Nitrogen Fixation Associated With Acacia dealbata Link. Seedlings as Estimated by the Acetylene Reduction Assay

Abstract

Seedlings of Acacia dealbata were grown in pots containing a sandy loam and inoculated with a suspension of homogenized nodules collected from naturally occurring A. dealbata trees. Plants were maintained in a controlled environment growth room. The rate of acetylene reduction by intact plants was determined by placing pots inside 1 25 litre jars. Experiments were conducted after 12 weeks of growth when the plants had attained a steady acetylene reduction activity.

The rate of ethylene production was linear for at least 3 h, and the Michaelis-Menten constant (K_m) value was 3 87 kPa acetylene. The seedlings showed little diurnal variation in acetylene reduction activity when day and night lengths were 12 h, but activity declined when the dark period was extended to 15 h. Acetylene reduction activity was markedly affected by soil moisture, the optimum rate occurring between 25 and 50% of field capacity (33 kPa).

A rapid decline in the rate of acetylene reduction occurred when 8 mg nitrate-N was added per pot. Acetylene reduction activity, dry matter yield and nitrogen uptake were adversely affected by induced soil salinity. The loss in acetylene reduction activity was highly correlated with the degree of soil salinization.