Limits to adaptive plasticity: temperature and photoperiod influence shade-avoidance responses

In plants, the ratio of red to far-red wavelengths (R:FR) reliably indicates neighbor proximity and influences stem elongation. Enhanced elongation increases light interception and fitness under crowded conditions. However, many environmental factors vary simultaneously such that responses to R:FR may be affected by abiotic conditions or maternal environmental conditions. This study examines the effects of temperature, photoperiod, and maternal environment on stem-elongation responses to R:FR. Four populations of Abutilon theophrasti (two from disturbed, weedy areas and two from cornfields) were used in factorial common-garden experiments of temperature x R:FR x population and photoperiod x R:FR x population. Seedling growth of greenhouse- and field-derived seed was compared to evaluate maternal effects. Maternal environment did not alter seedling elongation. Higher temperatures resulted in both a twofold increase in average elongation and increased responsiveness to R:FR. Significant three-way interactions in both experiments demonstrate that population responses to R:FR differ depending on temperature and photoperiod conditions. These results indicate that elongation responses to R:FR are more variable than previously realized. The observed variability in elongation also suggests that the outcome of competitive interactions in the natural environment will depend on ambient temperature, photoperiod length, and population origin.