Leaf Regulation of the Nitrogen Concentration in the Grain of Wheat Plants

The regulation of the final grain N concentration in wheat (Triticum aestivum) plants was studied through the alteration of the source/sink ratio. Plants were grown in a greenhouse in pots with soil, and fertilized with a supraoptimal N supply. The plants were divided into six groups. In one treatment, plants remained untouched as a control (Treatment 1). In another group, all the ears except that of the main tiller were removed at flowering (Treatment 2). All other plants were de-tillered after the emergence of the third leaf, leaving only one tiller per plant. At flowering, one plant set was left untouched (Treatment 3). In a second group, all the leaves were excised (Treatment 4). In another group half the spikelets of the ear were excised (Treatment 5) and in the last group three-quarters of the spikelets were excised (Treatment 6). Ear excision produced an increase in individual grain weight and the grain N concentration above the normal N concentration observed in this cultivar. The final N concentration was correlated with the concentration of free amino acids in the flag leaf 34 d after flowering. It is concluded that in intact plants grain protein synthesis is substrate-limited by the amino acid export pool in the leaves, and grain excision increases the availability of amino acids to be transported to the remaining grains.