Abstract

Climate change is one of the biggest challenges facing food production worldwide, particularly in Mediterranean climate regions where rainfall has been decreasing and temperature increasing. Alfalfa is a perennial forage crop that could play an important role in increasing the resilience and sustainability of rainfed production systems in these environments. In this study, we tested how a prolonged drought period of 5-6 months in a Mediterranean environment of central Chile affects the plant persistence and productivity of an alfalfa diversity panel of 69 landraces, cultivars, and diverse pre-breeding populations (hereafter accessions) from Kazakhstan, Azerbaijan, Spain, Australia, USA, and Chile, with putative drought tolerance. The objectives were i) to evaluate the phenotypic variability in plant survival and productivity among the alfalfa diversity panel, under rainfed and with supplemental irrigation, in a Mediterranean environment; and ii) to study the use of an NDVI sensor, and RGB and thermal cameras for field phenotyping and evaluation of canopy traits, such as leaf area index (LAI) and forage yield (FY). Results showed large phenotypic variability for plant survival after the three drought periods, ranging from 30 % to 76 % under rainfed conditions and from 48 % to 90 % with supplementary irrigation. The FY during winter was significantly different between cultivars, with more than 4 Mg ha-1 per year in some accessions. The average FY for three growing seasons (2018-2020) ranged between 2.26 and 10.80 Mg ha-1 under rainfed and 3.92-11.14 Mg ha-1 per year with supplementary irrigation. Among the most productive accessions under both water regimes were two populations of the Medicago arborea x sativa hybrid (AF3448 and AF3347), cultivars Genesis and Venus, and the landraces Aragon from Spain and Alta Sierra 2 from Chile. The normalized difference vegetation index (NDVI) showed exponential and positive relationships with the leaf area index (LAI) and FY. RGB images were obtained at 60 cm above the top of the canopy with a digital camera and at 30 m using a drone. The RGB indices "intensity", a* and greener area obtained from images taken at 60 cm above the top of the canopy, were highly correlated (r = 0.76 - 0.98; P 0.0001) with the NDVI and FY, under rainfed and with supplementary irrigation in 2019. The RGB indices "saturation", a* and u* obtained from a drone at 30 m, were also highly correlated (r > 0.73 - 0.94; P 0.0001) with the NDVI and FY of accessions growing under both water regimens in 2020. The canopy temperature (CT) obtained from thermal images from a drone was higher under the rainfed regime. The stress degree day (SDD), defined as the difference between canopy temperature (Tc) and air temperature (Ta) was negatively related to FY. Under the rainfed regimen, the population AF3348 and cultivars Genesis and Aragon exhibited the lowest SDD and the highest FY.