Salinity is one of the most serious agricultural problems for plant growth and production worldwide. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the main enzyme involved in CO2 fixation in C3 plants, and its activity is rapidly changing for adjusting the photosynthetic carbon reduction cycle under stress conditions. Therefore, monitoring of its activity provides information on limitations of carboxylation arising from adverse environmental conditions such as salinity. In this study, we investigated the effects of salt stress induced by 50 and 100 mM NaCl treatments on the initial and total activity of Rubisco of two Iranian melon genotypes, “Ghobadlu” and “Suski-e-Sabz”, as well as “Galia” F1 cultivar at 15 and 30 days of treatment. Rubisco activation state was also calculated by the ratio of initial to total Rubisco activities. Results showed that both the initial and total activities of this enzyme significantly decreased as a result of treatment with 50 or 100 mM NaCl after 15 and 30 days of commencing salt treatments. Moreover, the general trend observed in the genotypes was a decrease in activation state of Rubisco with increasing stress levels at 15 and 30 days of treatment. Our finding showed a notable reduction in biochemical components of carbon assimilation, i.e. Rubisco activity, in studied melon genotypes suggests that specific negative effects of imposed saline stress on photosynthetic machinery could be attributed to the decrease of non-stomatal pathways.
