Changes in Sexual Expression as Result of Defoliation and Environment in a Monoecious Shrub in Mexico: Implications for Pollination

Abstract

Folivory may indirectly impact plant reproduction through changes in sexual expression (i.e., number or proportion of male and female flowers produced), which influence plant-pollinator interactions via changes in pollinator preference or efficiency. This study is an experimental evaluation of the effect that defoliation has on sex expression in the monoecious shrub Cnidoscolus aconitifolius, how such effect varies across sites, as well as how such changes indirectly affect pollinator visitation rates. The present study used three populations of C. aconitifolius, each one located in a different site in Yucatán (México): pasture, deciduous forest and subdeciduous medium height forest and three levels of defoliation: 50 percent, 100 percent, and a control (no damage). Results showed that defoliation reduced significantly the total number of male flowers produced in two of the sites. Defoliation did not impact female flower production or the proportion of female flowers produced. Finally, floral visit rates were not affected by defoliation via changes in sexual expression and neither by site or by the interaction site  defoliation. Findings showed that defoliation had an effect on sex expression in C. aconitifolius, although apparently this change did not affect the plant-pollinator interactions.Abstract in Spanish is available at http://www.blackwell-synery.com/loi/btp.

Spatially inconsistent direct and indirect effects of herbivory on floral traits and pollination success in a tropical shrub

“…However, the few studies that have examined such effects for monoecious species have found contrasting results. For instance, chronic levels of herbivory caused an increase in the investment in the male function through a greater production of male reproductive structures in Pinus edulis (Cobb et al 2002), while previous studies with C. aconitifolius showed that artificial defoliation caused an increase in the proportion of female flowers (Parra‐Tabla et al 2004, Arceo‐Gómez et al 2009), which is similar to what was found for Cucumis sativus (Thomson et al 2004). Results for C. aconitifolius in this study also showed contrasting effects of defoliation depending on the flower type and size variable measured.…”

“…A common indirect effect of herbivory on pollination success can occur through a reduction in flower production because floral display size is one of the most important signals which plants use to attract pollinators (Strauss and Irwin 2004). However, a recent study by Arceo‐Gómez et al (2009) with C. aconitifolius that was conducted at the same study sites used here, showed that a decrease in flower number and changes in the female‐to‐male flower ratio resulting from defoliation did not affect pollinator visit rates at any region, suggesting that defoliation does not have an indirect effect on pollination success through these variables. However, results presented here suggest that changes in pollination success may have been indirectly mediated by the effect of defoliation on floral traits (i.e.…”

“…Such limits to compensation may help explain the observed pattern for male and female flower size of C. aconitifolius because the degree of investment in one function or the other may depend on water and nutrient availability (Parra‐Tabla et al 2004). Moreover, a previous study with this species showed that its capacity to increase the proportion of female flowers in response to defoliation was consistent across regions but variable in magnitude (Arceo‐Gómez et al 2009). Such evidence suggests a tradeoff between male and female flower production for this species which is mediated by nutrient availability and folivory.…”

“…Folivory is most intense at the start of the rainy season, when leaf production is greatest (Parra‐Tabla et al 2004). Previous defoliation experiments have shown that artificial clipping increases trichome production on leaves and stems of C. aconitifolius (Abdala‐Roberts and Parra‐Tabla 2005), alters the number and proportion of male and female flowers, and reduces fruit production (Parra‐Tabla et al 2004, Arceo‐Gómez et al 2009).…”

Sexual stability in the nearly dioeciousPinus johannis(Pinaceae)

“…Biotic factors such as herbivory alter sexual expression in plants by modifying the relative allocation to male or female function (Obeso, 2002; Thomson et al, 2004). Herbivory reduces male reproductive traits, relative to female, in several species of angiosperms (Hendrix and Trapp, 1981; Lehtila and Strauss, 1999; Parra‐Tabla et al, 2004; Arceo‐Gomez et al, 2009) and in the gymnosperm Taxus canadensis (Allison, 1990). In other angiosperms (e.g., Dactylorhiza maculata ) and gymnosperms (e.g., Pinus ponderosa and P. edulis ), female reproduction is reduced by herbivory (Snyder, 1993; Vallius and Salonen, 2000; Cobb et al, 2002).…”