Intraspecific pod and seed trait variation of two herbaceous legume seeds in response to competing neighbours and nutrient resource abundance

The presence of competing neighbours can exert pressure on parent plants, forcing them to adjust their reproductive strategies to maximize recruitment success (adaptive response); however, competitive neighbour effects may be suppressed if more nutrient resources are made available (passive response). In this study, we investigated if the response of some measured seed and pods traits of Lablab purpureus and Mucuna pruriens (focal plants) were adaptively determined by the presence of competitive neighbour species (using Megathyrsus maximus as the focal neighbour) or passively determined by nutrient resource availability. We measured four pod traits (pod weight, pod length, pod filling, and no of pods/ha) and eight seed traits (no of seeds/ha, seed mass, seed yield, dry matter content, Calcium content, Phosphorus content, Nitrogen content, and seed germination percentage) of the focal plants established as mixtures (neighbour treatment) or as monocultures (no-neighbour treatment) at different levels of phosphorus fertilizer application (0, 20, and 40 kg/ha applied on treatment basis). The data were analyzed using a linear mixed-effect model. Results showed that plant identity modified competitive neighbour and nutrient resource availability effects. Competitive neighbour species facilitated increased pod length response, but suppressed pod filling and number of pods/ha which were passively improved at higher levels of nutrient abundance. The no of seeds/ha and seed yield were also passively improved at higher levels of nutrient abundance, previously suppressed by competitive neighbour effects. The presence of focal neighbours and nutrient resource levels interacted positively to increase the Phosphorus content, Nitrogen content, and seed germination %. Other measured traits were not influenced by competitive neighbour effects or nutrient resource availability. In conclusion, competitive neighbour effects on the seeds and pod traits of the focal plants depended on plant identity and nutrient resource availability. Passive trait responses may magnify competitive effects on progenies after successful recruitment.