Effects of parental age on salt stress tolerance in an aquatic plant

Parental age influences components of offspring fitness in many species. The ability to tolerate stress also affects fitness, but less is known regarding changes in offspring stress tolerance with increasing parental age, especially in plants. We examined first and fifth-born clonal offspring (using birth order as a proxy for parental age), and compared their fitness in several sub-lethal concentrations of salt (NaCl), to investigate the interactive effects of birth order and salt stress on the offspring of the aquatic plant Lemna minor L. We found that increasing salt concentration reduced reproduction particularly at early ages, which detrimentally affected fitness, as measured by the intrinsic rate of natural increase. Fifth offspring had greater fitness than first offspring, potentially due to the hump-shaped relationship between offspring fitness and birth order observed in other studies on Lemna, with fifth offspring near the peak of the hump. We found no interactive effect of birth order and salt concentration on offspring fitness; however, there were interactive effects on the time to first reproduction and the size of fronds. Specifically, first offspring exposed to increasing salt concentrations exhibited longer delays to first reproduction and grew to a greater size, while fifth offspring showed little change in either variable with increasing salt concentration. Thus, variation in birth order affected offspring response to salt stress, although not in terms of fitness. These results help illuminate factors impacting the age-specific strength of natural selection and stress responses, and may be environmentally relevant in the context of environmental salinization.