2017 ASHS Annual Conference

Phosphorus (P) is an essential nutrient required for plant growth and survival. Excess P runoff from agricultural production sites is a primary cause of deteriorated water quality and other serious environmental problems. The objective of this study was to determine an optimum rate of P required for greenhouse crop production, and how a physiological shift from vegetative growth to reproductive growth changes P requirements in three plant species: two flowering plants, geranium ‘Bullseye Scarlet’ and French marigold ‘Bonanza Yellow’, and one foliage plant, coleus ‘Chocolate Scarlet’. Plants were grown in a 1:1 mixture of perlite and vermiculite with a range of P concentrations (3, 5, 10, 15, 20 and 30 mg·L^-1), while all other nutrient elements remained the same. Growth parameters (height, width, leaf number, branch number, and flower number) were measured during production. Plants were harvested at vegetative and reproductive stages, and dry weight (DW) of all plant parts and P contents were quantified at each harvest as well as root characteristics. During vegetative and reproductive growth, growth parameters logarithmically increased with increasing P rate in all plant species. Higher biomass allocation to shoots was observed during both vegetative growth (90%, 78%, and 95% in geranium, marigold, and coleus, respectively) and reproductive growth (72%, 65%, and 90%): stems and leaves in geranium, and leaves in marigold and coleus. Increased biomass of the tissues was associated with higher P accumulations. These results indicate that plant type and growth stage differently affect P accumulation patterns due to different biomass allocation patterns. Higher P had more predominant effects in coleus especially during vegetative growth, significantly increasing shoot DW compared to geranium and marigold. P content also linearly increased in the shoots of coleus with higher P. Geranium and marigold accumulated 92% and 90% P, respectively, in shoots during vegetative growth, and 89% and 72% during reproductive growth. Meanwhile, coleus accumulated 96% P in shoots during vegetative growth, and maintained 93% P during reproductive growth. It is clear that P requirements are higher during vegetative stage compared to reproductive stage regardless of the type of plants, and Coleus, a foliage plant, requires higher P compared to geranium and marigold, flowering plants due to higher vegetative shoot biomass production. P concentrations were optimal at 10 to 15 ppm for vegetative growth, and 10 ppm for reproductive growth in all the tested plants.