The partitioning of nitrate assimilation between root and sboot of higber plant species is indicated by tbe relative proportions of total plant nitrate reductase activity (NRA) in tbe two plant parts and the relative concentrations of nitrate and reduced N in tbe xylem sap. Tbese have been collated here from tbe literature and temperate and tropical species compared.Both the distribution of NRA and xylem sap nitrate: reduced N indicate tbat tbe following four generalizations can be made. 1. Temperate, perennial species growing in low external nitrate concentralions (about 1 mol m"^) carry out most of tbeir nitrate assimilation in tbe root. As external nitrate concentration increases (in the range found in agricultural soils, 1-20 mol m~~^), shoot nitrate assimilation becomes increasingly important. 2. Temperate, annual legume species growing in low external nitrate concentralions carry out most of their nitrate assimilation in tbe root. Sboot nitrate assimilation increases in imporlance as external nitrate concentration is increased. 3. Temperate, annual non-legume species vary greatly in tbeir partitioning of nitrate assimilation between root and sboot when growing in low external nitrate concentrations. Regardless of the proportion carried out in tbe root at low external nitrate concentrations, nitrate assimilation in tbe shoot becomes increasingly important as external nitrate concentration is increased. 4. Tropical and subtropical species, annual and perennial, carry out a substantial proportion of their nitrate assimilation in tbe shoot wben growing in low external nitrate concentrations. Tbe partitioning of nitrate assimilation between root and shoot remains constant as external nitrate concentration increases.It is proposed that a greater proportion of nitrate assimilation occurs in tbe sboot wben an increase in the rate of nitrate uptake does not induce an increase in NR level in the root. Thus, a greater proportion of the nitrate taken up remains unassimilated and is passed into tbe xylem. A constant partitioning of nitrate assimilation between root and sboot is •Present addie.ss and correspondence: acbieved by balancing NR levels in tbe root witb rates of nitrate uptake.Tbe advantages and disadvantages of assimilating nitrate in either tbe root or shoot are discussed in relation to temperate and tropical babitats.

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The nitrogen use efficiencies (NUE) of N 2 fixation, primary NH 4 + assimilation and NO 3 − assimilation are compared. The photon and water costs of the various biochemical and transport processes involved in plant growth, N-assimilation, pH regulation and osmolarity generation, per unit N assimilated are respectively likely to be around 5 and 7% greater for N 2 fixation than for a combination of NH 4 + and root and shoot NO 3 − assimilation as occurs with most crops.Studies on plant and rhizobial genes involved in nodulation and N 2 fixation may lead to more rapid nodulation, production of more stress-tolerant N 2 fixing systems and transfer of the hydrogenase system to rhizobium/legume symbioses which currently do not have this ability. The activity of an uptake hydrogenase is predicted to decrease the photon cost of diazotrophic plant growth by about 1%.

SUMMARYNitrate reductase activity (NRA) was measured in leaves, stems and roots of six grain legumes supplied with different nitrate concentrations. The contribution of the stem relative to total plant activity was assessed.In Lupinus angustifolius L., Pisum sativum L. and Vicia faba L., legumes of temperate origin, an increased proportion of total plant NRA occurred in the shoot as nitrate concentration was increased. In Cajanus cajan (L.) Wilisp., Glycine max (L.) Merr. and Phaseolus vulgaris L., legumes of tropical origin, the proportion of total plant NRA in root and shoot was relatively constant regardless of nitrate concentration.More than 20% of total plant NRA can occur in the stem of G. max, L. angustifolius, Pisum sativum, Phaseolus vulgaris and V.faba. In C. cajan 10 to 15 % of total plant NRA was found in the stem at all nitrate concentrations. It is concluded that stem tissues can contribute significantly to overall nitrate metabolism.

Background: Non-native invasive plants capable of fast growth may establish and develop rapidly under the high soil nitrogen (N) and increased irradiance levels that occur in gaps at disturbed sites within deciduous woodland. However, if disturbance is transitory, then the long-term success of an invasive species within deciduous woodland will be dependent on its ability to compete with native species and acclimate to the typical conditions of low N availability and irradiance. Aims: To gain greater understanding of the ecophysiological reasons for the success of Impatiens glandulifera as a weed species in deciduous woodland. Methods: We examined N form, N concentration and irradiance effects on seed dormancy, N nutrition and shade acclimation of Impatiens. Results: Under controlled conditions, Impatiens seed required a period of wet chilling to overcome dormancy. High nitrate ( ) concentration and light gave substantial increases in germination over 8-12 weeks chilling and decreased time to 50% emergence over a full winter's chilling. Under high irradiance in a glasshouse, Impatiens showed N nutrition characteristics similar to those of fast growing crop and weed species. Within woodland and on high and relatively low supply in a glasshouse, Impatiens showed a range of mechanisms of acclimation to low irradiance, in particular, increases in height, dry matter partitioning to the shoot, use of as osmoticum in stems, specific leaf area (SLA) and leaf chlorophyll concentration. Conclusions: High soil concentrations and light may be factors in the synchronisation of emergence of Impatiens in disturbed sites producing a high-density and competitively strong cohort of seedlings within a short period of time. Rapid growth under high irradiance and N supply will allow Impatiens seedlings to establish quickly in disturbed woodland sites. Acclimation to low irradiance, in particular, maintenance of height and development of an exceptionally high SLA at relatively low supply, as demonstrated could allow Impatiens to shade out competing field layer species at the low levels of N and irradiance typical of woodland.