AOBPreview originally published online on October 24, 2002
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Annals of Botany 90: 745-753, 2002
© 2002 Annals of Botany Company
The Influence of Phosphorus Deficiency on Growth and Nitrogen Fixation of White Clover Plants
1 Plant Nutrition and Soil Fertility Laboratory, Department of Agricultural Sciences, Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark, 2 Organic Farming Unit, Department of Agricultural Sciences, Royal Veterinary and Agricultural University, Højbakkegaard Allé 13, DK-2660 Taastrup, Denmark and 3 INRA – Unite d’Agronomie, 234 Av. du Brezet, F-63039 Clermont-Ferrand Cedex 02, France
* For correspondence at: Organic Farming Unit, Department of Agricultural Sciences, Royal Veterinary and Agricultural University, Højbakkegaard Allé 13, DK-2660 Taastrup, Denmark. E-mail hhj{at}kvl.dk
Received: 4 July 2002; Returned for revision: 26 July 2002; Accepted: 18 September 2002 Published electronically: 24 October 2002
The effects of P deficiency on growth, N2-fixation and photosynthesis in white clover (Trifolium repens L.) plants were investigated using three contrasting relative addition rates of P, or following abrupt withdrawal of the P supply. Responses to a constant below-optimum P supply rate consisted of a decline in N2-fixation per unit root weight and a small reduction in the efficiency with which electrons were allocated to the reduction of N2 in nodules. Abrupt removal of P arrested nodule growth and caused a substantial decline in nitrogenase activity per unit root weight, but not per unit nodule mass. Similarly, the rate of photosynthesis per unit leaf area was unaffected by abrupt P removal, whereas CO2 acquisition for the plant as a whole decreased due to a decline in total leaf area, leaf area per unit leaf weight and utilization of incoming radiation. These changes followed the decline in tissue P concentrations. The ratio between CO2-fixation and N2-fixation was maintained under short-term P deprivation but increased under long-term low P supply, indicating a regulatory inhibition of nodule activity following morphological and growth adjustments. It is concluded that N2-fixation did not limit the growth of clover plants experiencing P deficiency. A low P status induced changes in the relative growth of roots, nodules and shoots rather than changes in N and/or C uptake rates per unit mass or area of these organs.
Key words: Fodder legumes, grassland, phosphorus, phosphorus deficiency, photosynthetic efficiency, N2-fixation, nitrogenase activity, relative addition rate, specific shoot area, Trifolium repens L., white clover.
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