Professor University of Lethbridge Lethbridge, Alberta, Canada
Background/Question/Methods
Parental age influences components of offspring fitness in many species. Older parents may produce offspring with decreased fitness, as shown by decreased offspring survival or reproduction, shorter lifespans, or lower population growth rates. The ability to tolerate stress also affects offspring fitness, but less is known regarding changes in offspring stress tolerance with increasing parental age, especially in plants. To investigate the interactive effects of birth order and salt stress on the offspring of the aquatic plant Lemna minor L., we examined first and fifth-born clonal offspring (using birth order as a proxy for parental age), and compared their fitness when exposed to 0, 1, 2, or 4 g/L of salt (NaCl) added to a standard growth medium. The intrinsic rate of natural increase was used as the primary measure of fitness, with the time to first reproduction, total offspring number, and lifespan examined as components of fitness. Additionally, the physiological measurements of Na+ tissue concentration, dry mass and frond surface area were examined for effects of birth order, salt concentration, or interactions of these.
Results/Conclusions
We found that increasing salt concentration detrimentally affected fitness, as measured by the intrinsic rate of natural increase, primarily due to reduced reproduction at early ages. 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 slowed time 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.
