Some factors responsible for differences between plant species in absorption and utilization of phosphate

The ability of plants to absorb phosphate and grow in a soil system depends upon a number of soil and environmental factors as well as a number of plant factors determined by the particular plant species involved. The purpose of this study was to compare the growth and phosphate nutrition of four plant species, two grasses and two vegetables, and to identify the plant factors which control the responses of these species to applied phosphate. Glasshouse pot experiments using a krasnozem soil with a high capacity to fix phosphate showed that the four species differed in both their relative growth and phosphate uptake in unfertilized soil and also ii their responses to applied fertilizer phosphate. Cabbage grew relatively better than the other species at all levels of applied phosphate and produced near maximum yield at a lower level of applied phosphate. Of the four species, lettuce was found to produce the lowest relative yields at each phosphate treatment. Despite species differences in both the absolute and relative amounts of phosphate absorbed, the yield differences between species were largely the result of differences in the efficiency with I which absorbed phosphate was utilized in plant growth. This appeared to be partly due to differences in the nitrogen status of the four species at harvest. In addition, fractionation of the phosphate in shoots of lettuce and cabbage grown under phosphate deficient conditions showed a difference in the relative amounts of inorganic orthophosphate in the shoot. Cabbage was found to have a greater proportion of its phosphate in organic form. Sand culture experiments using organic phosphates and inorganic phosph9te-fertilizer reaction products as sources of phosphate showed that interspecific differences in the utilization of these compounds were apparently not involved in any differential species response to phosphate in the soil. However, some reservations were expressed concerning the extrapolation of these results to the soil situation. Depletion zone studies using \\(^{32}\\)P demonstrated that the diameter of the zones of phosphate depletion around roots of these species were significantly different between two soil types but no differences were observed between different species in either soil. Plant growth rates and rates of phosphate absorption in continuous-flow nutrient cultures of low phosphate concentration were found to differ widely between species especially at below optimum concentrations. These results suggested that the poor growth of lettuce in krasnozem soil was partly due to its limited rate of phosphate absorption from solutions of low concentration. The results of the continuous-flow nutrient culture experiments were confirmed in short-term phosphate uptake studies using excised roots. Kinetic analysis of the \\(^{32}\\)P absorption data for non-sterile excised roots revealed that all species possessed two separate absorption mechanisms. For the vegetables the relative contribution of each uptake mechanism to the total phosphate absorbed was similar, but the rates of phosphate uptake by both the \a\" and \"b\" mechanisms were always greater for cabbage than for lettuce. Differences in phosphate uptake between grass and vegetable excised roots were due to differences in both the rates of absorption and the relative magnitudes of the two uptake mechanisms. Phosphate uptake by sterile excised roots of cabbage and lettuce was also characterized by the operation of dual absorption mechanisms. In comparison with rates of phosphate uptake under non-sterile conditions the absence of micro-organisms had no significant effect on the rate of phosphate absorption by lettuce. Bowever phosphate uptake by excised roots of cabbage was reduced under sterile conditions indicating a positive interaction between cabbage roots and the microflora with respect to phosphate absorption under nonsterile conditions."