Salt tolerance in apple seedlings is affected by exogenous ABA application

Salinity imposes stress on plant tissues that can lead to water deficits or ion-specific stresses resulting from altered ion ratios. The effect of exogenous ABA on 'Fuji' apple seedlings exposed to sodium chloride (NaCl) was analyzed by measuring parameters such as water potential, stomata aperture, sodium (Na+) concentration, endogenous abscisic acid (ABA), and total polyamines as well as genes related to their metabolism. The results show that damage to the leaf surface such as sunburn, yellowing and hardening of the leaves became more prominent compared to untreated controls (no ABA or NaCl) as treatment progressed. Regarding endogenous ABA levels, there was a significant difference at 5 days between the ABA+NaCl and NaCl groups. At the end of the treatment, the NaCl group showed higher levels of chlorophyll when compared to the untreated control and ABA+NaCl group. The stomata of the groups under stress showed recovery at the end of the treatment; however, the ABA+NaCl group response more closely resembled the status of the untreated, non-stressed control. Water potential levels dropped and stomata closure was apparent at 5 days for groups under stress; however, while the ABA+NaCl group displayed recovery as treatment continued, the NaCl group remained low even when the plants had recovered from the initial shock. Total polyamine levels were higher for the ABA+NaCl group after ABA application and then decreased with time, whereas that for the NaCl group was higher after stress began, peaking at 5 and 10 days. Taken together, ABA, PA and gene activations suggest that the early response to stress induced by ABA might have helped the seedlings overcome it more efficiently.