Department of Plant Sciences

The objective of this study was to evaluate potato genotypes for P efficiency and to identify the mechanisms of P efficiency. Two efficient and two inefficient genotypes were selected from screening experiments and grown in soil as well in nutrient solution at different P lev-els. Genotypes CGN 17903 and CIP 384321.3 were clearly superior to genotypes CGN 22367 and CGN 18233 in terms of relative growth rate (RGR) of shoot d.m.. P-efficient genotypes had about 15% longer root hairs compared to P-inefficient genotypes but this did not enhance P uptake rate per cm root length. However, P efficiency of genotype CGN 17903 was related to higher P utilization efficiency and that of CIP 384321.3 to both higher P uptake efficiency in terms of root-shoot ratio and intermediate utilization efficiency. The higher RGR of the efficient genotypes resulted from enhanced net assimilation rate (NAR) rather than from leaf area ratio (LAR). Net photosynthetic rate was similar for all geno-types. However for the P-inefficient genotype CGN 22367, the lower NAR could be ex-plained by increased leaf dark respiration. For P-inefficient genotype CGN 18233 it is speculated that increased carbon cost of root respiration or carbon exudation or both caused low NAR, since dark respiration of this genotype was similar to that of P-efficient geno-types. Keywords: dark respiration, net assimilation rate, net photosynthetic rate, P utilization effi-ciency, P uptake rate, root hair, root-shoot ratio