Fruit Set, Herbivory, Fruit Reduction, and the Fruiting Strategy of Catalpa Speciosa (Bignoniaceae)

Abstract

Catalpa speciosa regularly initiates more fruits than it matures. Most of the fruits that fail to mature are abscised from 2—5 wk after the flowering period. Catalpa speciosa has a host—specific herbivore, Ceratomia cactic (Sphingidae), which severely defoliates some branches while causing little or no damage to leaves of other branches. The pattern of fruit abscission was examined in relation to the principal herbivore. Field observations and experiments show: (1) the larger the infructescence, the higher the probability that a given fruit will abort; (2) branches that have experienced simulated herbivory have significantly more abortions than control branches;(3) if one fruit is aborted from an infructescence, the remaining fruits have a higher probability of maturing; (4) fruits accumulate <10% of their mature dry weight and total protein during the 1st 5 wk of growth and the remaining 90% in the next 4 wk (after the period of abortion). These data suggest that (1) each branch supplies the energy necessary to mature the fruits that it hears, (2) environmental stress, especially herbivory, reduces the resources available for fruit production, (3) fruit abortion is a response to limited resources, and (4) the pattern of fruit growth reduces the "cost" of abortion. The overall pattern of fruit reduction is interpreted as an adaptive trade—off between seed number and seed quality which permits the parent plant to match fruit production with available resources.