The partitioning of nitrate assimilation between root and shoot of higher plant species is indicated by the relative proportions of total plant nitrate reductase activity (NRA) in the two plant parts and the relative concentrations of nitrate and reduced N in the xylem sap. These have been collated here from the literature and temperate and tropical species compared.

Both the distribution of NRA and xylem sap nitrate: reduced N indicate that the following four generalizations can be made.

1Temperate, perennial species growing in low external nitrate concentrations (about 1 mol m−3) carry out most of their nitrate assimilation in the root. As external nitrate concentration increases (in the range found in agricultural soils, 1–20 mol m−3), shoot nitrate assimilation becomes increasingly important.
2Temperate, annual legume species growing in low external nitrate concentrations carry out most of their nitrate assimilation in the root. Shoot nitrate assimilation increases in importance as external nitrate concentration is increased.
3Temperate, annual non-legume species vary greatly in their partitioning of nitrate assimilation between root and shoot when growing in low external nitrate concentrations. Regardless of the proportion carried out in the root at low external nitrate concentrations, nitrate assimilation in the shoot becomes increasingly important as external nitrate concentration is increased.
4Tropical and subtropical species, annual and perennial, carry out a substantial proportion of their nitrate assimilation in the shoot when growing in low external nitrate concentrations. The 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 the shoot when 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 the xylem. A constant partitioning of nitrate assimilation between root and shoot is achieved by balancing NR levels in the root with rates of nitrate uptake.

The advantages and disadvantages of assimilating nitrate in either the root or shoot are discussed in relation to temperate and tropical habitats.

The partitioning of nitrate assimilation between root and shoot of higher plants

Summary
The benefits which this paper addresses are those of maintaining the intracellular acid-base balance during growth, and of generating osmolarity related to regulation of turgor in environments of low water potential. These benefits may incur costs in terms of the quantity of potentially growth-limiting resources (photons, water, nitrogen) which are needed to produce unit quantity of ‘baseline’ plant biomass. The direction (excess H+ or excess OH−) and magnitude of acid–base perturbation during growth depends on the nature of the N-source (NH4+, N2 or NO3−), so that the costing of pH homoiostasis involves consideration of the costs of overall N-assimilation for comparison with the other costs of growth of a terrestrial C3 plant.

Photon costs for the various biochemical and transport processes involved in overall growth, N-assimilation, pH regulation and osmolarity generation are computed using known stoichiometries of coupled reactions. Water costs are deduced from the C-requirements for the various processes (including C lost in associated decarboxylations) by assuming a constant value of water lost in transpiration per unit net C fixed in an illuminated shoot. Nitrogen costs are deduced from the N-content of the plants or compounds under consideration. The computed costs for N-assimilation and the generation of osmolarity are referred to the costs of ‘baseline’ plant synthesis using the cheapest mechanisms (NH4+ as source for N-assimilation; inorganic ions as the basis for osmolarity generation) so that the increment of cost related to assimilation of N2or NO3−, or of osmolarity generation using an organic compatible solute, can be presented.

Photon costs of growth with N2 fixation and the processes associated with regulation of pH are (granted the assumptions made as to stoichiometries and plant composition) 9 % higher than are those of growth with NH4+ as N˜ source. The predicted cost of growth with NO3− as N source depends on the location of NO3− reduction and the mechanism of OH− disposal, and ranges from 5 to 12% more than that for growth with NH4+ as N source. H2O (transpiration) costs follow a similar pattern, with growth on N2 as N source costing 12% more, and growth on NO3− costing to 1–2 to 167 % more, than growth with NH4+ as N source.

The extra costs in photons of using compatible solutes (sorbitol, proline or glycine betaine) to generate an osmolarity of 500 osmol m−3 in all of the non-apoplastic water of the plant add 21·5 to 26·1 % to the total costs of growth, while use of compatible solutes to generate osmolarity in ‘N’ phases (i.e. cytosol, plastid stroma, mitochondrial matrix) alone would add 5·2 to 6·2% The costs of growth in terms of transpirational water are increased 7·9 to 98 % by the use of compatible solutes for osmolarity generation in the ‘N’ phases only. The increments for the N-containing solutes are higher when NO3− is the N-source rather than NH4+. The N-cost of growth with N-containing compatible solutes generating 500 osmol m−3 in ‘N’ phases increases the N cost of growth by 33%.

