Both freezing and drought stress cause cellular dehydration in plants, and there are similar increases in protective compounds, including various soluble sugars, amino acids and plant hormones. These responses suggest that these stresses could potentially interact, with exposure to the first stress increasing the tolerance to the second stress, in a phenomenon referred to as cross-acclimation. Plants previously exposed to drought stress had higher survival and growth after freezing stress, providing support for cross-acclimation. However, the reciprocal effects of freezing stress on subsequent drought tolerance have not been explored in the context of growth and survival. We froze Poa pratensis tillers in the late fall, early spring or late spring in controlled environment chambers at 0, -5 or -10 for 3 d, and then subjected them to no drought (-0.025 MPa), moderate drought (-0.140 MPa) or severe drought (-0.250 MPa) for 3 weeks in the summer. We assessed survival and total dry biomass after the 3 week post-drought period, and we determined soluble sugar concentrations (glucose, fructose and sucrose) in leaves before and 0 d, 30 d and 55 d after freezing.
Results/Conclusions
For both survival and biomass there were significant interactions between freezing and drought stress, with freezing in the fall and spring increasing summer drought tolerance. Specifically, under severe drought, survival was highest for spring (83%) and fall (73%) frozen tillers and was lowest for tillers that previously experienced low freeze stress (39%). For tillers that survived the severe drought treatment, spring frozen tillers had significantly higher dry total biomass (332 mg ± 50 mg) than fall (195 mg ± 40 mg) and low freeze stress (182 mg ± 30.2 mg) tillers. These responses were not associated with increased soluble sugar content; freezing effects on leaf sugar concentrations were absent 55 d post-freezing. Our results demonstrate that multiple stresses that occur over different seasons can interact, and highlights the importance of examining plants in the context of the annual cycle when assessing stress responses. These interactions are highly relevant to herbaceous species in northern temperate regions that are experiencing more intense and frequent stress as a result of changes in snow cover and extreme climatic events.