In vitro Evaluation and Selection for Salinity Tolerance in Some Citrus Rootstock Seedlings
Salinity is considered a critical problem especially in citrus trees since they are one of the most globally important horticultural salt sensitive crops. This study was conducted to evaluate the performance of some citrus rootstocks under different salinity concentrations. Seeds of seven citrus rootstocks [four accessions of sour orange Citrus aurantium L. (Balady, Brazilian, Spanish and Russian), Macrophylla (Alemow) Citrus macrophyla, besides, Trifoliate orange (Poncirus trifoliate (L.) Raf.); and Pomeroy trifoliate (Poncirus trifoliate (L.) Raf.)] were cultured in vitro on Murashige and Skoog salts (MS) supplemented with NaCl. The effects of NaCl concentrations at 0.0, 1000, 2000, 3000, 4000 and 5000 ppm were studied. Salinity tolerance of these rootstock seedlings were noticed in vitro and in vivo. Moreover, proline accumulation in leaves, protein and isozyme anding patterns through gel electrophoresis were analyzed. Results showed that, number of burned leaves increased during the first subculture more than the second subculture at 5000 ppm with Brazilian sour orange. Moreover, both seedling height and emerged shoots were enhanced by low levels of salts compared to control. Results also cleared that, the increment of sodium chloride levels (up to 5000 ppm) in culture medium lead to a significant accumulation of proline in leaves of citrus rootstocks. Moreover, Balady sour orange considered the most citrus rootstock tolerant to salinity stress since it exhibited the highest survival percentage and proline accumulation followed by Spanish sour orange while Brazilian, Russian sour orange, Alemow and Trifoliate orange found to be moderate to salt tolerance. On the other hand, Pomeroy trifoliate was the most sensitive rootstock, which gave the lowest values. Protein banding patterns revealed a total number of nineteen bands with molecular weight (MW) ranged from 22.2 to 287.45 KDa. Thirteen
monomorphic bands were detected while the remaining six bands were polymorphic with 31.51 % polymorphism. On the other hand, peroxidase and polyphenyl oxidase, isozymes banding patterns represent differences with the tested rootstocks in some bands existence or its density with the NaCl treatment of 4000 ppm compared with control treatment.