COLD-INDUCED CHANGES IN RIBULOSE 1,5-BISPHOSPHATE CARBOXYLASE-OXYGENASE AND PHOSPHOENOLPYRUVATE CARBOXYLASE IN CITRUS

The effects of cold acclimation on ribulose 1,5-bisphosphate carboxyla se-oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPCase) w ere examined in leaves of moderately cold-hardy 'Valencia' orange (Cit rus sinensis L. Osbeck) and in two cold-hardy genotypes, 'Trifoliata' orange (Poncirus trifoliata L. Raf) and 'FF-6-7-2' citrus hybrid (Citr us ichangensis Swing. X Fortunella margarita Lour. Swing.). Soluble pr otein extracted from leaves and analyzed by electrophoresis showed sig nificant differences in composition among the three citrus genotypes. A 12 kD polypeptide with unknown function was present in large quantit y in 'Valencia', but not in the other two genotypes. Probing with anti bodies specific for Rubisco and PEPCase indicated that 'Trifoliata' ha d two polypeptides of Rubisco small subunit (13 and 14 kD) whereas 'Va lencia' and 'FF-6-7-2' had only one (14 kD). Cold acclimation differen tially altered the expression of the two carboxylases. In all three ge notypes, cold acclimation resulted in increased amount of Rubisco larg e subunit but decreased amount of the small subunit. However, the amou nt of PEPCase was substantially enhanced by cold acclimation in the co ld-hardy genotypes ('FF-6-7-2' and 'Trifoliata'), but was largely redu ced by the cold treatment in the moderately cold-hardy genotype ('Vale ncia'). The enhanced expression of PEPCase during cold acclimation may be important for the induction of freezing tolerance in citrus.