The Effect Of Zinc Or Cadmium Stress On Photosynthesis And Endogenous Hormone Levels Of Sedum Alfredii

Sedum alfredii Hance, discovered for the first time in China as a Zn/Cd hyperaccumulator, has a broad application prospect in phytoremediation of heavy metal polluted soil in practice. In this paper, the effects of Zn or Cd stress on diurnal variation of photosynthesis index, diurnal variation of organic acid contents and endogenous hormone levels changes during48hours were investigated to explain the strong Zn/Cd tolerance and accumulation physiological machanism of S. alfredii. The main results of this study are as follows:1. The diurnal variation of stomatal conductance, net photosynthetic rate, transpiration rate and intercellular CO2concentration in leaves of S. alfredii’under zinc or cadmium stress were studied. The results showed that under metal stress, leaf stomata opened during day time (8:00-16:00) and closed during night time (0:00-8:00). At the same time, the diurnal variation of net photosynthetic rate did not show the typical "four stage" of crassulacean acid metabolism plant. Presumably, the hyperaccumulating ecotype of S. alfredii is not obligate crassulacean acid metabolism type.2. The correlation between the diurnal variation of leaf net photosynthetic rate and malic acid and citric acid levels ware analyzed. The results showed that malic acid and citric acid content under zinc or cadmium stress were higher than that in normal controls (except8:00), which indicated malic acid and citric acid accumulation benefited the hyperaccumulation of S. alfredii. Normal conditions or under heavy metal stress, the changes of malic acid and citric acid contents and diurnal variation of net photosynthetic rate showed no significant correlation, suggesting that the accumulation of malic acid and citric acidcoincided with the heavy metal hyperaccumulation of S.alfredii and Crassulacean acid metabolism has no direct relationship.3. The change of endogenous hormone auxin (IAA), gibberellin (GA), cytokinin (ZR) and abscisic acid (ABA) content of S. alfredii within48hours of zinc or cadmium stress were analyzed. Results show that, with zinc or cadmium treatments, IAA, GA, ZR and ABA showed a rapid increase within12hours, showing a high peak, which indicated that the four endogenous hormones of S. alfredii has fast response characteristics to heavy metal stress. Compared to the other two kinds of endogenous hormones, IAA and GA greatly increased under the stress of heavy metals, suggesting an important role of IAA and GA in the strong Zn/Cd tolerance and accumulation ability of S. alfredii.