Interactive Effects of Immediate and Ancestral Salt Stress on Fitness in Duckweed

Premise of research. Organisms that reproduce asexually must respond to abiotic stresses in their environment while contending with the potential disadvantage of the reduced ability for genetic variation through recombination. Common duckweed (Lemna minor) is an aquatic plant that reproduces predominantly asexually through the budding of ramets. As a freshwater plant, duckweed experiences stress from salt (e.g., NaCl), which detrimentally affects growth, photosynthesis, and cellular processes. We took a demographic approach to examine whether ancestral exposure to salt stress affects the ability of duckweed offspring to tolerate immediate exposure to the same stress.Methodology. We placed three consecutive clonal generations of duckweed in an environment with 2 g L-1 NaCl (nonlethal salt stress) or 0 g L-1 NaCl (control), followed by an additional zero to three generations in the control environment to vary the presence and schedule of ancestral stress. After these treatments, we used the offspring produced as focal plants for the experiment. Half were placed into the salt stress environment, and half were placed in control conditions; focal plants were tracked longitudinally.Pivotal results. Immediate stress decreased fitness as a result of a slowing of reproduction, suggesting a cost of stress tolerance. The effects of ancestral stress, and the interaction of immediate and ancestral stress, were more complex. Specifically, recent ancestral stress induced plants to produce offspring more quickly but with the potential cost of lowered offspring quality.Conclusions. Our results amplify findings that responses to, and fitness consequences of, current stressors can be contingent on past exposure.