Lateral root frequency decreases when nitrate accumulates in tobacco transformants with low nitrate reductase activity: consequences for the regulation of biomass partitioning between shoots and root1

Accumulation of nitrate in the shoot of low-nitrate reductase tobacco transformants leads to an increase of the shoot:root ratio to higher values than in nitrogen-sufficient wild-type plants, even though the transformants are severely deficient in organic nitrogen. In the present paper, wild-type plants and low- nitrate reductase transformants were grown on vertical agar plates to investigate whether this inhibition of root growth by internal nitrate (i) can be reversed by adding sugars to the roots and (ii) is due to slower growth of the main roots or to a decreased number of lateral roots. When grown with a low nitrate supply, the transformants resembled wild-type plants with respect to amino acid and protein levels, shoot-root allocation, lateral root frequency, and rates of growth. When the transformants were grown with a high nitrate supply in the absence of sucrose they grew more slowly and had lower levels of amino acids and protein than wild-type plants, but accumulated more nitrate and developed a high shoot:root ratio. Root length was not affected, but the number of lateral roots per plant decreased. The slower root growth was accompanied by an increase of the concentration of sugars in the roots. Addition of 2% sucrose to the medium partially reversed the high shoot:root ratio in the transformants, but did not increase the frequency of lateral roots. It is concluded that nitrate accumulation in the plant leads to decreased root growth via (i) changes in carbon allocation leading to decreased allocation of sugars to root growth, and (ii) a decrease in the number of lateral roots and a shift in the sensitivity with which root growth responds to the sugar supply. This revised version was published online in June 2006 with corrections to the Cover Date.     

Plant biomass partitioning and chemical defense: Response to defoliation and nitrate limitation

Summary Plant growth and allocation to root, shoot and carbon-based leaf chemical defense were measured in response to defoliation and nitrate limitation inHeterotheca subaxillaris. Field and greenhouse experiments demonstrated that, following defoliation, increased allocation to the shoot results in an equal root/shoot ratio between moderately defoliated (9% shoot mass removed) and non-defoliated plants. High defoliation (28% shoot mass or >25% leaf area removed) resulted in greater proportional shoot growth, reducing the root/shot ratio relative to moderate or non-defoliated plants. However, this latter effect was dependent on nutritional status. Despite the change in distribution of biomass, defoliation and nitrate limitation slowed the growth and development ofH. subaxillaris. Chronic defoliation decreased the growth of nitrate-rich plants more than that of nitrate-limited plants. The concentration of leaf mono- and sesqui-terpenes increased with nitrate-limitation and increasing defoliation. Nutrient stress resulting from reduced allocation to root growth with defoliation may explain the greater allocation to carbon-based leaf defenses, as well as the defoliation-related greater growth reduction of nitrate-rich plants.     

Growth pattern and carbon allocation to volatile leaf terpenes under nitrogen-limiting conditions in Heterotheca subaxillaris (Asteraceae)

Summary Rosettes of Heterotheca subaxillaris were grown at four levels of nitrate. Individual leaf volatile mono- and sesquiterpene content, leaf nitrogen content, and root and shoot dry weight were measured on individual leaves every two weeks for 18 weeks. Rosettes with the highest nitrate availability had 2.2-fold greater leaf nitrogen levels compared to plants with the lowest availability. As nitrate availability became increasingly limited, carbon allocation to both volatile leaf terpenes and root growht increased. Leaf mono- and sesquiterpene content was greatest in the young leaves of individuals growing at the lowest nitrate availability conditions. Higher levels of carbon-based herbivore-deterring chemicals in nitrate-limited plants may increase net productivity through retention of nitrogen that would otherwise be lost to herbivory.     

