Characteristics of phosphorus uptake kinetics of poinsettia and marigold

In a previous experiment it has been found that maximum uptake rate (Imax), Michaelis constant (Km), and minimum nutrient concentration (Cmin) as plant physiological characteristics may be important for phosphorus (P) uptake in peat-substrate. Thus, variation of P uptake parameters was evaluated with a series of depletion studies for poinsettia (Euphorbia pulcherrima) and marigold (Tagetes patula) under fluctuating climatic conditions and different developmental stages.Relative growth rate (RGR) of marigold was higher than that of poinsettia and declined for both crops with plant age. Lower air temperature reduced the RGR of poinsettia, but not of marigold. However, the lower light intensity reduced the RGR of marigold while it had no effect on the RGR of poinsettia. A short photoperiod reduced the RGR of poinsettia. Maximum uptake rate also decreased with plant age and with decrease of air temperature for both poinsettia and marigold; however, it was independent of light intensity. Maximum uptake rate of poinsettia was lower during short photoperiod than that of longer photoperiod. A close correlation between RGR and Imax was observed with both poinsettia and marigold throughout all treatments. The Km and Cmin were affected neither by plant age, air temperature, light intensity nor by day length. However, higher Imax, but lower Km and Cmin values were observed for marigold than for poinsettia during all treatments. The required P availability in the substrate was not much affected by short term fluctuations of growing conditions and photoperiod. However, it was clearly reduced with plant age for both crops which should be considered for fertilization.