Reproduction in an Orchid Can Be Resource-Limited over its Lifetime1

Abstract

Most orchids studied thus far show long-term resource adjustments to increases in fruit production within a flowering season, but none of these offers rewards to their potential pollinators. If nectar production is energetically expensive, then resources utilized to produce fruits and seeds may be even more limited in pollinator-rewarding orchids than in non-rewarding ones. Thus, resource adjustments may be more dramatic or entirely different in nectar producing plants. In this study, we performed artificial hand-pollinations for two consecutive flowering seasons in the nectar producing orchid Comparettia falcata, and tested whether or not fruit set, seed set, and seed viability were limited by the quantity of pollinations or by resources. In addition, we compared mechanisms of short-term (fruit abortion within seasons) and long-term consequences (percent change in leaf length and change in flower number per plant between seasons, probability of shoot and inflorescence production, and mortality) between hand-pollinated and unmanipulated plants. The relationships among plant traits related to vegetative size and reproduction also were examined. Hand-pollinations showed some negative effects. Fruit set was higher in hand-pollinated plants in the first season but was similar to the controls in the second. Seed set was significantly lower and abortions were higher than in unmanipulated plants. On the other hand, some of our measurements were unaffected by the hand-pollination treatment. Unexpectedly, there were no significant differences between groups in percent change in leaf length, change in flower number per plant between seasons, or the probability of shoot and inflorescence production. Although there was a strong correlation between leaf size and the number of flowers produced within a season, associations between leaf size and traits related to current or future reproduction were not consistent. Like other epiphytic orchids, pollination limitation occurred within a single season in C. falcata, but increases in fruit production also resulted in reduced lifetime fitness as estimated by a compounded fitness index. Contrary to all other epiphytic orchids studied, long-term adjustments to increased fruit production in C. falcata through reduction in future growth or flower and inflorescence production were either minor or lacking. Our results suggest that the nature of plant strategies associated with resource constraints during sexual reproduction may be dependent on whether or not plants have evolved traits that are costly.