Effects of Ultraviolet‐B Radiation Stress on the Abscisic Acid Status of Rumex patientia Leaves

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Abstract
To determine if increased abscisic acid (ABA) levels are associated with ultraviolet‐B (UV‐B) radiation (288–320 nm) stress, Rumex patientia L. plants were exposed to visible light with high or low (control) UV‐B irradiance and then assayed for ABA by gas chromatography. Though leaf growth was inhibited by the UV‐B irradiation, no differences in levels of free ABA were found between leaves from the two treatments after 1, 3 or 5 days of exposure. Unlike the situation in most environmental stresses, increased ABA levels are not associated with UV‐B radiation stress. Further, photolysis or isomerization are not likely explanations for the absence of ABA increase. Calculations indicate that if ABA in the leaf were fully exposed to solar UV‐B radiation only about 6.7% would be photolyzed by existing daily radiation at 40°N latitude, and 11.3% by radiation resulting from a 40% decrease in atmospheric ozone. The quantum yield for isomerization of cistrans‐ABA by radiation in the 288–312 nm waveband was determined. Existing daily solar UV‐B irradiation is capable of isomerizing ABA in solution to an equilibrium mixture of 50% cisjrans and 50%irans, trans‐ABA., However, because of radiation attenuation in the leaf, it is estimated that only a small fraction of the cistrans‐A.BA. would be isomerized.

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