Quantitative Model of the Translocation of Nitrogen to the Grain

Translocation of nitrogen was measured in wheat (Triticum aestivium L. cv SUN 9E) plants grown without an exogenous supply of nitrogen from the time that the flagleaf began to emerge, and a model of nitrogen translocation was constructed to describe translocation on one day during the linear period of grain growth. Nitrogen for grain development was derived entirely by the redistribution of nitrogen from vegetative organs. Leaves contributed 40%o, glumes 23%, stem 23%, and roots 16% of the nitrogen incorporated by the grains on the fifteenth day after anthesis. Less than 50% of the nitrogen exported from leaves was translocated directly to the grain via the phloem, the rest was translocated to the roots and was cycled in the roots and exported to the shoot in the transpiration stream. Nitrogen imported by leaves and glumes via the xylem was not accumulated in these organs but was transferred to the phloem for reexport from the organs. A large proportion (60%) of the nitrogen in the transpiration stream was cycled in the glumes. The glumes were also a major source of nitrogen for grain development. It was considered likely that this organ always plays an important role in nitrogen metabolism in wheat. In areas where cereals are grown, it is common for the level of available soil nitrogen to be low during the grain-filling period. Under such conditions, the nitrogen requirement ofthe developing grains is largely satisfied by mobilization of protein from vegetative organs (3, 26). In a previous paper, the amino acid compositions (%: molar basis) of phloem exudates from flag leaves and xylem sap from the roots and stem of wheat during grain filling were determined (19). During this period, nitrogen exported from the flag leaf in the phloem was derived from the hydrolysis of protein and from nitrogen imported by the leaf in the transpiration stream. Because of the quantity of nitrogen exported from the roots of those plants, it was considered likely that at least some of ' Supported by grants to M.J.D. from the Wheat Industry Research Council of Australia. During the course of this study H.L. was the recipient of a University of Melbourne Research Fellowship and R.J.S. held a Commonwealth Postgraduate Research Award. Additional funds were provided in the form of a Commonwealth Special Research Grant from the Committee on Research and Graduate Studies. 2Present address: Department of Agronomy, University of WisconsinMadison, 1575 Linden Drive, Madison, WI 53706. 3 Present address: Department of Plant Physiology, University of Groningen, 9750 AA Haren, The Netherlands. 4To whom reprint requests should be sent. 7 the nitrogen exported from the roots arose from nitrogen translocated to the roots from the shoot. Subsequently, cycling of nitrogen in the shoot and roots was investigated in wheat seedlings grown with an intact (20) and 'split' root system (8), and it was shown that nitrogen cycled in an organ may represent a significant proportion of the nitrogen exported from the organ. In this paper, these studies are extended to the grain-filling period of wheat. The redistribution and translocation of nitrogen was measured for one day during the linear phase of grain growth and the extent to which cycled nitrogen and mobilized nitrogen contributed to export from plant organs was determined. MATERIALS AND METHODS Plant Material. Wheat (Triticum aestivum L. cv SUN 9E) was grown in 15 x 8 cm diameter plastic pots containing 950 g of washed quartz sand. Plants were initially grown in a glasshouse under natural light during the months of January and February (Melbourne, Australia). Maximum daily temperature was 25°C and minimum night temperature was about 10°C. Tillers were removed immediately after they appeared. During this period, pots were watered to field capacity twice weekly with a complete nutrient solution containing 3.0 mm MgSO4, I mnm KH2PO4, 5 mM CaCl2, 10 nms K2SO4, 50 tIM Fe Na EDTA, 5 mm Ca(NO3)2, 10 mM KNO3, 2.5 mm (NH4)2 SO4, and micronutrients as described by Lewis and Powers (10) and were watered with tap water on other days. When flag leaves began to emerge, nitrogen was omitted from the nutrient solution and the concentration of the other nutrients was progressively reduced over three waterings. The final composition of the nitrogen-free nutrient solution was 0.3 mM MgSO4, 0.1 mm KH2PO4, 0.5 mm CaCl2, 1 mm K2SO4, 5 pw Fe Na EDTA, and micronutrients as described by Lewis and Powers (10). Plants were watered with this nutrient solution until grain maturity. The sand held no nitrogen at anthesis and grain growth was therefore dependent on mobilization of nitrogen from other organs. When plants reached anthesis, asjudged by the first appearance of anthers, they were labeled and transferred to a controlledenvironment growth chamber with an 18-h, 24°C day and a 6-h, 16°C night. Light was provided by 65-w fluorescent tubes (Thorn Industries, Melbourne, Australia) and 150-w incandescent lamps. The photosynthetic photon flux density (400-700 nm) at plant height was 450 ,imol m 2 s-1 and the fluence rate (400-1,200 nm) was 440 w.m-2. RH was about 65%. Three major harvests of 30 plants each were made on days 9, 15, and 17 after anthesis. The plants were dissected into grain, glumes (palea, lemma, sterile glume, and rachis), flag leaf, leaf 2, leaf 3, other leaves, stem, and roots (Fig. 1). Leaves were removed from the stem at the node and included the leaf blade and sheath. Data from these harvests www.plantphysiol.org on July 15, 2017 Published by Downloaded from Copyright © 1983 American Society of Plant Biologists. All rights reserved. Plant Physiol. Vol. 71, 1983 Table I. Accumulation of Nitrogen in Plant Organs during the Period 9 to 17 Days after Anthesis The equations were derived from data of three major harvests of 30 plants at 9, 15, and 17 d from anthesis. The dry weight of each plant part was determined (n = 30), and six independent determinations of the nitrogen content of each organ at each harvest, made by sampling from replicate groups of five plant parts. Variance of the increments or decrements of nitrogen in each organ during the study period were calculated by summing the variances at the first and final harvests. The decrements determined by this method agree well with those predicted from nitrogen loss over the grain-filling period (Fig. 3). N = nitrogen content of the organs (mg); t = time from anthesis (days).