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Photoperiodism or water stress can shift young leaves of Kalanchoe blossfeldiana from C3-type photosynthesis to Crassulacean acid metabolism (CAM). CAM induction implies synthesis of phosphoenolpyruvate carboxylase (EC 4.1.1.31, PEPC). Present results show that, in plants as well as in isolated leaves, an increase in endogenous levels of abscisic acid (ABA) preceded PEPC increase, independently of the nature of the CAM inductive parameter. Exogenously applied ABA was taken up and accumulated by isolated leaves to a level similar to the internal ABA concentration in the leaf tissues. Both in entire plants and in isolated leaves, exogenous ABA mimicked the effects of water stress or short days in triggering PEPC capacity. Using a heterologous cDNA probe (derived from Sorghum leaf PEPC gene) it could be shown that an early step of the leaf response to drought or ABA treatment consisted in an increase in the amount of PEPC transcripts. Depending on the duration of ABA application, C3 photosynthesis, typical CAM or CAM-idling could be triggered in isolated leaves. In each case, similar increases in PEPC capacity (maximum extractable activity) were obtained, suggesting that the type of photosynthesis performed by the leaves is not controlled by the level of the PEPC capacity. The results suggest that the photosynthetic behaviour of the leaves is primarily governed by the progressive drop in availability of external carbon caused by the ABA-controlled closure of the stomata. Effects of ABA on PEPC synthesis proceed independently of CAM performance. |
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