Two field experiments were conducted to compare 15 wheat genotypes at two phosphorus (P) levels (zero-P control or low P levelwithout application of P fertilizer on soil with 8 mg extractable P kg-1, and adequate P levelwith P fertilizer applied at 52kg P ha-1) for yield, P uptake, and P utilization efficiency (P efficiency ratioPER, P harvest indexPHI, and P physiological efficiency indexPPEI). On the average of two experiments, substantial and significant differences were observed among wheat genotypes for both grain and straw yields at both P levels. Grain yields ranged from 2636 to 4455 kg ha-1 in the zero-P control, and from 2915 to 4753 kg ha-1 at adequate P level. Genotype 5039 produced the maximum grain yield, while 6529-11 had the minimum grain yield at both P levels. Relative reduction in grain yield due to P deficiency stress (PSF) ranged from none to 32%, indicating differential P requirements of genotypes. Genotypes 4943, Pasban 90, Inqlab 91, PB 85, Lu 26s, 4770, Chakwal 86, 4072, 6544-6, and 5039 had little or no response to P application. Phosphorus responsive genotypes included FSD 83, Kohinoor 83, Parvaz-94, Pak 81, and 6529-11. A non-significant correlation (r = -0.466, P 0.05) between grain and PSF in zero-P control treatment also indicated the least effect of P deficiency on some wheat genotypes. A wide range of PPEI (270-380 kg grain kg-1 P absorbed in grain + straw at control P level, and 210-330 kg grain kg-1 P absorbed in grain + straw at adequate P level) indicated differential utilization of absorbed P by the genotypes for grain production. This indicated that wheat genotypes differed considerably in their P requirement for growth and responsiveness to P application. The findings also suggested that PPEI was a better parameter for measuring P efficiency than other parameters, and can be used for selecting P efficient genotypes, because it relates to the internal concentration of a nutrient and genetic makeup of plant. It is concluded that genotypes having ability to produce relatively high grain yield, good command to tune P within plant and high PPEI are suited to low P soil conditions. Genotypes 4072, Inqlab 91, 4943, Pak 81 and 5039 were P efficient and had above mentioned abilities, while genotypes FSD 83, 6544-6, and 6529-11 were P inefficient. It should be noted that traits related to P efficiency are inheritable and can be used to improve P use efficiency of a genotype through back cross breeding programs.
