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Three field experiments were carried out during 1987-88 (1 site) and 1988-89 (2 sites) with Cream Gold onions grown on siliceous sands, to investigate the effect of nitrogen (N), at rates up to 475 kg N/ha on total-N, nitrate-N, potassium (K) and phosphorus (P) concentrations in youngest fully expanded blades (YFEB), bulked blades, necks and developing bulbs. The plant samples were collected when the largest bulbs were 25-30 mm in diameter. Nitrate-N concentrations were in the order WEB> bulked blades>necks = developing bulbs. For total-N the order was YFEB = bulked blades>necks> developing bulbs. Nitrate-N was more sensitive to variations in N supply than total-N in all tissues sampled. Potassium concentrations were in the order bulked blades > YFEB > necks > bulbs. At N rates <75 kg N/ha, P concentrations were in the order YFEB = bulked blades > bulbs > necks. Coefficients of determination (r2) for the relationships between nitrate-N and total-N concentrations and relative marketable yield of bulbs were in the range 0.73-0.98. At sites 1 and 3, the relationships between total-N and relative marketable yield were 'C-shaped' or showed the Piper-Steenbjerg effect. Critical concentrations (values at 90% relative marketable yield) for nitrate-N varied between plant parts (375-590 mg/kg) and sites (590-940 mg/kg for YFEB). Critical total-N concentrations also varied between the different plant parts (1.2-2.9%) but less so between sites (2.4-2.9% for YFEB) compared with nitrate-N. Based on sensitivity (as indicated by the range in tissue concentrations in response to variations in N supply) and on the correlations between nitrate-N and total-N concentrations and per cent relative marketable yield, we concluded that nitrate-N and total-N concentrations in YFEB were suitable indicators of the N status of onion plants. The YFEB is easily identified, and compared with bulked blades, necks or bulbs, samples of 50-100 can be collected without destroying plants and will also not result in excessive plant material to dry. Based on the variation in critical values between sites (reproducibility), total-N is preferred to nitrate-N. Correlations between nitrate-N and total-N concentrations in YFEB and bulb quality attributes (scale thickness, glucose concentration, fructose concentration, soluble solids and dry matter) were poor (72 values 10.48) and of little predictive value. |
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