Abstract
SUMMARYDiurnal changes in the concentration of abscisic acid (ABA) which occur in leaves of droughted, field‐grown plants of pearl millet (Pennisetum americanum [L.] Leeke) are not always correlated with changes in bulk leaf water potential (Φ). A rapid decline in ABA content of the leaves following its rise to a peak level in mid‐morning, was observed in several time‐course studies despite continued water stress.The possibility that the reduction in ABA in leaves was due to an elevated rate of its export was examined in two ways: (i) by measuring ABA concentrations in developing panicles (possible sinks for leaf‐produced ABA) and in leaves, and (ii) by comparing the amounts of ABA in ungirdled leaves and in leaves heat‐girdled at the base of the lamina to block export. ABA concentrations in panicles generally paralleled those in leaves, though the peak level of ABA in the morning in panicles occurred later than in the leaves in some samplings. Although girdling initially increased ABA concentration, it did not prevent a subsequent fall which generally paralleled the decline observed in untreated leaves. The decrease in ABA that occurred despite the block to export and despite continuing stress was, therefore, attributed to changes in the synthesis or metabolism of ABA within the leaf.The probable rate of export of ABA from leaves, calculated from the changes in its concentration due to girdling, was highest at the time of most rapid ABA accumulation and declined thereafter. The percentage export of recently assimilated carbon declined similarly. However, the probable absolute rate of export of photosynthate, computed from stomatal conductance and [14C]‐export measurements, was not uniquely related to that of ABA.
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