Critical nitrate-nitrogen and total nitrogen concentrations for vegetative growth and seed yield of Linola (edible-oil linseed) as affected by plant age
Edible-oil linseed (Linola, CSIRO Australia) was grown in a sand culture experiment in a glasshouse to develop tissue tests for assessing the nitrogen (N) status of the crop. Seven rates of N, provided as nitrate, were used to obtain critical N concentrations. Plants were tissue-tested at 3 developmental stages: early tillering (TL), flower buds visible (BV), and the start of flowering (SF). Suitable tissues for tests based on nitrate-N were the upper half of the main stem and the whole main stem. Leaves were unsuitable as their nitrate-N concentration was unresponsive to N supply until well above the rate for maximum growth. For tests based on total N, suitable tissues were upper stem, upper 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 mg/L. Excessive N supplies (350, 700 mg N/L) reduced both seed oil percentage and seed yield. Critical nitrate-N concentrations in fresh, upper 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 concentration for seed yield could only be obtained for fresh stem tissue at TL, and this value was 50% lower than that for vegetative growth. Critical nitrate-N concentrations [mg/g dry weight (DW)] in dried stem tissue 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). Critical 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). Critical total N concentrations (% DW) for vegetative growth at TL, BV, and 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 total N values (% DW) for seed yield at TL, BV, and SF, respectively, were 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 convenient tissue for total N. Tissue testing for N status of Linola needs to be matched closely to plant age or stage of development because of the decline in critical N concentrations between early tillering and flowering.