Early phosphorus nutrition of eight forms of two clover species, Trifolium ambiguum and T. repens

Abstract

Eight ecotypes of Trifolium ambiguum (Caucasian clover) representing three ploidy groups were grown from seed on a phosphorus-deficient soil in a glasshouse. Six levels of P (as NaH₂PO₄) were applied in order to obtain a measure of the responses in yield of dry matter and uptake of P after 6, 9, and 12 weeks' growth. No differences in external P requirement (as μg applied P per g soil to produce 90% of maximum yield) or effectiveness of P utilisation (as dry matter produced per unit of P added) could be demonstrated between the ecotypes. However, the internal P requirement (as the critical P percentage in the foliage) decreased as the chromosome number increased. A solution-culture experiment with eight ecotypes (four diploid and four hexaploid) also failed to show substantial differences in external P requirement and efficiency of P utilisation, but, higher chromosome number was again associated with a lower internal P requirement. One of the most productive ecotypes, a hexaploid (to be released as cv. Monaro) was ranked highly with respect to efficiency of P utilisation, and had marginally lower internal and external P requirements than the others. Closer investigation of this genotype seems justified.

Seeds of Trifolium repens (white clover) were obtained to represent four cultiyars used commercially on soils of relatively high fertility and four ‘ecotypes’ surviving on soils of low fertility. Seed of each was sown in a series of pots containing phosphorus-deficient soil after six levels of added phosphorus (as NaH₂PO₄) had been mixed through the potted soil. Dry matter production and P concentration in the plants were measured after 6, 9, and 12 weeks' growth under controlled conditions of temperature and moisture. Differences within and between the two groups in external P requirement, internal P requirement, and efficiency of utilisation of P were quite small. The most efficient form in overall P nutrition was cv. Louisiana S1; the ‘ecotypes’ from low-fertility localities were the least efficient.

When the external and internal P requirements of the two species and their efficiency of P utilisation were compared in the above-mentioned soil cultures and in solution cultures, interspecific differences in the amount of applied P required to produce 90% of the maximum yield were negligible in plants of the same age, but the external requirement of T. repens became the greater when plants of T. ambiguum were grown on to produce a similar amount of dry matter. On the latter basis, measures of the efficiency of P utilisation (g DM per μg added P) favoured T. ambiguum. However, the estimate of P-scavenging ability and the critical P concentration in the foliage (i.e., internal P requirement) were similar for the two species.

Keywords:

• Trifolium ambiguum
• Trifolium repens
• P requirements
• P utilisation
• ecotypes
• ploidy groups