The Adaptive Nature of Seed Germination in Nemophila Menziesii Aggr.

Abstract

Nemophila menziesii aggr, includes three closely—related winter annual plants found in many communities in cismontane California and Oregon. The germination response is adapted to the Mediterranean climate that characterizes the region, i.e., wet winters and dry summers. The seeds germinate in late fall or early winter and are inhibited from germinating during late spring, summer, and early fall by high temperatures, long photoperiods, and low available moisture. Temperature and photoinhibition are proportional to the length of the inhibitory period and, in the case of photoinhibition, to light intensity. In combination these factors interact synergistically to inhibit the germination response. Seeds of all ages are able to germinate but germination of young seed is inhibited at temperatures that do not affect mature seed. Seed maturation is a function of high summer temperatures and aging. Populations from more northern and/or higher—elevation communities require more water and lower temperatures to germinate than more southern and/or lowest—elevation communities. These adaptations reflect the greater precipitation and cooler temperatures of the former sites as compared to the latter sites. The ecological parameters and germination responses change gradually both latitudinally and elevationally; hence the adaptations are ecoclinal in nature.