Elevated pCO(2 )favours nitrate reduction in the roots of wild-type tobacco (Nicotiana tabacum cv. Gat.) and significantly alters N-metabolism in transformants lacking functional nitrate reductase in the roots

The impact of elevated pCO(2 )on N-metabolism of hydroponically grown wild-type and transformed tobacco plants lacking root nitrate reduction was studied in order to elucidate the effects on (i) nitrate uptake, (ii) long-distance transport of N, (iii) nitrate reduction with emphasis on root-NR, and (iv) the allocation of N between the root and shoot. The findings were related to alterations of growth rates. At elevated pCO(2 )the wild type exhibited higher growth rates, which were accompanied by an increase of NO(3)(-)-uptake per plant, due to a higher root:shoot ratio. Furthermore, elevated pCO(2 )enhanced nitrate reduction in the roots of the wild type, resulting in enhanced xylem-loading of organic N (amino-N) to supply the shoot with sufficient nitrogen, and decreased phloem-transport of organic N in a basipetal direction. Transformed tobacco plants lacking root nitrate reduction were smaller than the wild type and exhibited lower growth rates. Nitrate uptake per plant was decreased in transformed plants as a consequence of an impeded root growth and, thus, a significantly decreased root:shoot ratio. Surprisingly, transformed plants showed an altered allocation of amino-N between the root and the shoot, with an increase of amino-N in the root and a substantial decrease of amino-N in the shoot. In transformed plants, xylem-loading of nitrate was increased and the roots were supplied with organic N via phloem transport. Elevated pCO(2 )increased shoot-NR, but only slightly affected the growth rates of transformed plants, whereas carbohydrates accumulated at elevated pCO(2 )as indicated by a significant increase of the C/N ratio in the leaves of transformed plants. Unexpectedly, the C/N balance and the functional equilibrium between root and shoot growth was disturbed dramatically by the loss of nitrate reduction in the root.     

Tobacco plants that lack expression of functional nitrate reductase in roots show changes in growth rates and metabolite accumulation.

When tobacco is provided with a high nitrate supply, only a small amount of the nitrate taken up by the roots is immediately assimilated inside the roots, while the majority is transported to the leaves where it is reduced to ammonium. To elucidate the importance of root nitrate assimilation, tobacco plants have been engineered that showed no detectable nitrate reductase activity in the roots. These plants expressed the nitrate reductase structural gene nia2 under control of the leaf-specific potato promoter ST-LS1 in the nitrate reductase-mutant Nia30 of Nicotiana tabacum. Homozygous T2-transformants grown in sand or hydroponics with 5.1 mM nitrate had approximately 55-70% of wild-type nitrate reductase acivity in leaves, but lacked nitrate reductase acivity in roots. These plants showed a retarded growth as compared with wild-type plants. The activation state of nitrate reductase was unchanged; however, diurnal variation of nitrate reductase acivity was not as pronounced as in wild-type plants. The transformants had higher levels of nitrate in the leaves and reduced amounts of glutamine both in leaves and roots, while roots showed higher levels of hexoses (3-fold) and sucrose (10-fold). It may be concluded that the loss of nitrate reductase acivity in the roots changes the allocation of reduced nitrogen compounds and sugars in the plant. These plants will be a useful tool for laboratories studying nitrate assimilation and its interactions with carbon metabolism.     

The periplasmic nitrite reductase of Thauera selenatis may catalyze the reduction of selenite to elemental selenium

Thauera selenatis grows anaerobically with selenate, nitrate or nitrite as the terminal electron acceptor; use of selenite as an electron acceptor does not support growth. When grown with selenate, the product was selenite; very little of the selenite was further reduced to elemental selenium. When grown in the presence of both selenate and nitrate both electron acceptors were reduced concomitantly; selenite formed during selenate respiration was further reduced to elemental selenium. Mutants lacking the periplasmic nitrite reductase activity were unable to reduce either nitrite or selenite. Mutants possessing higher activity of nitrite reductase than the wild-type, reduced nitrite and selenite more rapidly than the wild-type. Apparently, the nitrite reductase (or a component of the nitrite respiratory system) is involved in catalyzing the reduction of selenite to elemental selenium while also reducing nitrite. While periplasmic cytochrome C ₅₅₁ may be a component of the nitrite respiratory system, the level of this cytochrome was essentially the same in mutant and wild-type cells grown under two different growth conditions (i.e. with either selenate or selenate plus nitrate as the terminal electron acceptors). The ability of certain other denitrifying and nitrate respiring bacteria to reduce selenite will also be described.     

