Phosphorus nutrition of barley, buckwheat and rape seedlings. I. Influence of seed-borne P and external P levels on growth, P content and 32P/31P-fractionation in shoots and roots

Schjørring, J. K. and Jensén, P. 1984. Phosphorus nutrition of barley, buckwheat and rape seedlings. I. Influence of seed-borne P and external P levels on growth, P content and ³²P/³¹P-fractionation in shoots and roots. Seedlings of barly (Hordeum vulgare L. cvs Salka and Zita), buckwheat (Fagopyrum esculentum Moench) and rape (Brassica napus L. ssp. napus ev. Line) were grown at 8 or 10 different external P levels in the range 0-2000 μM. Apart from P, the nutrient solutions were complete. In some experiments with barley and rape, ³²P-labelled phosphate was used. Root fresh weights of buckwheat and rape decreased when the external P supply exceeded the level required for maximal root development. In all three species, the roots constituted a decreasing proportion of the total plant fresh weight as the external P level increased. The shoot/root fresh weight ratio increased linearly with the P concentration of the roots. The ratio between the P concentration in shoots and roots increased with the P status of the seedlings grown at low to intermediate external P levels, but decreased at higher P levels. The proportion of total seedling-P held in roots consequently reached a minimum value and thereafter increased as the P status of the seedlings increased. This indicates that some control mechanism counteracted the accumulation of harmful P levels in the shoots. ³²P-Phosphate uptake by seedlings of barley and rape grown in solutions with 2 μM P overestimated the actual net phosphorus uptake by a factor of 6 to 7, indicating a marked fractionation of ³²P and ³¹P. For seedlings grown in solutions with 25 μM P (barley) or 50 μM (rape) no fractionation occurred. The relative excess of ³²P in high P seedlings accumulated in the roots. It is suggested that the fracionation was caused by efflux of low specific activity phosphorus and by diffusion of free phosphate ions across the plasmalemma of the root cells in response to a difference in the concentration gradient between the two P isotopes.