SALT STRESS-INDUCED CHANGES IN SUPEROXIDE-DISMUTASE ISOZYMES IN LEAVES AND MESOPHYLL PROTOPLASTS FROM VIGNA-UNGUICULATA (L) WALP

The effect of salt stress (NaCl and/or KCl) on the specific activity o f superoxide dismutase (SOD) isozymes in leaves from cowpea plants and in mesophyll protoplasts from cowpea leaves was examined. In leaves, mitochondrial Mn-SOD was significantly decreased by 35 mM (up to 35%) and 100 mM NaCl (up to 60%), whereas, under the same conditions, cytos olic and mitochondrial Cu,Zn-SOD I was slightly diminished and chlorop lastic Cu,Zn-SOD II did not show a response. In protoplasts, the speci fic activity of SOD isozymes was decreased as a function of both the s alt concentration and the type of salt used. Mn-SOD was again the most salt-sensitive isozyme, but in contrast to whole leaf preparations, C u,Zn-SOD II in protoplasts was markedly inhibited by NaCl and Cu,Zn-SO D I showed an intermediate sensitivity. KCl concentrations equivalent to those of NaCl were less effective in inhibiting the SOD isozymes, s uggesting the existence of specificity for certain ions. In protoplast s salt inhibition of SODs was reversible. However, Mn2+ was necessary for total recovery of Mn-SOD activity in NaCl-treated protoplasts. The in vitro results suggested a competitive inhibition of SOD isozymes b y salts. A possible competitive inhibition of SOD activity in salt-tre ated cowpea plants was also suggested. Finally, the possible correlati on between plant response to NaCl and the potential of SOD isozymes wa s analyzed.