Phenotypic Variation of Cold Stress Resistance-Related Traits of White Clover Populations Naturalized in Patagonian Cold Environments

White clover (Trifolium repens L.) is the main forage legume species in grazing pastures in central, central-southern, and southern regions of Chile and many other temperate regions of the world. The selection of cold stress resistance genotypes is an important step for breeding cultivars to improve the clover growth curve and extend the grazing season in early spring and autumn. The objective of this work was to phenotypically characterize a population of 192 white clover genotypes selected for cold stress resistance under three contrasting regional winter-cold conditions (mean daily minimum air temperatures of 2.5, 1.6, and -1.4 degrees C) during the coldest month. An a-lattice experimental field design was used, with 24 incomplete blocks and two replicates. Morphological, physiological and biochemical traits were evaluated at different dates over three growing seasons from 2013 to 2016. A high degree of variability was found among the genotypes for all the characteristics studied. There was no effect of environment when the experimental sites were considered as a source of variation, but comparison of measurements taken at dates separated by a winter showed that there was a decrease in stolon length and diameter, petiole length, leaflet size, and dry matter accumulation in response to cold conditions. The normalized difference and medium resolution imaging spectrometer terrestrial chlorophyll vegetation indices (NDVI and MTCI, respectively) and water-soluble carbohydrate concentrations in stolons also varied among genotypes and were affected by winter cold. The high degree of phenotypic variability allows for the use of this population as a genetic source for cold stress resistance in white clover.