This study was conducted to examine plant biomass and phosphorus (P) accumulation and partitioning in response P availability and to determine the optimal P concentration during growth phases of two plant species with contrasting growth characteristics: geranium (Pelargonium
Bailey) “Bullseye Scarlet”, a flowering plant, and coleus (Solenostemon scutellarioides
(L.) Codd) “Chocolate Mint”, a foliage plant. Plants were grown in inert media (1:1 mixture of perlite and vermiculite) with complete nutrient solutions containing a range of P concentrations considered low (3 and 5 mg/L), intermediate (10 and 15 mg/L), and high (20 and 30 mg/L). Higher P rates logarithmically increased shoot and root dry mass of geranium and coleus plants regardless of the growth phase, but linearly enhanced flower dry mass of reproductive geranium plants resulting from the accelerated flower development. During the vegetative phase, the intermediate-P increased the shoot biomass production of geranium plants, but high-P was more effective for coleus plants. During the reproductive phase, however, the intermediate-P increased shoot biomass production of both geranium and coleus plants to the level achieved by high-P. The change from vegetative to reproductive phase increased the relative biomass to flowers, roots, and shoots of reproductive geranium plants and roots and shoots of reproductive coleus plants in decreasing orders, resulting in an increased root-to-shoot ratio. The P content of all plant parts showed a logarithmical increase with higher P rates for reproductive geranium plants but a linear increase for reproductive coleus plants. During the reproductive phase, a higher proportion of acquired P was allocated to flowers of low-P geranium plants than the roots of high-P coleus. Our results demonstrate that geranium plants require intermediate-P throughout the growth phases, while coleus plants require high-P during the vegetative phase and intermediate-P during the reproductive phase. P-use efficiency (PUE) ranged from 5 to 15% in high-P, which was improved with intermediate-P by 36 to 70%. To further improve PUE, the application method also needs to be taken into consideration such that the fertigation volume is reduced during the vegetative phase and increased before the reproductive phase.
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