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L-3,4-Dihydroxyphenylalanine (L-DOPA) is an allelochemical released by roots of velvet bean (Mucuna pruriens) that affects the growth of several plant species. However, its mechanism of action is inconclusive. In this work, we compared the effects of L-DOPA (0.01–1.0 mmol L−1) and of aqueous extracts (300, 1200, and 3000 mg L−1) of velvet bean on growth and photosynthesis (gas exchange and chlorophyll a fluorescence) of soybean (Glycine max). Overall, the results showed that both L-DOPA and aqueous extracts of velvet bean reduced the growth, leaf area, photosynthetic rate (A), stomatal conductance (gs), transpiration (E), and quantum yield of electron flow through photosystem II (PSII) in vivo (ΦF). In addition, L-DOPA and aqueous extracts increased the internal CO2 concentration (Ci) and the leaf wax and trichome density on the leaf surface, while the maximum quantum yield of PSII (Fv/Fm) was not changed. These results suggest that the reduction of A should not be related exclusively to the stomatal closure, but also to limitations of the carbon metabolism, as indicated by the increase of Ci and decrease of ΦF. Briefly, we concluded that soybean growth inhibition by L-DOPA and aqueous extracts of velvet bean is due to the combination of damage in the root meristem and reduction in A.

Changes in soluble and cell wall bound peroxidases activities, phenylalanine ammonia-lyase activity and phenolic compounds and lignin contents in roots of calcium-treated soybean (Glycine max (L.) Merr.) seedlings and their relationships with root growth were investigated. Three-day-old soybean seedlings were cultivated in nutrient solution with or without 0.025–5.0 mM calcium for 24 h. In general, length and fresh and dry weights of roots increased, while activities of enzymes (soluble and cell-wall peroxidases and phenylalanine ammonia-lyase) and phenolic compounds and lignin contents decreased against calcium concentrations. In the absence of calcium, phenylalanine ammonia-lyase and peroxidases activities increased by accumulating phenolic compounds and lignin due to restricted growth of roots. Enhanced calcium supply reduced the production of phenolic compounds and lignification due to low phenylalanine ammonia-lyase and peroxidases activities, reinforcing the essential role of calcium to improve the soybean root growth.