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Economic and environmental concerns have increased the need for quantitative advice on fertilizer rates. In addition, it would aid researchers to be able to estimate the degree to which nutrient availability is affecting yield in a wide variety of field experiments. All of these needs can, in principle, be addressed using the new PARJIB model. PARJIB retains the functional simplicity of much earlier analytical models of crop responses to soil test values and fertiliser application rates. However, in a key departure from previous approaches, response to scaled nutrient supply indices is dictated by the potential yield adjusted for plant population and water stress. The version currently being evaluated simulates responses to supply of N, P, K and Mg, varying either singly or in combination. We have calibrated the model for sweet corn, carrots, and snap bean crops grown under temperate conditions in a wide range of soils. Simulated yields agreed well with observed values; the root mean square error was 8% to 13%, and regressions of observed against simulated yields passed through the origin with slopes that were not significantly different from 1. After calibration, the model predicted strong interactions between nutrient supply, plant population and water stress. PARJIB appears to have substantial potential to improve nutrient management for horticultural crops.
Solenostemon scutellarioides (coleus) were grown in drainage lysimeters in concurrent experiments to evaluate effects of irrigation quantity and frequency on growth responses, leaf gas exchange, and nitrate leaching. Lysimeters in Expt. 1 were irrigated either with 13 mm daily or 13 mm every other day. Daily irrigation increased mean leachate and doubled nitrate leached compared with every other day (22.9 kg·ha−1 N versus 10.8 kg·ha−1 N, respectively). In Expt. 2, lysimeters were irrigated every 2 days with 13 mm or every 3 days with 18 mm such that total depth applied was equivalent. Irrigation frequency had no effect on irrigation quantity or nitrate leached. In these experiments, assimilation rates, stomatal conductance, and transpiration rates were influenced by day since irrigation with values lower on days without irrigation. However, neither irrigation quantity nor frequency affected final shoot dry weight, root dry weight, height or growth indices (P > 0.05).
Abstract
A range of tomato (Lycopersicon esculentum Mill.) cultivars was examined for changes during ripening in firmness, endopolygalacturonase (PG) activity and the molecular forms of polygalacturonase, Ca concentration, and the extractability of the Ca. Firm cultivars were firmer than the soft cultivars throughout ripening, and generally they contained less PG activity at each stage examined. In all cultivars, PG was predominately or entirely in the high molecular weight form (PG1) early in ripening, with the PG2 forms being increasingly prominent as ripening progressed. Differences in firmness were established while PG1 was the predominant PG. Uronic acid polymers in isolated cell walls were degraded rapidly by endogenous PG when citrate was present to complex Ca. In the presence of sufficient citrate, cell wall uronic acids of a firm and soft cultivar were equally susceptible to hydrolysis, suggesting that differences in the digestion of the walls by PG were dependent upon differences in Ca content or distribution. However, neither total, water, nor saline-extractable Ca showed consistent correlations with fruit firmness, and they also showed no progressive change during ripening.