The Effects of Phosphorus Supply on The Rates of Intake of Phosphorus and Nitrogen and Upon Certain Aspects of Phosphorus Metabolism in Gramineous Plants

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Abstract
For expts. previously described, new data are presented relating to the intake of P by gramineous plants, its distribution within the plant, and its partition in the leaves between alcohol-soluble, nucleic-acid, and "residual" P. Major determinants of the rate of intake of P by the plant are: (a) the demand set up by the growth and normal functioning of various plant parts, and (b) the concn. of the nutrient in the medium. It is considered that the indirect effect of P treatment on growth, and hence on demand, is more important than the direct effects of external concn. of P on the rates of intake of that nutrient. For the oat expt., rates of intake of P and N are expressed per unit wt of root system. With N, the supply of which was the same for all treatments, large initial effects of P treatment on the rates of intake are accounted for in terms of differences in the ratios of roots to shoots. P-deficient oat plants derived only 30% of their inflorescence P from other plant parts; those with an excessive supply derived 93% of their inflorescence P from these sources. The percentage P contents of the stems, leaves, and roots fell ultimately to lower values with a moderate supply of P than they did with the deficient supply. A more efficient re-utilization of P in the plants receiving the greater supply is favored as an interpretation of this "dilution" effect From an examination of the data for protein-N and nucleic-acid P in the leaves of young plants, it is concluded that the effects of P treatment on protein-N content at this stage are due primarily to variation in nucleoprotein content With P deficiency, oat seedlings soon exhausted their seed reserves of P, and this was followed by a drastic change in the partitioning of leaf P, such that absolute nucleic-acid P was reduced to 1/5 of the value obtaining 11 days earlier, alcohol-soluble P decreased slightly, and the water-soluble fractions represented by "residual" P increased by 30%. Total P was virtually unchanged in amt. During this same period of 11 days, the whole of the P intake from the medium was retained by the roots, and there was a stimulation of root growth relative to leaf growth. These facts suggest that the high root wt. ratios found with P deficiency may be due to the fixation in organic forms of a greater proportion of the absorbed P, so that relatively little is available for shoot growth. Nucleo-cytoplasmic ratios of oat and Phalaris leaves are presented for a wide range of growth stage and nutrient treatment No obvious correlations with growth data such as the relative leaf growth rates were revealed.