Sexual dimorphism in response to herbivory and competition in the dioecious herb Spinacia oleracea

Sexual dimorphism is common in dioecious plant species and is usually attributed to different cost of reproduction associated with male and female functions. Differences in growth and performance between male and female plants may be accentuated under stress, potentially leading to sex-ratio biases and affecting population growth. Environmental stress involves multiple factors that often occur simultaneously. Among different stress combinations that occur in field conditions, competition and herbivory and their interaction are key biotic factors that can affect plant growth and performance. Here, we conducted a glasshouse experiment in Cardiff, UK, using the cultivated spinach, Spinacia oleracea, as a model system to study sexual dimorphism in intra- and inter-specific competitive ability and in response to herbivory by a generalist herbivore Helix aspersa. We found stronger inter- than intra-specific competition: growth (above-ground biomass) and chlorophyll content of male and female plants was reduced when growing with Brassica oleracea, but not when growing with conspecifics. In the absence of herbivory, females growing with same-sex neighbour had greater root biomass than males; whilst herbivory reduced root biomass significantly only in females competing with same sex neighbours. Plant damage caused by herbivores was similar when growing with male or female conspecifics but greater when growing with B. oleracea. Finally, plant damage caused by herbivores did not differ between male and female plants; however, males increased their allocation to roots and reduced their chlorophyll content after damage. Our results showed that sexual dimorphism occurs in S. oleracea, despite being a worldwide crop, selectively bred for its edible leaves. In particular, our results suggest stronger same-sex competition for females and greater tolerance to herbivory in males than in females of S. oleracea.