Pak. J. Agri. Sci., Vol. 42(3-4) 2005 DIFFERENCES IN PHOSPHORUS ABSORPTION, TRANSPORT AND UTILIZATION BY TWENTY RICE (ORYZA SATIVA L.) CULTIVARS Tariq Aziz ', Pahrnatuttah', M. Aamer Maqsood' and Tahir Mansoor" 1 Institute of Soil & Environmental Sciences, University of Agriculture, Faisalabad. 2Ayub Agricultural Research Institute, Faisalabad. Plants have adopted a wide range of morphological and physiological mechanisms to cope with P deficiency in soil. Crop species and even cultivars within the species differ genetically in these mechanisms and hence in their response to P deficiency stress. We evaluated growth response and P utilization efficiency of twenty rice cultivars grown in hydroponics with adequate (260 ~M P) as well as deficient (26 ~M P) levels of P. The cultivars differed significantly (p<0.001) in biomass accumulation at both P levels. Phosphorus contents varied significantly (P<0.01) among rice cultivars at both levels of P supply. Positive correlation (p-, 0.01) of shoot dry matter production in rice cultivars with root biomass (r = 0.70, n=160), P uptake (r = 0.82, n=160) and utilization rate (r = 0.46, n=160) indicated that these are the main morphological and physiological parameters for maximizing shoot production in P-starved condition. Greater efficiency in dry matter production per unit amount of P absorbed was obvious in IR 24-PK, 77-74-5-2 and PK 3362-2-1, whereas 33897-11, PK 1818-4-1-7 were least efficient in terms of specific utilization rate of P. Keywords: Genetic variability, P use efficiency, P utilization rate, rice INTRODUCTION Phosphorus (P) is the second limiting nutrient for crop production after N; and >30% of the world's arable land is deficient in available P (Vance, et al., 2003). Notoriously low use-efficiency (15-20 %) of native soil P, sub-optimal rates of P application in tropics and subtropics (Vance, et al., 2003) and fear of depletion of world's resources of inexpensive rock P, demands to development or adoption of strategies aimed at increased P acquisition and utilization efficiency by plants. Plants have evolved a diverse array of strategies to obtain adequate P under conditions of limited P supply. These strategies can be grouped into two broad categories. First are those aiming at increased P acquisition from soil and second aimed at increased P utilization within the plants. These includes decreased growth rate, increased growth per unit of P uptake, remobilization of internal P (Vance, et al., 2003; Vance, 2001; Plaxton and Carswell, 1999), increased production and secretion of phosphatases, exudation of organic acids (Raghothama, 1999) and increased root surface area due to more root growth, etc. (Lynch and Brown, 2001). Species and cultivars within species differ greatly in adopting these strategies as most of these processes are under genetic control. Exploitation of plant genetic capacity to produce cultivars efficient in P acquisition and utilization would sustain agriculture in developing countries. Identification of traits/ mechanisms responsible for these differences is a prerequisite for long term breeding experiments. I Rice is a major cereal crop grown around the globe and is an important staple food in Pakistan. It commonly suffers from P deficiency, because >90% soils of the country are deficient in available P. Furthermore, farmers are generally reluctant to apply P fertilizers because of high price, lack of availability and low recovery efficiency. Hence there is a dire need to categorize existing rice cultivars according to their P use efficiency to sustain rice production in the country and for long term breeding experiments to produce more P efficient rice cultivars. We evaluated twenty rice cultivars for their growth, P uptake and P utilization efficiency under deficient and adequate P levels in hydroponics for their categorization according to P use efficiency. MATERIALS AND METHODS The experiment was conducted in a green-house of Institute of Soil & Environmental Sciences, University of Agriculture, Faisalabad, Pakistan. Seeds of 20 rice cultivars were germinated in pre-washed riverbed sand in polyethylene lined iron trays. Fifteen days old seedlings were transplanted in foam plugged holes of thermo pol sheets, floating on Yoshida nutrient solution (Yoshida et al., 1976) containing 26 and 260 ~M P in two polythene lined iron tubs of 200-L capacity. The pH of the solution was maintained daily at 6.0 ± 0.5 with 0.05 M HCI or NaOH. The experiment was laid out according to completely randomized factorial design (Steel et el., 1996) with eight replicates of each cultivar, consisting of two plants per replicate. The plants were harvested 20 and 32 days after 8