CAN EARLY WILTING OF OLD LEAVES ACCOUNT FOR MUCH OF THE ABA ACCUMULATION IN FLOODED PEA-PLANTS

When pea plants (Pisum sativum L.) were subjected to flooding, abscisic acid (ABA) content in shoots and roots increased up to 8-fold in the following days and stomatal conductance significantly decreased. Although young leaves of flooded plants had a slightly higher water potential than those of the unflooded plants, old leaves had lower water potential and lost turgor at the time when a substantial ABA increase was detected. In plants where the old leaves were clipped off, flooding did not cause any ABA increase during 7 d of the experimental period, except under conditions of higher transpiration demand, when the increase in ABA content was both delayed end small in scale (only 1-fold). When intact plants were flooded and ABA was assayed separately in both old and young leaves, the ABA increase in old leaves preceded that in young leaves. Evidence here suggests that the flooding-induced ABA increase mainly results from the wilting of old leaves. This suggests that young leaves may be protected from wilting by ABA originating in old leaves under unfavourable environmental conditions.