Accumulation of grain nitrogen was studied in the wheat cultivars Argentine IX and Insignia. The pattern of nitrogen removal from several tissues of each cultivar was compared with the pattern of acid proteinase activity. There was a highly significant relation between the rate of nitrogen loss from the tissues and the rate estimated from the enzyme activity measurements. This suggests an important role for acid proteinase enzymes in leaf senescence. Redistribution of nitrogen present in the plant at anthesis accounted for 78.5 and 80.6 % of the final grain nitrogen yield of Argentine IX and Insignia respectively.

Gel electrophoretic studies have revealed that crude extracts from various tissues of wheat seedlings contain two major enzymes capable of hydrolysing aminoacyl �-naphthylamides. A third enzyme which exclusively hydrolyses the �-naphthylamides of the imino acids proline and hydroxyproline has also been demonstrated in wheat leaves. The pH optimum for �-naphthylamidase activity against phenylalanine �-naphthylamide in crude extracts was 7.4. The two major enzymes differ with respect to their substrate specificities; the more anionic enzyme, APl, hydrolyses a relatively narrow range of hydrophobic aminoacyl substrates including the �-naphthyIamides of leucine, phenylalanine, methionine, tyrosine and tryptophan, while the enzyme of lower electrophoretic mobility, AP2, hydrolyses a broad range of substrates. The two enzymes also differ in their sensitivity to the metal chelator 1,10-phenanthroline. The AP2 enzyme from wheat, like that from pea, appears to be a metallo-enzyme, but APl does not show any sensitivity to phenanthroline. The results of developmental studies performed with wheat seedling tissues are consistent with the view that the naphthylamidase enzymes function as aminopeptidases in vivo. A close association was observed between total enzyme activity and soluble protein content, with the highest naphthylamidase activities being found in actively growing tissues.

The seasonal patterns of leaf nitrate reductase activity were compared in five wheat cultivars which differ widely in their capacities to accumulate grain nitrogen. Significant differences in the average levels of nitrate reductase activity were observed between cultivars. Total seasonal nitrate reductase activity was closely related to total plant nitrogen at maturity. Grain nitrogen was only related to total seasonal nitrate reductase activity when allowance was made for significant differences between cultivars in nitrogen redistribution patterns. The significance of these results with respect to the possible use of nitrate reductase activity levels as a selection criterion for nitrogen productivity is discussed.

Factors affecting the reduction of acetylene to ethylene by nodules of Acacia pellita O. Schwarz were studied on 3-year-old trees in the field. The specific activity of rooted nodules (22.8 ± 3.3 nmol mg-1 fresh wt h-1) was greater than that of detached nodules alone (13.5 ± 2.1 nmol mg-1 h-1). Acetylene reduction started without a lag phase and remained linear for about 4 h. The apparent Km value for acetylene was derived to be 11.2 x 10-3 atm. Nodule material could be kept successfully without loss of activity when moist and shaded from direct sunlight for periods of up to 1 h. Acetylene reduction ceased when the assay temperature was 0°C, activity increasing to 20.3 ± 2.3 nmol mg-1 h-1 at 22°. The Acacia plants exhibited a diurnal cycle of nitrogen (acetylene) fixation; the greatest specific activity at midday was 27.7 ± 4.8 nmol mg-1 h-1 when the shaded soil temperature was 32°C and photosynthetically active radiation was 2550 μE m-2 s-1. By using a regression of nodule number per tree on stem diameter to obtain the total number of nodules per hectare, the total nitrogen fixation per wet season was estimated to be 12 ± 4 kg ha-1 where the planting density in the study area was 1110 trees ha-1. The work is discussed with reference to its implications for management of the restored areas and the role of acacias as nitrogen-fixing plants in the pre-mining native Eucalyptus tetrodonta open-forest.

The yield of grain nitrogen of 24 wheat cultivars grown in the field was found to be related to the activity of the enzyme nitrate reductase (EC1.6.6.1) in the first vegetative leaf of 21-day-old plants grown in a controlled environment. It was concluded that a cultivar's potential for yielding grain nitrogen could be predicted from the activity of nitrate reductase at the seedling stage. This could enable the plant breeder to identify, early in the selection program, lines worth keeping for more advanced testing. Harvest index for nitrogen varied considerably between cultivars, but was usually largest in cultivars with large yields of grain nitrogen. Improvement in harvest index for nitrogen should go some way towards breaking the inverse relation between grain yield and grain nitrogen percentage in plants grown on a limited supply of soil nitrogen.