Experimental defoliation affects male but not female reproductive performance of the tropical monoecious plant Croton suberosus (Euphorbiaceae)

Abstract

Background and aims: Monoecious plants have the capacity to allocate resources separately to male and female functions more easily than hermaphrodites. This can be advantageous against environmental stresses such as leaf herbivory. However, studies showing effects of herbivory on male and female functions and on the interaction with the plant's pollinators are limited, particularly in tropical plants. Here, the effects of experimental defoliation were examined in the monoecious shrub Croton suberosus (Euphorbiaceae), a wasp-pollinated species from a Mexican tropical dry forest.

Methods: Three defoliation treatments were applied: 0 % (control), 25 % (low) or 75 % (high) of plant leaf area removed. Vegetative (production of new leaves) and reproductive (pistillate and staminate flower production, pollen viability, nectar production, fruit set, and seed set) performance variables, and the abundance and activity of floral visitors were examined.

Key results: Defoliated plants overcompensated for tissue loss by producing more new leaves than control plants. Production of staminate flowers gradually decreased with increasing defoliation and the floral sex ratio (staminate : pistillate flowers) was drastically reduced in high-defoliation plants. In contrast, female reproductive performance (pistillate flower production, fruit set and seed set) and pollinator visitation and abundance were not impacted by defoliation.

Conclusions: The asymmetrical effects of defoliation on male and female traits of C. suberosus may be due to the temporal and spatial flexibility in the allocation of resources deployed by monoecious plants. We posit that this helps to maintain the plant's pollination success in the face of leaf herbivory stress.

Fig. 1.

The effect of defoliation treatments on leaf production rate (A) and on pollen production per flower (B) in plants of Croton suberosus. Values are back-transformed means ± s.e. Bars with different letters are significantly different at P < 0·05.

Fig. 2.

Production of pistillate and staminate flowers per inflorescence in plants of Croton suberosus under the three defoliation treatments. Values are back-transformed means ± s.e. Different letters indicate significant differences between treatments at P < 0·05. All comparisons remain significant at the sequential Bonferroni-corrected alpha level (0·05/3 = 0·017).

Fig. 3.

Floral sex ratio (staminate/pistillate flowers) of plants of Croton suberosus from the three defoliation treatments. Plants were ordered by the relative value. Horizontal continuous and dashed lines represent the mean and median, respectively.

Fig. 4.

The effect of defoliation treatments on the quantity of sugar produced by pistillate and staminate flowers plants of Croton suberosus. Values are back-transformed means ± s.e. Bars with different letters are significantly different at P < 0·05.