Interaction of sulfur and nitrogen nutrition in tobacco (Nicotiana tabacum) plants: significance of nitrogen source and root nitrate reductase

Abstract: The significance of root nitrate reductase for sulfur assimilation was studied in tobacco (Nicotiana tabacum) plants. For this purpose, uptake, assimilation, and long-distance transport of sulfur were compared between wild-type tobacco and transformants lacking root nitrate reductase, cultivated either with nitrate or with ammonium nitrate. A recently developed empirical model of plant internal nitrogen cycling was adapted to sulfur and applied to characterise whole plant sulfur relations in wild-type tobacco and the transformant. Both transformation and nitrogen nutrition strongly affected sulfur pools and sulfur fluxes. Transformation decreased the rate of sulfate uptake in nitrate-grown plants and root sulfate and total sulfur contents in root biomass, irrespective of N nutrition. Nevertheless, glutathione levels were enhanced in the roots of transformed plants. This may be a consequence of enhanced APR activity in the leaves that also resulted in enhanced organic sulfur content in the leaves of the tranformants. The lack of nitrate reductase in the roots in the transformants caused regulatory changes in sulfur metabolism that resembled those observed under nitrogen deficiency. Nitrate nutrition reduced total sulfur content and all the major fractions analysed in the leaves, but not in the roots, compared to ammonium nitrate supply. The enhanced organic sulfur and glutathione levels in ammonium nitrate-fed plants corresponded well to elevated APR activity. But foliar sulfate contents also increased due to decreased re-allocation of sulfate into the phloem of ammonium nitrate-fed plants. Further studies will elucidate whether this decrease is achieved by downregulation of a specific sulfate transporter in vascular tissues.     

Cultivar differences in the rate of nitrate uptake by intact wheat plants as related to growth rate

Six Argentinian wheat ( Triticum aestivum L.) cultivars grown in nutrient solutions in controlled environment were compared for their nitrate uptake rates on a root dry weight basis. Up to 3-fold differences were observed among the cultivars at 16, 20 and 24 days from germination, either when measured by depletion from the nutrient solution in short-term experiments, or by total N accumulation in the tissue during 8 days.
No differences in total N concentration in root or shoots were found among cultivars. Although the different cultivars showed significant differences in shoot/root ratio and nitrate reductase activity (EC 1.6.6.1) in the roots, none of these parameters was correlated with the nitrate uptake rate. However, nitrate uptake was found to be positively correlated (r = 0.99) with the shoot relative growth rate of the cultivars. The three cultivars with the highest nitrate uptake rates and relative growth rates showed a positive correlation between root nitrate concentration and uptake. However, this correlation was not found in the cultivars with the lowest growth and uptake rates.
Our results indicate that the difference in nitrate uptake rate among these cultivars may only be a consequence of their differences in growth rate, and it is suggested that at least two mechanisms regulate nitrate uptake, one working when plant demand is low and another when plant demand is high.     

The Distribution of Nitrate Assimilation Between Root and Shoot in Vicia faba L.

Nitrate assimilation was examined in two cultivars (Banner Winterand Herz Freya) of Vicia faba L. supplied with a range of nitrateconcentrations. The distribution between root and shoot wasassessed. The cultivars showed responses to increased applied nitrateconcentration. Total plant dry weight and carbon content remainedconstant while shoot: root dry weight ratio, total plant nitrogen,total plant leaf area and specific leaf area (SLA) all increased.The proportion of total plant nitrate and nitrate reductase(NR) activity found in the shoot of both cultivars increasedwith applied nitrate concentrations as did NO₃^–: Kjeldahl-Nratios of xylem sap. The cultivars differed in that a greaterproportion of total plant NR activity occurred in the shootof cv. Herz Freya at all applied nitrate concentrations, andits xylem sap NO₃^–: Kjeldahl-N ratio and SLA were consistentlygreater. It is concluded that the distribution of nitrate assimilationbetween root and shoot of V. faba varies both with cultivarand with external nitrate concentration. Vicia faba L., field bean, nitrate assimilation, nitrate reductase, xylem sap analysis     

Influence of Vesicular-Arbuscular Mycorrhizal Fungi on the Response of Potato to Phosphorus Deficiency

