Changes in the Concentration and Composition of Peduncle Sieve Tube Sap During Grain Filling in Normal and Phosphate-Deficient Wheat Plants

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Abstract
Exuding aphid stylets were used to sample the contents of peduncle sieve tubes over 24-h periods at approximately weekly intervals during grain filling in normal and phosphate-deficient wheat plants. Samples were assayed for sucrose, total amino acids and potassium and for total solute concentration (osmolality). Sieve tube sap osmolality typically increased for about 2-4 h following illumination and then slowly declined. However, the response to illumination eventually disappeared in phosphate-deficient plants as grain filling progressed. These daily changes were superimposed on a progressive increase in sieve tube sap concentration by about 500 milliosmolal units during grain fill in both normal and deficient plants. Sieve tube sap concentration was always greater in phosphate-deficient plants. These results suggest that transport into the grains takes place via a developmentally controlled pathway of steadily increasing resistance. The molar ratio of sucrose to total amino acids declined substantially in the 2-week period after anthesis and then remained constant at about 2 : 1. The proportion of potassium in the exudate also declined markedly during the same period. Relative proportions of sucrose, amino acids and potassium were very similar in normal versus phosphate-deficient plants of comparable ages. There were no evident differences between normal and phosphate-deficient plants in exudate amino acid composition. Only minor changes occurred during grain filling, except during the last week when glutamine, the predominant amino acid at all times, increased even more in relative importance. In general, the results are characterised by the absence of any simple relationship between grain development or growth rates and sieve tube sap concentration or composition. The composition of the sieve tube sap may be used to calculate values of overall grain composition (N and K as a percentage of dry matter) which are in good agreement with reported values for wheat grains.