A study was made of the effects of calcium and hydrogen ions on the nodulation of subterranean clover (Trifolium subterraneum L.) in water culture. A compound interaction was found. At pH 4.0 or less no nodules were formed at any calcium concentration. At a 0.01mM calcium concentration no nodules were formed at any pH used. Above these critical values, almost maximum nodulation could be obtained by an increase in either calcium concentration or pH, so that each factor was to a large degree replaceable by the other. Calcium and hydrogen ions in the range of concentrations which produced these marked interactions on nodulation had no measurable effect on plant growth (nitrate was supplied in solution). The effects of calcium and hydrogen ions on the growth of the Rhizobium strain used were also markedly different from their effects on nodulation. Hydrogen ions had a dominating effect on Rhizobium growth, while calcium ions had no effect at any pH and were required at most in trace amounts. Hydrogen ions depressed calcium uptake by the plants. It is suggested that, in the range of concentrations where they were replaceable, the effects of calcium and hydrogen ions on nodulation were through their influence on the level of calcium in the plants. It is concluded that the calcium requirement for nodulation of subterranean clover is higher than for growth of the host plant or for growth of Rhizobium.
A simple and rapid method is presented for the assessment of the phosphorus status of subterranean clover. The method rests on extracting fresh leaf tissue (400 mg) with five drops 10 N H2SO4 and measuring phosphorus in the filtered extract by a molybdenum blue colour method, visually or calorimetrically. No special skills or equipment are required and the method is therefore potentially suitable for use by advisers and farmers. Two standard blue colours would be sufficient for a visual separation between non-deficient, moderately deficient or severely deficient plants. The method was tested in early spring with leaf samples obtained from two field experiments (five phosphorus levels), a sand culture experiment, and a pot experiment with phosphorus deficient soil (five phosphorus levels in each). Close relations were obtained between relative yields (yields as a percentage of the maximum in each experiment) and extractable phosphorus (R2 = 0.93). The curve fitted to the relation had a clearly defined inflexion point, indicating a critical value of 150 ppm extractable phosphorus at 90% of the fitted asymptote for relative yield. The relation for total phosphorus and yields was not as close (R2=0.77) and the critical value not clearly defined. Extractable phosphorus was closely related to total phosphorus below the critical value for extractable phosphorus, but not above this value. Likely physiological are discussed.
During the first 3 days after transfer of moderately sulphur-deficient plants (S1) to full nutrient solutions, the relative growth rate (Rw) was considerably lower than that of plants raised at higher sulphur levels (S2 and Sa). This was reflected in a lower leaf area ratio of the S1 plants, and particularly in a reduction of nearly 50% in the net assimilation rate (EA). Net losses in dry matter from younger emerged leaves and petioles accounted for 25% of the dry matter in new leaves and petioles of S1 plants produced during this period.Shading of the five oldest trifoliate leaves on the day of transfer reduced E A at all sulphur levels by about the same absolute amount. However, EA of the shaded S1 plants was now only 21 % of EA for the shaded S3 plants. In S1 plants dry matter losses from younger emerged and shaded leaves and petioles accounted for 64% of the dry matter in new leaves and petioles produced during the first 3 days of recovery. No net losses, occurred in S2 and S3 plants~ Removal of the five older trifoliate leaves at transfer caused a reduction in R w at S1 and S2 levels, but only a reduction in E A of the S1 plants. E A of the defoliated S1 plants, attributable to the younger emerged leaves, was only 32% of that for the defoliated S8 plants.Assimilates for new growth during-the first 3 days of recovery from a sulphur stress were considered to arise from current photosynthesis in mature, relatively unimpaired leaves and from mobilization in the younger emerged leaves. Although these early treatment effects were small, th~ consequent changes in growth patterns were large.There were no, net losses of dry matter from the roots.
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