ACTIVITY OF NITRATE REDUCTASE IN TRIFOLJUMSUBTERRANEUM: EFFECTS OF MYCORRHIZAL INFECTION AND PHOSPHATE NUTRITION

Abstract

SummaryActivity of nitrate reductase (NADH‐dependent), measured by in vitro methods, was present in both roots and shoots of non‐mycorrhizal Trifolium subterraneum. As expected, the activity was dependent upon the amount of nitrate supplied and was closely related to the concentration of nitrate in the tissues.Mycorrhizal plants had a greater capacity to synthesize the enzyme in both roots and shoots than non‐mycorrhizal plants when both were grown under phosphate‐limiting conditions. This conclusion was reached from the results of two types of experiment: (1) when constant (relatively low) amounts of nitrate were supplied to the plants each day, the specific activity of the enzyme in the roots declined over the period of the experiment (52 days) as the plants grew larger. There was also a decline in shoots of mycorrhizal plants. However, when the nitrate supply was increased fourfold, the stimulation of activity was considerably greater in the roots of mycorrhizal plants than in non‐mycorrhizal plants; (2) interactions between mycorrhizal infection, phosphate nutrition and nitrate reductase activity were investigated in plants which received different amounts of nitrate according to size. At the two lowest levels of additional soil phosphate (0 and 0.2 mmol phosphate per kg), there was a large mycorrhizal growth response accompanied by reduction in root: shoot ratio, increase in the phosphate concentration in the tissues and considerable stimulation of nitrate reductase activity, both on fresh weight and whole plant bases. This stimulation (together with changes in root: shoot ratio and phosphate concentration) was also apparent at 0.4 mmol phosphate per kg in the absence of any overall mycorrhizal growth response. At 0.67 mmol phosphate per kg, there was neither a growth response nor any stimulation of enzyme activity.NADPH‐dependent nitrate reductase activity, more characteristic of fungi than NADH‐dependent activity, was barely detectable and similar (very low) activity was present in both mycorrhizal and non‐mycorrhizal roots. The conclusions are that mycorrhizal effects on nitrate reductase activity are largely indirect, mediated via improved phosphate nutrition and that direct involvement of the fungi in nitrate assimilation is likely to be very slight.