Morphological and biochemical interactions between a vesicular-arbuscular mycorrhizal (VAM) fungus (Glomus fasciculatum [Thaxt. sensu Gerdemann] Gerdemann and Trappe) and potato (Solanum tuberosum L.) plants during the development of P deficiency were characterized. Nonmycorrhizal (NM) plants grown for 63 d with low abiotic P supply (0.5 mM) produced 34, 52, and 73% less root, shoot, and tuber dry matter, respectively, than plants grown with high P (2.5 mM). The total leaf area and the leaf area:plant dry weight ratio of low-P plants were substantially lower than those of high-P plants. Moreover, a lower shoot:root dry weight ratio and tuber:plant dry weight ratio in low-P plants than in high-P plants characterized a major effect of P deficiency stress on dry matter partitioning. In addition to a slower rate of growth, low-P plants accumulated nonreducing sugars and nitrate. Furthermore, root respiration and leaf nitrate reductase activity were lower in low-P plants than in high-P plants. Low abiotic P supply also induced physiological changes that contributed to the greater efficiency of P acquisition by low-P plants than by high-P plants. For example, allocation of dry matter and P to root growth was less restricted by P deficiency stress than to shoot and tuber growth. Also, the specific activities of root acid phosphatases and vanadate-sensitive microsomal ATPases were enhanced in P-deficient plants. The establishment of a VAM symbiosis by low-P plants was essential for efficient P acquisition, and a greater root infection level for P-stressed plants indicated increased compatibility to the VAM fungus. By 63 d after planting, low-P VAM plants had recovered 42% more of the available soil P than low-P NM plants. However, the VAM fungus only partially alleviated P deficiency stress and did not completely compensate for inadequate abiotic P supply. Although the specific activities of acid phosphatases and microsomal ATPases were only marginally influenced by VAM infection, VAM roots characteristically had a higher protein concentration and, consequently, enhanced microsomal ATPase and acid phosphatase activities on a fresh weight basis compared with NM roots. Morphological and ultrastructural details of VAM plants are discussed in relation to the influence of the VAM symbiosis on P nutrition of potato.     

Natural variation of Arabidopsis response to nitrogen availability

Our understanding of plant growth in response to nitrogen (N) supply is mainly based on studies of mutants and transformants. This study explored the natural variability of Arabidopsis thaliana first to find out its global response to N availability and secondly to characterize the plasticity for growth and N metabolism among 23 genetically distant accessions under normal (N+), limited (N-), and starved (N0) N supplies. Plant growth was estimated by eight morphological traits characterizing shoot and root growth and 10 metabolic parameters that represented N and carbon metabolism. Most of the studied traits showed a large variation linked to genotype and nutrition. Furthermore, Arabidopsis growth was coordinated by master traits such as the shoot to root ratio of nitrate content in N+, root fresh matter and root amino acids in N-, and shoot fresh matter together with root thickness in N0. The 23 accessions could be gathered into four different groups, according to their growth in N+, N-, and N0. Phenotypic profiling characterized four different adaptative responses to N- and N0. Class 1 tolerated N limitation with the smallest decrease in shoot and root biomass compared with N+, while class 2 presented the highest resistance to N starvation by preferential increased root growth, huge starch accumulation, and high shoot nitrate content. In contrast, class 3 plants could tolerate neither N limitation nor N starvation. Small plants of class 4 were different, with shoot biomass barely affected in N- and root biomass unaffected in N0.     

Metabolism of Urtica dioica as dependent on the supply of mineral nutrients

Plants of Urtica dioica L., a very nitrophilous species, were grown in a nutrient solution containing either high (100%) or low (2%) nutrient supply. Part of these plants were subjected to a sudden switch from 100% to 2% or vice versa. Plant weight, sugar and organic nitrogen (both soluble and insoluble) and nitrate content were measured during growth. The activities of two nitrogen assimilating enzymes, nitrate reductase (NR) and glutamine synthetase (GS) were determined.
Growth of Urtica dioica was retarded at low nutrient supply. Root growth was limited by another factor than nitrogen. This was shown by a higher protein content. In the first period after a switch from 100% to 2%, redistribution of nitrogen from shoot to root could be demonstrated, and leakage from the root into the nutrient solution. It is suggested that in these conditions GS in the root reacted to this downward flux. Comparison with earlier findings on the less nitrophilous Plantago lanceolata showed that at 100% nutrient supply a correlation occurs between nitrate reduction and glutamine synthetase activity in that plant part which exported reduced nitrogen: the root in P. lanceolata and the shoot in U. dioica. In the importing plant part, glutamine synthetase was influenced by nitrate reduction as well as by imported reduced nitrogen.     

