Net CO2-uptake of sets of clover plants (Trifolium subterraneum L.) was measured over 3 weeks in ambient air and in a highly CO2-enriched atmosphere (400 Pa CO2). Phosphate (P) in the nutrient solution was varied between 0·05 mol m−3 P (reduced P) and 2·0 mol m−3 P (high P). In ambient air, the daily increments of the daily rate of net CO2-uptake (DICU; a parameter related to relative growth) were higher at reduced P than at high P. Stimulation by high CO2 of net CO2-uptake in the first day was less at reduced P than at high P. In the following days, high CO2 markedly inhibited DICU at reduced P, and thus growthstimulation by high CO2 ceased after between 4 and 12 d. By contrast, at high P, DICU increased more than 2-fold upon CO2-enrichment, and thus growth stimulation by high CO2 was maintained. Intermediate results were obtained with half-strength Hoagland's solution (0·5 mol m−3 P).

Leaf pools of inorganic ortho P, soluble esterified P, and total P declined markedly in high CO2 when P-nutrition had been reduced. Considerable decline also occurred in high CO2 when P-nutrition had been increased suggesting that P-uptake was not well tuned with net CO2-uptake (growth).

It is proposed that high CO2 can perturb the P-metabolism of clover, the impairment being less at high levels of P-nutrition. With regard to high CO2 as a growth stimulus, these results demonstrate that increasing P-nutrition to a level supraoptimal in ambient air can considerably improve the growth of a C3-plant in high CO2.

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