Response to phosphorus availability during vegetative and reproductive growth of chrysanthemum: I. Whole-plant carbon dioxide exchange

Whole-plant CO₂ exchange and root-shoot interactions during transition from vegetative to reproductive growth of 'Coral Charm' chrysanthemum (Dendranthema xgrandiflorum Ramat.) were investigated over a range of P concentrations considered to be deficient (1 μM), adequate (100 μM), or high (5 mM). Transition from vegetative to reproductive growth resulted in reduced photosynthate production, root respiration, biomass accumulation, and starch accumulation in leaves. Root respiration was low in high-P plants regardless of growth stage. Reduced root respiration may indicate changes in source-sink relationships during the transition from vegetative to reproductive growth, making roots less competitive sinks than developing flowers. Plant responses to P deficiency included decreased CO₂ assimilation and shoot biomass accumulation but increased root respiration, root:shoot ratio, specific leaf mass (SLM), and starch accumulation in leaves. Reduced root respiration activity in high-P plants was presumably due to differences in root architecture resulting in proportionately fewer root apices in high P. Daily CO₂ assimilation, shoot biomass, SLM, and root:shoot ratio were similar in plants grown with adequate-P and high-P availability, although plant P accumulation increased with P availability. Our results suggest that the excessive P fertilization often used in ornamental production systems is detrimental to root activity.