Effects of salinity, diurnal cycle and age on nucleotide pools of bean leaves

In the present study, we investigated the relationship between salt stress and nucleotide levels in the shoot of Phaseolus vulgaris L. cv. Stringless Green Pod to determine if reduced growth was correlated with reduced nucleotide levels. Overall fresh weight of 25-d-old plants after having been on full salt treatment for 7 d was 33% lower compared with untreated plants. Shoot fresh weight decreased by 40% compared with 22% for the roots thus increasing the root to shoot ratio from 0.7 to 0.9. We examined young and juvenile leaves as well as mature leaves in order to compare growing tissue to fully enlarged tissue. To ascertain whether the effects of salt stress on nucleotide pools were more severe during the day than at night, we studied the combined effects of diurnal cycle and salt stress on these nucleotide pools. Salt treatment selectively affected certain nucleotide pools with the adenine nucleotides (AdN) being the most affected. We found large diurnal fluctuations of AdN pools in all leaves. During the day, AMP and ADP increased while ATP decreased. The sum, ATP + ADP, tended to remain constant and in mature leaves total AdN increased with AMP, an indication of net synthesis. At night, ATP increased in all leaves. However, salt stress prevented this night-time increase in mature leaves while enhancing it in juvenile and young leaves. In the daytime, salt stress caused a nearly 2-fold increase in AMP of young leaves and a large increase in the adenylate kinase mass action ratio (K). At night, the excess AMP disappeared with no change in total AdN. It is clear from these results that salt stress did not reduce shoot growth by depleting ATP in growing leaves. It did, however, reduce the ATP level of mature leaves and perhaps their ability to supply essential metabolites for growing regions.