Nodule and Plant Development in Stylosanthes humilis H.B.K.: Symbiotic Response to Phosphorus and Sulphur

Abstract

The effect of phosphorus and sulphur on the initiation and subsequent course of development of effective nodulation was determined for young seedlings of Stylosanthes humilis over the period days 1 1 to 26 from sowing. The plants were inoculated with an effective strain of Rhizobium and grown under controlled conditions in a small amount of nitrogen-deficient soil. Phosphorus had a beneficial effect on the initiation of nodules, which were first detected at day 11 in high-phosphorus plants, but not until day 14 in low. Thereafter, nodule development was greatly enhanced by phosphorus, nodule numbers, volumes and dry weights being increased. Nodule relative growth rates were stimulated from 0 . 3 g/g/day at low phosphorus levels to 0 . 7 g/g/day at high phosphorus levels over days 23-26. The nodules became pink earlier, and developed more rapidly as nitrogen-rich organs, in response to phosphorus. This suggested that from the earliest stages, phosphorus not only promoted the development of an increased mass of nodular tissue but also favoured an effective symbiosis. This enhanced nodule development led to greatly stimulated growth, with increasing amounts of total nitrogen and phosphorus in high-phosphorus plants. The assimilation of nitrogen by the whole plant was increased from 17 mg/g nodule dry weightlday at low to 53 mg/g/day with high phosphorus over days 23-26. Sulphur caused an increase in dry weight, but the response to sulphur mainly occurred late in development and was smaller than the response to phosphorus in the young seedlings of this trial.