CRITICAL NITRATE-NITROGEN AND TOTAL NITROGEN CONCENTRATIONS FOR VEGETATIVE GROWTH AND SEED YIELD OF LINOLA (EDIBLE-OIL LINSEED) AS AFFECTEDBY PLANT-AGE

Edible-oil linseed (Linola, CSIRO Australia) was grown in a sand cultu re experiment in a glasshouse to develop tissue tests for assessing th e nitrogen (N) status of the crop. Seven rates of N, provided as nitra te, were used to obtain critical N concentrations. Plants were tissue- tested at 3 developmental stages: early tillering (TL), flower buds vi sible (BV), and the start of flowering (SF). Suitable tissues for test s based on nitrate-N were the upper half of the main stem and the whol e main stem. Leaves were unsuitable as their nitrate-N concentration w as unresponsive to N supply until well above the rate for maximum grow th. For tests based on total N, suitable tissues were upper stem, uppe r leaves, total stem, total leaves, and whole shoot. Critical N supply rates for vegetative growth at TL, BV, and SF, respectively, were 85, 145, and 145 mg/L. The critical N supply rate for seed yield was 65 m g/L. Excessive N supplies (350, 700 mg N/L) reduced both seed oil perc entage and seed yield. Critical nitrate-N concentrations in fresh, upp er stem tissue for vegetative growth decreased from 0.26 to 0.16 mg/g fresh weight (FW) between stages TL and BV. A critical nitrate-N conce ntration for seed yield could only be obtained for fresh stem tissue a t TL, and this value was 50% lower than that for vegetative growth. Cr itical nitrate-N concentrations [mg/g dry weight (DW)] in dried stem t issue for vegetative growth at TL, BV, and SF, respectively, were 2.3, 1.7, and 0.7 (upper stem); and 2.1, 1.1, and 0.6 (whole stem). Critic al nitrate N values (mg/g DW) for seed yield at TL, BV, and SF were 1. 1, 0.8, and 0.3 (upper stem); and 1.0, 0.7, and 0.2 (whole stem). Crit ical total N concentrations (% DW) for vegetative growth at TL, BV, an d SF, respectively, were 3.0, 2.3, and 2.2 (upper stem); 5.3, 5.8, and 4.5 (upper leaves); 2.2, 1.7, and 1.6 (whole stem); 5.5, 4.9, and 4.5 (total leaves); and 4.5, 3.1, and 2.7 (whole shoot). Corresponding to tal N values (% DW) for seed yield at TL, BV, and SF, respectively, we re 2.9, 2.2, and 2.0 (upper stem); 5.2, 4.8, and 4.3 (upper leaves); 2 .1, 1.4, and 1.4 (whole stem); 5.2, 4.4, and 4.2 (total leaves); and 4 .3, 2.8, and 2.6 (whole shoot). The upper stem is the preferred tissue when testing for nitrate-N, and the whole shoot is the most convenien t tissue for total N. Tissue testing for N status of Linola needs to b e matched closely to plant age or stage of development because of the decline in critical N concentrations between early tillering and flowe ring.