Diurnal Changes in Endogenous Abscisic Acid in Leaves of Pearl Millet (Pennisetum americanum(L. ) Leeke) under Field Conditions

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Abstract
Diurnal variation in leaf abscisic acid (ABA) content was investigated in pearl millet (Pennisetum americanum (L. ) Leeke) growing in the field in the semi-arid tropics and subjected to varying degrees of water stress. There was a two- to three-fold change in ABA content during the photoperiod in three groups of ‘severely’ stressed plants of the genotype BJ 104. Maximum ABA occurred mid-morning (1030 h). ABA levels then declined to a minimum at 1500 h. Changes in ABA content of ‘moderately’ stressed and fully irrigated plants were smaller, but still significant. Though, when averaged over the day, levels of ABA of the five groups were positively related to the degree of water stress, relationships between ABA concentration and total water (Ψ) or turgor (Ψp) potentials varied considerably with time of sampling. Within groups, changes in ABA contents during the day were not always accounted for by changes in Ψ or Ψp. Temporal changes in leaf ABA content similar to those found in BJ 104, and largely unrelated to Ψ, were observed in the genotypes Serere 39 and B282 in a subsequent year. Leaf ABA content of droughted plants (BJ 104) did not decline appreciably overnight despite a marked increase in Ψ. However, a large reduction in ABA content with increase in Ψ did occur following heavy rainfall. Diurnal changes in stomatal conductance (g1) of BJ 104 could not be simply accounted for by temporal changes in total leaf ABA content, even when allowance was made for effects of irradiance and other environmental variables on g1. It is suggested that the sensitivity of stomata to ABA, or accessibility of the hormone to the stomatal complex, changes during the day.