Allocation responses to CO2 enrichment and defoliation by a native annual plant Heterotheca subaxillaris

Among plants grown under enriched atmospheric CO₂, root:shoot balance (RSB) theory predicts a proportionately greater allocation of assimilate to roots than among ambient‐grown plants. Conversely, defoliation, which decreases the plant's capacity to assimilate carbon, is predicted to increase allocation to shoot. We tested these RSB predictions, and whether responses to CO₂ enrichment were modified by defoliation, using Heterotheca subaxillaris, an annual plant native to south‐eastern USA. Plants were grown under near‐ambient (400 μmol mol^?1) and enriched (700 μmol mol^?1) levels of atmospheric CO₂. Defoliation consisted of the weekly removal of 25% of each new fully expanded, but not previously defoliated, leaf from either rosette or bolted plants. In addition to dry mass measurements of leaves, stems, and roots, Kjeldahl N, protein, starch and soluble sugars were analysed in these plant components to test the hypothesis that changes in C:N uptake ratio drive shifts in root:shoot ratio. Young, rapidly growing CO₂‐enriched plants conformed to the predictions of RSB, with higher root:shoot ratio than ambient‐grown plants (P < 0.02), whereas older, slower growing plants did not show a CO₂ effect on root:shoot ratio. Defoliation resulted in smaller plants, among which both root and shoot biomass were reduced, irrespective of CO₂ treatment (P < 0.03). However, H. subaxillaris plants were able to compensate for leaf area removal through flexible shoot allocation to more leaves vs. stem (P < 0.01). Increased carbon availability through CO₂ enrichment did not enhance the response to defoliation, apparently because of complete growth compensation for defoliation, even under ambient conditions. CO₂‐enriched plants had higher rates of photosynthesis (P < 0.0001), but this did not translate into increased final biomass accumulation. On the other hand, earlier and more abundant yield of flower biomass was an important consequence of growth under CO₂ enrichment.     

Growth and reproduction of Arabidopsis thaliana in relation to storage of starch and nitrate in the wild-type and in starch-deficient and nitrate-uptake-deficient mutants

We have investigated the interactions between resource assimilation and storage in rosette leaves, and their impact on the growth and reproduction of the annual species Arabidopsis thaliana. The resource balance was experimentally perturbed by changing (i) the external nutrition, by varying the nitrogen supply; (ii) the assimilation and reallocation of resources from rosette leaves to reproductive organs, by cutting or covering rosette leaves at the time of early flower bud formation, and (iii) the internal carbon and nitrogen balance of the plants, by using isogenic mutants either lacking starch formation (PGM mutant) or with reduced nitrate uptake (NU mutant). When plants were grown on high nitrogen, they had higher concentrations of carbohydrates and nitrate in their leaves during the rosette phase than during flowering. However, these storage pools did not significantly contribute to the bulk flow of resources to seeds. The pool size of stored resources in rosette leaves at the onset of seed filling was very low compared to the total amount of carbon and nitrogen needed for seed formation. Instead, the rosette leaves had an important function in the continued assimilation of resources during seed ripening, as shown by the low seed yield of plants whose leaves were covered or cut off. When a key resource became limiting, such as nitrogen in the NU mutants and in plants grown on a low nitrogen supply, stored resources in the rosette leaves (e.g. nitrogen) were remobilized, and made a larger contribution to seed biomass. A change in nutrition resulted in a complete reversal of the plant response: plants shifted from high to low nutrition exhibited a seed yield similar to that of plants grown continuously on a low nitrogen supply, and vice versa. This demonstrates that resource assimilation during the reproductive phase determines seed production. The PGM mutant had a reduced growth rate and a smaller biomass during the rosette phase as a result of changes in respiration caused by a high turnover of soluble sugars ( Caspar et al. 1986 ; W. Schulze et al. 1991 ). During flowering, however, the vegetative growth rate in the PGM mutant increased, and exceeded that of the wild-type. By the end of the flowering stage, the biomass of the PGM mutant did not differ from that of the wild-type. However, in contrast to the wild-type, the PGM mutant maintained a high vegetative growth rate during seed formation, but had a low rate of seed production. These differences in allocation in the PGM mutant result in a significantly lower seed yield in the starchless mutants. This indicates that starch formation is not only an important factor during growth in the rosette phase, but is also important for whole plant allocation during seed formation. The NU mutant resembled the wild-type grown on a low nitrogen supply, except that it unexpectedly showed symptoms of carbohydrate shortage as well as nitrogen deficiency. In all genotypes and treatments, there was a striking correlation between the concentrations of nitrate and organic nitrogen and shoot growth on the one hand, and sucrose concentration and root growth on the other. In addition, nitrate reductase activity (NRA) was correlated with the total carbohydrate concentration: low carbohydrate levels in starchless mutants led to low NRA even at high nitrate supply. Thus the concentrations of stored carbohydrates and nitrate are directly or indirectly involved in regulating allocation.     

