In the cultivation of medicinal plants, salinity and light can affect the yield and quality of biomass. The objective of this study was to evaluate the effect of light and salinity levels of the irrigation water on the growth and salt tolerance of three medicinal plant species of the genus Plectranthus. A completely randomized split-plot design with five repetitions was used. The plots were formed by the environment of cultivation (full sunlight and greenhouse), the subplots by the salinity in the irrigation water - ECw (0.7, 1.9, 3.1, 4.3 and 5.5 dS m-1), and the subsubplots by the three plant species (P. amboinicus, P. barbatus and P. grandis). The following variables were evaluated: relative chlorophyll index (RCI), leaf area (LA), leaf area ratio (LAR), specific leaf area (SLA), shoot dry mass (SDM), root dry mass (RDM), total dry mass (TDM), RDM/SDM ratio, relative salt tolerance, and concentration of Na+ and K+ in leaves. Salinity reduced plant growth, with the greatest reductions observed in plants exposed to full sunlight. Salinity affected the dry matter partitioning, and the roots were more affected than the shoot. With increasing ECw, there was a significant increase in the concentration of Na+ in the leaves, while K+ and RCI decreased. However, the increase in Na+ accumulation was lower in P grandis. The three species were moderately tolerant to salinity up to 3.1 dS m-1, considering the total dry mass production, except the P. grandis grown in greenhouse, classified as tolerant. Regarding the relative reduction in shoot dry mass (part of commercial interest), we could recommend the cultivation of P. grandis when the available irrigation water presents values of ECw up to 3.1 dS m-1.
salt stress; inorganic ions; environmental factors; dry biomass
