PHOSPHORUS FRACTIONS IN RELATION TO GROWTH IN PIGEON PEA (CAJANUS-CAJAN (L) MILLSP) AT VARIOUS LEVELS OF P SUPPLY

In order to evaluate the tolerance of pigeon pea (Cajanus cajan (L) Millsp. ICPL 316) to low-P conditions in relation to the utilization of P among the major biochemical fractions, soybean (Glycine max (L) Merr cv. Tamahomare) was used as a reference crop. Pigeon pea and soybean were cultured hydroponically in a glasshouse for 28 days with three levels of orthophosphate (P(i)). Low, control, and high-P plants were supplied with 0.1, 1.0, and 10 ppm P respectively. In another experiment, pigeon pea seedlings were grown in a solution culture for 48 days with four levels of P(i) concentrations ranging from 0 to 30 ppm. Results of the two experiments indicated that pigeon pea requires relatively low concentrations of external P(i) for growth compared to soybean. Low-P treatment significantly reduced leaf area, leaf dry weight, and shoot weight in both species but the reduction was more pronounced in soybean than in pigeon pea. Low-P treatment did not significantly affect the root dry weight in both species. Dry matter partitioning to soybean leaves at the low-P treatment was far lower than that to pigeon pea leaves. These results suggest that the decrease in dry matter partitioning to leaves led to a greater reduction in total dry weight in soybean compared to pigeon pea. Although the reduction in the total lipid-P amount associated with the low-P treatment was similar in both species, the low-P treatment led to a 40 and 64% reduction in residue-P (protein-P, nucleic acid-P, and phytin-P) in pigeon pea and soybean respectively. For the soluble-P (mainly P(i)) fraction, the reduction was more severe in soybean than pigeon pea. The proportion of the soluble-P fraction did not increase much with increasing P supply in pigeon pea whilst in soybean, a greater proportion of P was present as soluble-P at the high P levels. The results suggest that the "pool size" of inorganic-P fraction in pigeon pea may be smaller than that of soybean. Our results suggest that pigeon pea is more tolerant to low-P treatment compared to soybean because it maintains relatively low concentration of inorganic-P at the low-P conditions, presumably due to the efficient incorporation of the external P(i) into residue-P.