Amino Acids Translocated from Turgid and Water-stressed Barley Leaves

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Abstract
The phloem exudation technique of King and Zeevaart was modified for use with barley [Hordeum vulgare cv. Proctor] plants, to investigate the effect of water stress upon amino acid translocation at seedling and grain-filled stages. Seedling leaves and flag leaves from unstressed and moderately water-stressed plants exuded 14CO2 assimilates, sugars and amino acids when their sheaths were cut and immersed in a 5 mM solution of Na2EDTA (pH 7.0). By including PEG [polyethylene glycol] 6000 (-10 bars) in the Na2EDTA solution, leaves severed from moderately water-stressed plants were maintained in a wilted state. Such leaves produced about as much exudate as turgid leaves of unstressed plants. The following observations suggest a phloem origin for most of the exudate. Exudation was markedly stimulated by light and by CO2 enrichment. The release of NO3- declined after cutting and did not parallel exudation of 14CO2 assimilates, sugar and amino acids. The relative quantities and specific radioactivities of sugars and amino acids in the exudate differed from those of sugars and amino acids extracted from sheath tissue. Major amino acids in exudate from unstressed seedling and flag leaves were glutamine, glutamate, serine, alanine and aspartate; proline was virtually absent. Exudate from water-stressed leaves contained relatively more serine and also some proline and .gamma.-aminobutyric acid.