Photosynthetic Characteristics of Indian Rhubarb (Rheum emodi Wall.): An Endangered Medicinal Plant

Rheum emodi Wall. ex Meissn (Polygonaceae) is a perennial herb distributed in altitudes ranging from 2800 to 3800m in the temperate and subtropical regions of Himalayas. The herb has been traditionally used to treat pathological ailments like fevers, ulcers, bacterial and fungal infections, jaundice and liver disorders. In addition to that, roots of R. emodi are reported to have biological activities such as laxative, diuretic, and in vivo inhibitory effect towards P388 leukemia in mice models. Due to commercial exploitation and its unscientific extraction from nature, Rheum emodi has been categorized as an endangered and top-priority species for conservation by Threatened Plants Species Committee of the Survival (TPSSC) of International Union for Conservation of Nature (IUCN). In this regard, strategies are being developed for the conservation and cultivation of this species at comparatively lower altitudes. In the present study, photosynthetic characteristics of R. emodi Wall were studied to assess its conservation and cultivation possibility at comparatively lower altitude. Plants of R. emodi, collected from its natural habitat (Tungnath, 3600m altitude), grown at lower altitude (550m) were examined for their growth and photosynthetic characteristics at different light (0, 100, 600, 900, 1200, 1500 and 2000µmol m^-2 s^-1), and temperature (10, 15, 20, 25, 30 ^oC) and CO₂ (350 and 700µmol mol^-2) concentrations. Optimum temperature for photosynthesis in this species was observed at 25 ^oC, which appears to be much higher than the average temperature of its natural habitat (3600m) during active growth period. A gradual increase in the rate of photosynthesis with light intensity up to 2000µmol m^-2 s^-1 at both (ambient and elevated) levels of CO₂ indicates its adaptability potential to survive under high irradiance. Under optimum conditions, a considerable increase (˜ 30%) in its water use efficiency was observed when exposed to elevated CO₂ concentration. Since lower altitude is normally characterized with higher temperature and CO₂ conditions as compared to alpines, this species can perform much better at lower altitudes as it is able to acclimate under higher temperature and CO₂ conditions. Within its limits, this study suggests that R. emodi can be cultivated at lower altitudes in sun-exposed sites for its conservation and cultivation to meet its ever increasing industrial demand.