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Impatiens (Impatiens × hybrida `Impulse Orange'), and marigold (Tagetes × hybrida `Janie Tangerine') plants grown under low phosphorus were more resistant to drought stress than plants grown with a conventional, high-P fertilization program. Low concentrations of P were supplied using alumina-buffered P incorporated into the peat media. Alumina was charged with two levels of P, giving two levels of P-desorption. The alumina-buffered P amendment amounted to 2% by volume of the medium. Control plants (high-P treatment) were fertilized with a nutrient solution containing a P concentration of 1.5 mm. Phosphorous leaching was reduced by 96% to 99.4% in the low-P treatments compared with controls. Low-P plants showed no signs of P deficiency or aluminum toxicity. Impatiens plant diameter was significantly reduced by low-P fertilization, and leaf area was reduced by low P in both species. In marigold plants, roots were confined to a small volume beneath the drip tube in high-P plants, while in low-P plants they were well distributed through the medium. Impatiens roots showed no obvious differences in root distribution. Plants at the marketing stage were exposed to drought. The low-P plants of both species wilted more slowly and recovered more quickly when irrigated than the high-P controls. The reduced leaf area on the low-P plants may account for the improvement in drought tolerance.
A series of experiments was conducted to investigate the response to drought stress of marigold (Tagetes patula L. `Janie Tangerine') plants grown with reduced phosphorus. Plants were grown with convention al phosphorus fertilization (1 mm, control) or one of two levels of alumina-buffered phosphorus (Al-P), 21 or 5 μm. Plants supplied with 21 μm Al-P produced plants with equal total dry weight, more flowers and reduced leaf area compared to control plants. Whole-plant photosynthetic CO2 assimilation expressed on a leaf area basis was nearly twice as high in 21 μm Al-P plants as in controls, probably as a result of reduced intraplant shading. In plants supplied with 21 μm Al-P, smaller leaf area resulted in reduced whole-plant transpiration. Moreover, the relative water content of the growing medium was significantly lower at wilting with 21 μm Al-P than for control or 5 μm Al-P regimes. The improved water acquisition with 21 μm Al-P could be explained by increased root proliferation via longer main roots and less densely distributed lateral roots. The results indicate that optimizing phosphorus nutrition with solid-phase buffered-phosphorus fertilizer improves drought tolerance by reducing transpiration and increasing water acquisition from the medium.
Bedding plants are frequently exposed to water stress during the postproduction period, resulting in reduced quality. We demonstrated that alumina-buffered P fertilizer (Al-P) provides adequate but much lower P concentrations than conventionally used in soilless mixes. When impatiens (Impatiens wallerana Hook. f. `Impulse Orange') and marigold (Tagetes patula L. `Janie Tangerine') plants were grown with reduced phosphorus using Al-P, P leaching was greatly reduced and plant quality was improved. Diameter of impatiens plants and leaf area of plants of both species were reduced by Al-P. Marigold plants grown with Al-P had more flowers and fewer wilted flowers. Flower wilting was also reduced for impatiens plants grown with Al-P. In marigold plants, roots were confined to a small volume beneath the drip tube in control plants, while roots of Al-P plants were well distributed through the medium. There was no obvious difference in impatiens root distribution. When plants at the marketing stage were exposed to drought, the Al-P plants of both species wilted more slowly than the conventionally fertilized controls. The reduced leaf area in both species and the improved root distribution of marigold may account for the improvement in drought tolerance of the Al-P plants.