The relationship between photorespiration and senescence of S. polyrrhiza P143

The relationship between senescence process and photorespiration was investigated in Spirodela polyrrhiza P143. The senescence of half-fronds cultivated under long-day conditions was either delayed by elevating CO2 or enhanced by elevating O2 composition of the surrounding environment, which implicated the involvement of photorespiration in this process. Application of sucrose into the cultivation medium was not found to be able to postpone the senescence process. In testing the effect of upstream substances of photorespiratory pathway on senescence, it was observed that applied at 0.5 mM, l-glycine and l-serine, but not glycollate, glyoxylate or formate, did induce senescence of both half-fronds and intact plants. No similar effect was observed of other amino acids, such as Arg, Trp and Cys, etc. As the role of l-serine in inducing senescence has been reported earlier, the possible role of l-glycine in inducing senescence of half-fronds under long-day conditions was further investigated. It was found that the peak value of endogenous glycine accumulated during senescence occurred later than that of serine and that the application of MOA, a specific inhibitor of glycine decarboxylase, inhibited the accumulation of serine and resulted in the accumulation of glycine, but did not cause the irreversible process of senescence. Therefore, a causative role of l-glycine in the senescence of half-fronds under long-day conditions could be ruled out. Measurement of changes in the endogenous serine level demonstrated that the appearance of endogenous serine peak value was postponed when half-fronds were cultivated at elevated CO2, which provides further evidence that l-serine is a specific factor of inducing senescence in duckweed.