Amino Acids Translocated from Turgid and Water-stressed Barley Leaves: I. Phloem Exudation Studies

The phloem exudation technique of King and Zeevaart (Plant Physiol 1974 53: 96-103) was modified for use with barley plants, to investigate the effect of water stress upon amino acid translocation at seedling and grainfilled stages.Seedling leaves and flag leaves from unstressed and moderately water-stressed plants exuded (14)CO(2) assimilates, sugars, and amino acids when their sheaths were cut and immersed in a 5 millimolar solution of Na(2)EDTA (pH 7.0). By including PEG 6000 (-10 bars) in the Na(2)EDTA solution, leaves severed from moderately water-stressed plants could be 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 CO(2) enrichment. The release of NO(3) (-) declined after cutting, and did not parallel exudation of (14)CO(2) 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.