Decreased Rubisco activity leads to dramatic changes of nitrate metabolism,amino acid metabolism and the levels of phenylpropanoids and nicotine in tobacco antisense RBCS transformants

Tobacco transformants that express an antisense RBCS construct were used to investigate the consequences of a lesion in photosynthetic carbon metabolism for nitrogen metabolism and secondary metabolism. The results show that an inhibition of photosynthesis and decrease in sugar levels leads to a general inhibition of nitrogen metabolism, and dramatic changes in the levels of secondary metabolites. The response was particularly clear in plants that received excess nitrogen. In these conditions, a decrease of Rubisco activity led to an inhibition of nitrate reductase activity, accumulation of nitrate, a decrease of amino acid levels that was larger than the decrease of sugars, and a large decrease of chlorogenic acid and of nicotine, which are the major carbon- and nitrogen-rich secondary metabolites in tobacco leaves, respectively. Similar changes were seen when nitrogen-replete wild-type tobacco was grown in low light. The inhibition of nitrogen metabolism was partly masked when wild-type plants and antisense RBCS transformants were compared in marginal or in limiting nitrogen, because the lower growth rate of the transformants alleviated the nitrogen deficiency, leading to an increase of amino acids. In these conditions, chlorogenic acid always decreased but the decrease of nicotine was ameliorated or reversed. When the changes in internal pools are compared across all the genotypes and growth conditions, two conclusions emerge. First, decreased levels of primary metabolites lead to a dramatic decrease in the levels of secondary metabolites. Second, changes of the amino acid : sugar ratio are accompanied by changes of the nicotine:chlorogenic acid ratio.     

Partitioning of inorganic nitrogen assimilation between the roots and shoots of cerrado and forest trees of contrasting plant communities of South East Brasil

Summary Woody plants growing in cerrado and forest communities of south-east Brasil were found to have low levels of nitrate reductase activity in their leaves suggesting that nitrate ions are not an important nitrogen source in these communities. Only in the leaves of species growing in areas of disturbance, such as gaps and forest margins, were high levels of nitrate reductase present. When pot-grown plants were supplied with nitrate, leaves and roots of almost all species responded by inducing increased levels of nitrate reductase. Pioneer or colonizing species exhibited highest levels of nitrate reductase and high shoot: root nitrate reductase activities. Glutamine synthetase, glutamate synthase and glutamate dehydrogenase were present in leaves and roots of the species examined.¹⁵N-labelled nitrate and ammonium were used to compare the assimilatory characteristics of two species:Enterolobium contortisiliquum, with a high capacity to reduce nitrate, andCalophyllum brasiliense, of low capacity. The rate of nitrate assimilation in the former was five times that of the latter. Both species had similar rates of ammonium assimilation. Results for eight species of contrasting habitats showed that leaf nitrogen content increased in parallel with xylem sap nitrogen concentrations, suggesting that the ability of the root system to acquire, assimilate or export nitrate determines shoot nitrogen status. These results emphasise the importance of nitrogen transport and metabolism in roots as determinants of whole plant nitrogen status.     

