Heavy Metal Cadmium Tolerance on the Growth Characteristics of Industrial Hemp (Cannabis sativa L.) in China

The effects of different concentrations of cadmium (Cd) on the growth physiological parameters of industrial hemp, including growth, the photosynthetic characteristics, the chlorophyll fluorescence and so on, were studied with hydroponic greenhouse experiments.The results showed that with the increase of Cd concentration, the levels of chlorophyll a, chlorophyll b and carotenoids in hemp leaves decreased, and the decreasing rates were in the following order: carotenoid> chlorophyll b>chlorophyll a.For the plants under Cd stress, net photosynthetic rate (Pn), transpiration rate (Tr) and stomatal conductance (Gs) showed a downward tendency as the concentration of Cd increased, while intercellular CO 2 concentration (Ci) increased slowly.During the stress process, the maximum primary photochemical efficiency (Fv/Fm) and the photosynthetic electron transport rate (ETR) decreased.The actual quantum yield (Yield) and the photochemical quenching coefficient (qP) showed an apparent middle low point at 50 μmol/L, increased with increasing Cd concentration, and then decreased rapidly after the concentration reached 75 μmol/L.The non-photochemical quenching coefficient (NPQ) reached its maximum value at Cd level of 50 μmol/L.The Cd stress has significant impacts on the photosynthetic mechanism of industrial hemp.With a high concentration of Cd (>75 μmol/L), photosynthetic metabolism of hemp was inhibited and damaged.The Cd tolerance of hemp was related to the protection mechanism of chlorophyll a.Under Cd stress, there was a self-protection mechanism to reduce the impact of Cd stress on the photosynthesis in hemp via sacrificing the carotenoids to protect the chlorophyll in hemp leaves.