Seed Germination and Early Seedling Growth Responses to Drought Stress in Annual Medicago L. and Trifolium L. Forages

Abstract

Climate change in the Mediterranean-like regions of South Africa has resulted in increased rainfall variability, a delayed start to the wet winter season, and increased occurrences of out-of-season summer rainfall events. These changes in bioclimatic conditions are predicted to become more pronounced and, therefore, could result in poor seedling establishment and false breaks from the soil seed bank, followed by seedling desiccation in annual medic and clover pastures. This study therefore aimed to quantify and compare the responses of three annual Medicago species and five annual Trifolium species to drought stress imposed at the seed germination, seedling establishment, and early seedling growth stages. Three separate controlled trials were conducted. Firstly, the seeds were germinated in seed germination chambers at constant temperatures ranging from 5 to 30 °C, in 5 °C increments, with five osmotic treatments within each temperature, and germination was recorded daily. For the second experiment, seeds were planted at 100, 70, 50, and 30% of the soil’s moisture-holding capacity without subsequent watering, and emergence was recorded daily. For the third experiment, one-month-old seedlings were subjected to two water-limitation periods (15 or 30 days), and their subsequent morphological responses were measured. The results from these experiments indicated that the species differed significantly in their responses to drought, and the best-performing species often differed when drought was imposed at different development stages. Five species—M. polymorpha L., M. truncatula Gaertn., T. alexandrinum L., T. vesiculosum Savi., and T. subterraneum L. ssp. subterraneum—were able to tolerate incidences of drought better than other species and, thus, should be prioritized for further research into the variation in drought tolerance between cultivars within these species.