Improved drought tolerance in marigold by manipulation of root growth with buffered-phosphorus nutrition

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 conventional 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.