Ontogeny of annual grass grain protein fractions and amino acids affected by nitrogen supply

Mechanisms involved in uptake, assimilation, and distribution of nitrogen ( N) in plants may reflect the effectiveness of various physiological functio ns developed under the conditions a species was evolved. The purpose of the se studies was to determine the influences of N nutrition on the partitioni ng of different N forms within the Avena sativa grain during its maturation . Plants were grown in a glasshouse using nutrient solution sand culture wi th four N treatments: two high-N (5.33 mM), a mid-N (2.67 mM) and a low-N ( 5.33 mM before anthesis and no N after anthesis). One high-N series was lab eled with N-15 at anthesis to distinguish the N in the grain taken up after anthesis (exogenous N) from that absorbed prior to anthesis and translocat ed to the grain (endogenous N). Exogenous N accumulated in a sigmoidal patt ern for the measured grain protein fractions and total grain-N, reaching 35 -40% of the N for these three fractions/classes of N. The grain prolamin-gl utelin protein fraction responds more to differences in N provided after an thesis than does the albumin-globulin protein fraction. A measure of consti tuent deposition in the grain, relative accumulation rate, is represented a s RAR = 1/x (.) dx/dt, where x is the concentration of a fi action and t is time. Albumin-globulin RAR had two increases, while prolamin-globulin RAR had a single longer increase during grain maturation. At individual samplin gs the distribution of exogenous N between the two protein fractions coinci ded with their RAR's. For oat grain uptake, exogenous N was the most import ant source of N in the fastest accumulating protein fraction. When comparin g grain of high-N with mid-N, individual amino acid concentrations ranged f rom an increase of 16.3% to a decrease of 17%. The average glucose requirem ent for the synthesis of amino acids which increased at high-N was less tha n that of the amino acids which decreased. The bisynthethic efficiency and production values (reciprocal of construction cost referenced to glucose re quired by the metabolic product synthesis) were significantly related to th e significantly skewed distribution of amino acids in the grain protein. Th e most bioenergetically efficient amino acids were the most utilized in the grain proteins; energy expenditure regulation is a possible reason.