Same-sex neighbours grow better than opposite-sex neighbours across three plant species

Plants recognize their neighbours through above- and belowground signaling, adjusting their growth strategies to maximize individual performance and maintain species coexistence. Dioecious plants are common in nature, but it is unknown whether they can recognize the sex of neighbouring plants and how they affect their performance.

We analyzed the root-based sexual recognition mechanisms in the seedlings of three dioecious species with different life forms including Spinacia oleracea (herb), Siraitia grosvenorii (vine) and Diospyros morrisiana (tree). Seedlings were planted in treatments that tested for the interaction of plant sex (female/male) and neighbour density (a single, two same-sex and two opposite-sex plants). The sexes of the three dioecious species seedlings were identified by the improved CTAB method and three previous reported methods relatively. The chemical components and relative contents of the root exudates were measured for each sample via solid-phase microextraction gas chromatography‒mass spectrometry (SPME‒GC‒MS).

The three dioecious species detected their neighbours through root-based signaling by generally similar patterns; that is, opposite-sex neighbours inhibited growth, but same-sex neighbours did not affect growth. The sensitivity of seedlings to a neighbouring plant's sex varies by sex and species. Among the female plants, the herbaceous plants presented the greatest sensitivity, whereas the trees presented the lowest sensitivity. While for male plants, trees presented the greatest sensitivity, whereas herbs presented the lowest sensitivity. Root recognition may be regulated by candidate allelopathic root exudate compounds (such as 2,4-di-tert-butylphenol, dimethylsilane glycol, and 3,5-dimethylbenzaldehyde) and their concentrations in the rhizosphere.

This study revealed consistent sex recognition via root communication in three dioecious plants. This finding suggests that niche differentiation differs among sexes and may interpret the formation of plant population sex ratios and spatial patterns of dioecious plants in natural communities.