LIFE-HISTORY VARIATION IN THE ANNUAL ARABLE WEED DIPLOTAXIS ERUCOIDES(CRUCIFERAE)

Variation in life-history traits such as emergence, survival, time of flowering, and fecundity were studied in Diplotaxis ercoides, a medite rranean winter annual weed, by analyzing cohorts that emerged in autum n, early spring, and spring. The response of the plants to the environ ment, as reflected by plant architecture and pattern of biomass alloca tion, was also studied. Seedlings that germinate in autumn produced fr om 3 to 10 times more seeds than those that germinated in spring. The main factor affecting the number of seeds produced appears to be the l ife-span. Reduction of the growing period led to a decrease in both nu mber and length of modular units, which resulted in decreased numbers of leaves, flowers, and fruits of each module. In semelparous D. eruco ides plants, differences in the pattern of biomass allocation to repro duction are related to plant size. Our field data indicate that an inc rease of reproductive effort with size occurs in small individuals; ho wever, a decrease occurs for vegetative biomass greater than 2 g and l ess than 5 g. Little variation in reproductive effort occurs when vege tative biomass is greater than 5 g. From a strategic point of view, si ze-dependent variation of reproductive effort in D. erucoides can be i nterpreted as good tactics to favour a higher proportion of resources devoted to reproduction in small individuals, thus ensuring some offsp ring. However, structural, developmental, and physiological constraint s lead to stabilization or even a decrease in reproductive effort abov e a certain threshold size, when production of offspring is already en sured.