Abiotic stress can directly limit plant growth, and plants modify resource allocation to defense and reproduction accordingly.  Few studies, however, have addressed the potential for delayed, “latent” responses to stress signals.  Delayed plasticity based on earlier stress signals may allow a plant to respond to expectations regarding environmental quality – by preemptively prioritizing defense over growth, for example.  We investigated allocation patterns over the entire life cycle of rapid-cycling Brassica rapa after transient stress applied in early development.  Germinating seedlings were subjected to one of four abiotic stresses: either 5 days of sodium chloride addition to nutrient solution (salinity stress), 3 days of dried-medium conditions after germination (drought stress), 5 days of deionized water irrigation (nutrient stress), or 4 days of buffered nutrient solution irrigation at a pH of 4.2 (acid stress).  After these stress periods, stressed plants and unstressed controls were allowed to grow under normal conditions.  Later differences in development (flowering time, plant height, total stem length) were measured, as were differences in reproduction and glucosinolate concentrations in true leaves (collected 18 days post-sowing) and seeds.

Results/Conclusions

Flowering time was significantly delayed in plants experiencing salinity, drought, or acid stress.  However, all stressed treatments were indistinguishable from controls by the end of their life cycles in terms of total stem length and seed count – measures of growth and reproduction respectively.  Stressed plants did show differences in allocation to glucosinolates.  Acid-treated plants showed approximately one-quarter higher concentrations of the major aliphatic glucosinolate, gluconapin, in leaves and seeds.  Plants that experienced nutrient deficiency, on the other hand, showed lower levels of gluconapin in their seeds.  Thus B. rapa changes later defense allocation based on particular stresses early in development, leading to a temporally flexible plastic response to environmental signals.  This delayed effect of abiotic stress suggests that long-term latent responses should be considered in future ecological and physiological research, especially in plants where responses to sustained environmental conditions are often plastic.