Selenium accumulation in floral tissues of Brassica juncea: Implications for pollination ecology

Background/Question/Methods Selenium (Se) contamination from agricultural irrigation and runoff has caused the buildup of toxic levels of selenium (Se⁺⁶) in water and soil worldwide, particularly in the central valley of California. In these contaminated areas, insect pollination can be critical to the functioning of both agricultural and natural ecosystems, yet we know very little about how pollutants such as Se impact pollination ecology. The overall goal of this study is to investigate the possible effects of Se on the pollination ecology of Se-accumulating and phytoremediating plants. We tested the hypothesis that ecologically relevant concentrations of Se will accumulate in the pollen of a phytoremediating plant species and negatively affect floral traits and plant fitness. Brassica juncea (L.) Czern (Indian mustard) is a Se accumulator species that is used for phytoremediation of Se contaminated soils and is foraged upon by honeybees as well as other pollinators. Brassica juncea plants were grown in the greenhouse using an irrigation tank system and were watered with concentrations of 1.5 ppm and 3 ppm sodium selenate (NaSeO₄). These concentrations are commonly found in polluted agricultural waters of the San Joaquin Valley in CA as well as in naturally seleniferous soils, making the Se treatments ecologically relevant for both agricultural and natural ecosystems. Plants irrigated with 0 mg/mL NaSeO₄ served as controls.

Results/Conclusions  Plants irrigated with 1.5 and 3 ppm NaSeO₄ produced pollen that contained 65.67 ± 16.42 and 669.40 ± 45.35 ug Se/g dry weight (n = 10 and 8, respectively). Control plants produced pollen with only trace amounts of Se. Se treatment significantly reduced floral display width (F _2,55 = 22.58, P < 0.0001), anther length (ANOVA, F _2,55 = 24.34, P < 0.0001), petal length (F _2,55 = 6.43, P = 0.0031), and petal width (F _2,55 = 21.91, P < 0.0001). Se uptake by non-hyperaccumulator plants (such as B. juncea) may negatively alter floral traits, rendering them unattractive to pollinators or even reducing pollen deposition and reproduction. Se accumulation in leaf tissues reduces the fitness of several types of herbivores, but there are currently no studies to date examining the effects of floral tissues containing Se on pollinator fitness. If pollinators cannot detect and avoid toxic Se compounds in the plant tissues they are foraging upon and collecting for their progeny, they may suffer similar adverse effects to those reported in other insect guilds.