultivated species. Thus, it is crucial to consider the effects of these climatic changes in human activities such as agriculture and livestock. Brazil has the largest commercial herd in the world. It is estimated that most of the animals are fed exclusively by pastures, which are subjected to daily and seasonal conditions of drought stress because they are rarely irrigated. Forage species such as Panicum maximum Jacq. cv. Mombaça (Poaceae) (C4) and Stylosanthes capitata Vogel. (Fabaceae) (C3) are widely used as pasture in the Brazilian agricultural production. In this scenario, the present study aims to evaluate in an integrated approach the isolated and combined effects of soil water deficit and warming (+2 °C) on water relations, gas exchange, leaf anatomy and leaf temperature of P. maximum and S. capitata. Evaluations will be performed under field conditions using the T-FACE system (Temperature Free-Air Controlled Enhancement). The water relations will be assessed by measurements of stomatal conductance, transpiration rate, net photosynthesis rate, water use efficiency, water potential, relative water content, free proline content and leaf area index. The anatomical responses will be evaluated using optical microscopy, confocal electron microscopy and transmission electron microscopy. It will be evaluated stomata density, stomata index and stomatal size, thickness and size of the different leaf tissues and its cells, the size and number of chloroplasts and the ultrastructure of organelles and subcellular membranes. The leaf and canopy temperature will be determined using thermographic analysis by a thermal camera and micro-climatic variables such as temperature and humidity and soil water content will be monitored using sensors. It will be tested the hypothesis that the increase in temperature under adequate soil moisture conditions will have a beneficial effect on the growth of both species, however the combined effects of water deficiency and high temperature will be harmful for the growth of both forage species under study.