Gradients in the Nitrogenous Constituents of the Sap extracted from Apple Shoots of Different Ages

Abstract

A comparison was made of the nitrogenous constituents in the sap extracted under vacuum from apple shoots of different ages, as regards changes both with season and in response to fertilizer nitrate applied in summer or autumn. Before blossoming the N concentration of the sap changed markedly with age of shoot, with the values doubling in the samples from the proximal half of the 3-year-old wood to the distal 2-year-old section, followed by a significant decrease in the 1-year-old shoot. After blossoming the gradients in sap concentration were less pronounced but usually the lowest values were found in the youngest part of the shoot. Fourteen days after a soil application of nitrate in July there was a marked increase in the concentration of asparagine in the sap, but only in the 3-year-old section of the shoot. Seven days later the xylem sap from all parts of the shoot contained increased levels of asparagine, aspartic acid, and glutamine. No changes in the xylem sap of shoots in response to fertilizer applied in October were observed until the following April. Then increased amounts of asparagine and glutamine were found in all sections, with the greatest increase being seen in the youngest part. It is suggested that this was due to accelerated mobilization of N reserves into the xylem sap in response to growth regulators originating in the roots rather than to movement of recently absorbed nutrients.