These results are consistent with the
finding that a mutant Arabidopsis which accumulates a
large amount of proline is salt-hypersensitive (Liu and Zhu
1997). The negative correlation between proline accumulation
and osmotolerance in some plant species does not rule
out an adaptive role for proline; rather, it may reflect the
importance of proline in the mechanisms of salt tolerance
other than osmotic adjustment (Delauney and Verma
1993). In fact, the level of proline in turfgrasses was too
low to have any significant effect on osmotic adjustment
(Torello and Rice 1986). Exogenous d-proline did not
function as an osmotic adjuster in antisense transgenic
Arabidopsis which was produced by downregulation of a
gene for P5CS (Nanjo et al. 1999). In contrast, proline could
protect lactate dehydrogenase from freeze–thaw cycles,
high temperatures, and chemicals (Rajendrakumar et al.
1994). Proline functioned in scavenging and/or reducing the
production of hydroxyl radicals in salt-stressed plants
(Smirnoff and Cumbes 1989; Alia et al. 1993). Thus, the
accumulation of proline in acacia species might contribute
to the cellular adaptation to salt stress or be a result of
metabolic changes induced by salt stress. In conclusion, the
level of proline accumulation after salt stress is not related
to the degree of salt tolerance in acacia species.