These predicted costs are under-estimates of ‘real’ costs which take into account the occurrence of alternate oxidase activity under some growth conditions and the production of additional organic acid anions with N2 as opposed to NH4+ as N source. Nevertheless, the predicted minimum costs of attaining the benefits of pH regulation and of turgor generation are of use in suggesting where selectively significant (i.e. low requirement for a scarce resource) alternative mechanisms may occur. Examples include a possible photon saving by using NH4+ rather than N2 or NO3− where all three are available; a possible water saving by use of photoreduction of NO3− in leaves in arid environments; and a possible N saving by use of non-N-containing compatible solutes (polyols) in environments of low water potential. Proof of these suggestions involves comparisons of inclusive fitness of genotypes possessing the trait under consideration with that of genotypes lacking the trait.

TANSLEY REVIEW No. 2

The literature on nitrogen (N) form effects on plants at different stages of their development has been critically reviewed, assessing the possible mechanisms of these effects. In particular, nitrate (NO3−) was compared with the other forms of N utilised by plants. It is concluded that the form of N available to plants can affect their time and rate of seed germination, leaf expansion and function, dry matter partitioning between shoot and root, and root architecture. The magnitude of these effects is dependent on environmental factors outside the supply of N. The mechanism of these effects is variable. Assessment of the importance of root or shoot NO3− assimilation under different environmental conditions is an important area for further study.

Do plants need nitrate? The mechanisms by which nitrogen form affects plants

The literature on the relations between plant nitrogen (N) assimilation enzymes and plant/crop N assimilation, growth and yield is reviewed to assess if genetic manipulation of the activities of N assimilation enzymes can result in increased yield and/or increased N use efficiency. The available data indicate that (I) levels of N assimilation enzymes do not limit primary N assimilation and hence yield; (II) root or shoot nitrate assimilation can have advantages under specific environmental conditions; (III) for cereals, cytosolic glutamine synthetase (GS1) is a key enzyme in the mobilisation of N from senescing leaves and its activity in senescing leaves is positively related to yield; and (TV) for rice (Oryza sativd), NADH-glutamate synthase (NADH-GOGAT) is important in the utilisation of N in grain filling and its activity in developing grains is positively related to yield. In our opinion, selection of plants, from either a genetically manipulated population or genetic resources, with expression of nitrate reductase/nitrite reductase primarily in the root or shoot should increase plant/crop growth and hence yield under specific environmental conditions. In addition for cereals the selection of plants with high GS1 in senescing leaves and in some cases high NADH-GOGAT in developing grains could help maximise the retrieval of plant N in seeds.

Can genetic manipulation of plant nitrogen assimilation enzymes result in increased crop yield and greater N-use efficiency? An assessment

Plants differ in tissue localization of nitrate reduction and assimilation. Some species reduce nitrate primarily in the leaves, whereas other species localize nitrate reduction and assimilation in the roots. We determined how nitrate assimilation is partitioned among leaves, stems and roots of poplar (Populus tremula L. x P. alba L.) by comparing tissue differences in in vivo nitrate reductase activity (NRA), nitrate reductase abundance and tissue nitrate concentration. Compared with stems or roots, NRA was greater in leaves, and the highest leaf NRA was found in young leaves. Leaf and root NRA increased with increasing nitrate supply, whereas stem NRA remained constant. Leaf NRA was at least 10-fold greater than root NRA at all external nitrate concentrations. Nitrate reductase abundance increased in all tissues with increasing nitrate availability, and nitrate reductase abundance was at least 10-fold greater in leaves than in stems or roots at all nitrate availabilities. Tissue nitrate concentration increased with increasing nitrate supply and was greater in roots than in stems and leaves. Photoperiod influenced NRA, with leaf NRA declining in nitrate-fertilized plants with short daily photoperiods (8-h). We conclude that different tissues of poplar vary in nitrate assimilation with little nitrate assimilation occurring in roots and the most nitrate assimilation taking place in leaves.

Partitioning of nitrate assimilation among leaves, stems and roots of poplar.

Summary
Nitrate 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 Viciafaba 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.

Distribution of nitrate reductase activity in six legumes: the importance of the stem

Nitrate 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 Viciafaba 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.

Distribution of nitrate reductase activity in six legumes: the importance

Effects of temperature on the vegetative and early reproductive growth of a cold-tolerant and a cold-sensitive line of nitrate-fed Phaseolus vulgaris L.

R. J. Thomas

Papers

Distribution of nitrate reductase activity in six legumes: the importance of the stem

Distribution of nitrate reductase activity in six legumes: the importance

Effects of temperature on the vegetative and early reproductive growth of a cold-tolerant and a cold-sensitive line of nitrate-fed Phaseolus vulgaris L.