Premise of study

Analyses of the variation in the abortion rate and weight of seeds of tropical trees are scarce, despite their importance in modulating an important factor (seed production) and maintaining the functioning of tropical ecosystems. The different factors that affect the survival and development of seeds in tropical trees are largely unknown.

Methods

We studied the patterns of intrafruit ovule and seed loss and seed weight in fruits of the Guanacaste tree, Enterolobium cyclocarpum, in samples of trees from 5 populations in Mexico and Costa Rica. We analyzed the effects of developmental phase, fruit and maternal tree effects, and seed position within the fruit on the abortion rate and weight of seeds from mature and immature fruits. Naturally, pollinated flowers were studied to determine the degree of ovule fertilization within the ovaries.

Key results

Overdispersion of the abortion rate per fruit within trees and a higher frequency of abortions in the extreme positions of the fruit are common patterns in all populations. This pattern is observed from the beginning of fruit formation. Ovules in the stylar position do not present higher survival rates, despite their precedence in fertilization. A negative relationship was found between the number of abortion events and seed weight. The greatest sources of variation in abortion and seed weight were observed within trees.

Conclusions

Fruit-specific fertilization and developmental conditions seem to be the main factors driving the probability of abortion and the seed weight within Guanacaste fruits, causing high variability in seed survival and vigor within maternal tress. In contrast to the findings of other studies in legumes, the fertilization of ovules is not important for seed development. The correlation between the fruit abortion rate and seed weight may be related to the sink capacity of the fruit, which affects the development of all the seeds in the fruit.

Five tree populations were sampled: three in Mexico (San Lucas (SL), Chamela-Cuixmala (CEC) and Veracruz (VER)) and two in Costa Rica, Central America (El Rodeo (ROD) and Santa Rosa (SR)). During 2021 we collected fruits from the ground under the tree canopy during the fruiting season (February–May) at the mentioned locations. An average of 10 trees were sampled at each location, with 10 fruits collected per tree. The fruits were taken to the laboratory, where they were opened, the seeds were individually weighed, and their positions in the fruit were recorded. The positions of the aborted and mature seeds were noted. An abortion event was recorded when the locule was empty or contained a small, deformed seed (wrinkled at the edges and very flat); most aborted seeds weighed less than 0.2 g, although some slightly heavier seeds were classified as abortions due to their shape. Normal fill seeds and abortion events were classified by their position in the fruit into three categories: stylar position (closest to the style position), basal (closest to the flower pedicel) or intermediate (any position between these two extreme positions). The total number of locules (empty and with abortions) and the total number of filled seeds per fruit were counted.  Mature seeds were weighed using an 1 mg electronic precision balance (Ohaus).