NITROGEN UPTAKE MECHANISMS OF TRIFOLIUM SUBTERRANEUM, BROMUS MOLLIS AND ERODIUM BOTRYS; AND SENESCENCE AND DECOMPOSITION IN T. SUBTERRANEUM (HEAVY ATOM, GLYCERALDEHYDE-3-PHOSPHATE DEHYDROGENASE)

Two areas of research are encompassed by this thesis to elucidate little-studied mechanisms of the ecophysiology of Trifolium subterraneum L. (subclover) that are important to the forage production process: (1) the comparative kinetics of nitrate and ammonium ion uptake and (2) components of the shoot and root senescence throughout the life cycle.; Understanding the ability of clover seedlings to compete for nutrients would aid management of clover in pastures. Thus, nitrogen uptake kinetics of the subclover were compared with those of soft chess (Bromus mollis L.) and filaree (Erodium botrys Vav. Bertl.). Seedlings were grown in aerated nutrient solutions in a controlled environment chamber for 14 days. Nitrogen uptake rates were measured by tracking nitrate or ammonia depletion at high and low nitrogen levels over a 24-hour period using a sensitive conductimetric procedure.; The maximal nitrate and ammonium uptake rates for soft chess and filaree were both much greater than for subclover. The affinities for nitrate of soft chess and filaree were much lower than subclover, whereas the affinities for ammonium of soft chess, filaree, and subclover were all about the same and at an intermediate level. These results are consistent with the hypothesis that subclover is a poorer competitor for both the nitrate and the ammonium forms of nitrogen than the nonlegumes tested.; Previous studies of the production of annual clovers have neglected the estimation of senescent and senescing material, where it occurs in the canopy and its fate.; In this study plants of subclover were grown in large plastic containers under greenhouse conditions. Two media treatments were imposed: (1) soil (Yolo clay loam) and (2) washed sand, to ensure a dry surface to allow litter to accumulate and minimize decomposition. Adequate nutrition was supplied.; Senescence was found to be a continual process starting with the cotyledons at 18 days after emergence. Senescence of trifoliate leaves was first detectable between days 40-62 (LAI 4). The maximal rate of leaf senescence was equivalent to 82 kg/ha/day and occurred during the late flowering stage. The maximal rate of litter decomposition was estimated to be 7 kg/ha/day. Below ground productivity was maximal at the onset of flowering. The percentage of the root in the total biomass ranged from 35% at day 18 to 18% at day 128 for the soil treatment and 29% to 6% respectively for the sand.