CARBON INTEGRATION IN PLANTAGO-ARISTATA (PLANTAGINAICEAE) - THE REPRODUCTIVE EFFECTS OF DEFOLIATION

Patterns of carbon integration in aclonal species are poorly understoo d in spite of their potential to influence individual fitness. To prov ide more information about these patterns, we performed a defoliation experiment with P. aristata. We examined, at the metameric level, the reproductive responses to the removal of the major carbon sources with in metamers. Bracts on marked reproductive spikes and leaves subtendin g these spikes were removed at three stages of reproductive maturity: spike elongation, flowering, and fruiting. Spike dry weight and length , capsule number, seeds per capsule, and seed weight were measured. We tested the hypothesis that seed weight would respond least to defolia tion. We also performed a complementary C-14 translocation experiment to measure the amount of radioactive carbon moving into the marked spi kes from outside the metamer. Defoliation depressed all components of reproduction within marked spikes, and little C-14 was translocated fr om outside the metamer into the reproductive spikes, even those that w ere defoliated. Both results support the view that reproductive metame rs in this species are largely autonomous with respect to their carbon budget. Defoliation during spike elongation most depressed reproducti on, and bract removal depressed reproduction more than did leaf remova l. The data suggest that bracts compensate for leaf removal by increas ing their photosynthetic rate; however, the ability to compensate diff ers among plant populations. Of all the reproductive components, seed weight was least affected by defoliation. The data show, however, that the time of defoliation relative to reproductive development influenc es which reproductive components are affected.