Growth, photosynthesis and root reserpine concentrations of two Rauvolfia species in response to a light gradient

Growth and secondary metabolites responses to light environment can be useful measurements to determine favourable habitat conditions for the cultivation and conservation of medicinal plants. We analyzed the growth, photosynthesis and root reserpine concentrations in seedlings of Rauvolfia vomitoria Afzel and Rauvolfia verticillatae (Lour.) Baill, two important medicinal plants yielding anti-hypertension alkaloids, at four different light levels (20%, 52%, 75% and 100% of full sunlight) in a shade house. Across all light intensities, seedlings of R. vomitoria grew faster with higher relative growth rate (RGR) than R. verticillatae, attributed to its higher photosynthetic capacities (light-saturated photosynthetic rate, A_max) and leaf area ratio (LAR). Typical shade-sun morphological responses to increasing light levels included decreased specific leaf area and LAR, whereas RGR and A_max was highest at median light levels for both species. R. vomitoria allocated higher leaf mass fraction and fine-root mass fraction, similar coarse root mass fraction, but lower stem biomass fraction than R. verticillatae. For fine roots, R. vomitoria had greater specific root length and small diameter than R. verticillatae, indicating its higher resource (water and nutrition) capture abilities. Both species had higher nitrogen concentration and lower reserpine concentration in fine roots than those of coarse roots. Neither reserpine concentration nor nitrogen was affected by light intensities. The reserpine concentration in coarse roots of R. vomitoria increased, whereas that of R. verticillatae decreased with increasing irradiance. Although not significantly, reserpine concentrations in coarse roots were positively correlated with A_max and RGR for both species across all light intensities. Shade or photosynthesis inhibition apparently did not increase alkaloid synthesis, which contrasted with the carbon/nutrient balance theory of plant defense. These results suggested that R. vomitoria and R. verticillatae could attain high biomass and yield of reserpine in high-light habitats (with max. in 75% sunlight), and intermediate-light habitats (with max. in 25–52% sunlight), respectively.