Biochemical genetics of further chlorate resistant, molybdenum cofactor defective, conditional-lethal mutants of barley

Summary Three plants, R9201 and R11301 (from cv. Maris Mink) and R12202 (from cv. Golden Promise), were selected by screening M₂ populations of barley (Hordeum vulgare L.) seedlings (mutagenised with azide in the M₁) for resistance to 10 mM potassium chlorate. Selections R9201 and R11301 were crossed with the wild-type cv. Maris Mink and analysis of the F₂ progeny showed that one quarter lacked shoot nitrate reductase activity. These F₂ plants also withered and died in the continuous presence of nitrate as sole nitrogen source. Loss of nitrate reductase activity and withering and death were due in each case to a recessive mutation in a single nuclear gene. All F1 progeny derived from selfing selection R12202 lacked shoot nitrate reductase activity and also withered and subsequently died when maintained in the continuous presence of nitrate as sole nitrogen source. All homozygous mutant plants lacked not only shoot nitrate reductase activity but also shoot xanthine dehydrogenase activity. The plants took up nitrate, and possessed wild-type or higher levels of shoot nitrite reductase activity and NADH-cytochrome c reductase activity when treated with nitrate for 18 h. We conclude that loss of shoot nitrate reductase activity, xanthine dehydrogenase activity and withering and death, in the three mutants R9201, R11301 and R12202 is due to a mutation affecting the formation of a functional molybdenum cofactor. The mutants possessed wild-type levels of molybdenum and growth in the presence of unphysiologically high levels of molybdate did not restore shoot nitrate reductase or xanthine dehydrogenase activity. The shoot molybdenum cofactor of R9201 and of R12202 is unable to reconstitute NADPH nitrate reductase activity from extracts of the Neurospora crassa nit-1 mutant and dimerise the nitrate reductase subunits present in the respective barley mutant. The shoot molybdenum cofactor of R11301 is able to effect dimerisation of the R11301 nitrate reductase subunits and can reconstitute NADPH-nitrate reductase activity up to 40% of the wild-type molybdenum cofactor levels. The molybdenum cofactor of the roots of R9201 and R11301 is also defective. Genetic analysis demonstrated that R9201, but not R11301, is allelic to R9401 and Az34 (nar-2a), two mutants previously shown to be defective in synthesis of molybdenum cofactor. The mutations in R9401 and R9201 gave partial complementation of the nar-2a gene such that heterozygotes had higher levels of extractable nitrate reductase activity than the homozygous mutants.We conclude that: (a) the nar-2 gene locus encodes a step in molybdopterin biosynthesis; (b) the mutant R11301 represents a further locus involved in the synthesis of a functional molybdenum cofactor; (c) mutant Rl2202 is also defective in molybdopterin biosynthesis; and (d) the nar-2 gene locus and the gene locus defined by R11301 govern molybdenum cofactor biosynthesis in both shoot and root.     

Partitioning of Sugar between Growth and Nitrate Reduction in Cotton Roots

The level of endogenous sugars was inversely related to nitrate availability in young cotton (Gossypium hirsutum L.) plants, with high nitrate causing a greater decline in sugar content of roots than of shoots. High nitrate (low sugar) plants also displayed relatively more shoot growth and less root growth than low nitrate (high sugar) plants. These data are consistent with the theory that roots are poor competitors for sugar, and that sugar supply is a major factor limiting root growth in vivo.

The effects of endogenous sugar level on root growth and on nitrate reductase activity in the root were different. When root sugar level was experimentally controlled by varying nitrate concentration in the nutrient solution, root growth was less sensitive than nitrate reductase activity to sugar deficiency. Also, in sterile root tips cultured on media containing a wide range of sucrose concentrations, growth rate was considerably less sensitive to endogenous sugar deficiency than was nitrate assimilation rate. Similarly, in plants which were detopped or girdled, nitrate reductase activity in the roots declined more rapidly than did root sugars, especially glucose and fructose. These results suggest that when sugar is deficient, cotton roots preferentially use it for growth at the expense of nitrate reduction.

     

Effects of root pruning of wheat at different nitrogen levels

In two experiments, wheat plants growing in solutions of different nitrogen concentration were subjected to root pruning. In higher concentrations of nitrogen the growth rate was higher, and the proportional allocation of growth to shoot higher, but pruning did not affect the allocation of growth at either level of nitrogen. This result gives no support to Thornley's source-sink model of the control of shoot: root ratio.