Indian Journal of Experimental Biology Vol. 39, February 200 1, pp. 101-110

Review Article

Absorption and accumulation of nitrate in plants: Influence of environmental factors

Bandana Bose Department of Plant Physiology, Institute of Agricultural Sciences, Banaras Hindu University, Varanasi, India

and

H S Srivastava*

Department of Plant Science, M.J.P. Rohilkhand University, Bareilly 243006, India

Plants adopt various strategies to fulfil their nitrogen nutrition requirement, the most important being the uptake of nitrate from the soil and its subsequent assimilation in to amino acids. The uptake of nitrate is energy dependent and is an active process involving high affinity and low affi nity transport systems. The net uptake of the anion depends upon both innux as well as on its passive efnux. When the uptake far exceeds over its assimilation in the plant, there is considerable accumulation of nitrate in the plant parts making them unfit for human and cattle consumption. Various environmental factors affect the uptake and accumulation of nitrate, which along with the genetic component of the plant affecting the net uptake and accumulation of the nitrate, need to be considered and carefully manipulated for effective nitrogen management in the plant, soil and aquatic environment.

Nitrogen is an essential plant nutrient and is of prime importance in the productivity of crops as it exceeds all other elements, as a percentage of plant's dry weight with the exceptions of carbon, hydrogen and oxygen. It is often a limiting nutrient for crop growth and productivity in most agricultural soils. Watson 1, as early as in 1947, showed that nitrogen deficiency reduced the plant growth by restricting leaf area development . A number of researches have shown that nitrogen limits seed yield at maturity. Application of nitrogen fertiliser is always found to improve the growth and development of plants which eventually improves the grain/seed yield . Nitrate or ammonium ions are absorbed by the plant roots and their nitrogen is incorporated eventually in to a variety of vital organic molecules such as amino acids, proteins, nucleic acids etc. Thus, total organic nitrogen content and the major nitrogenous metabolites increase quantitatively during N03- or ammonium supply in plants growing in nitrogen deficient medium. This is also reflected in increased growth and productivity of the plants with nitrogen supply. Among three forms of inorganic nitrogen viz. nitrate, ammonium and di nitrogen, N03- is of the greatest importance to plants raised on arable soil except for legumes and rice. The ammonium and di

*Correspondent author

nitrogen forms of nitrogen are of less importance because their availability and utilisation is often restricted by the physico-chemical nature and the microbial population of the soil. In fact, the N03-assimilation is estimated to produce 2xl04 megatons of organic nitrogen and it is about 100-folds greater than the rate of annual biological nitrogen fixation 2. Most plants prefer N03- over NH/, as a source of nitrogen. This is in spite of the fact that acquisition and assimilation of N03- is more energy demanding than ammonium. Beside, N03- assimilation occurs in various micro-organisms also including bacteria, yeast, fungi, algae etc.

In several instances, the stimulatory effect of N03-on growth and productivity and some related metabolic aspects could not be directly correlated with the assimilation of N03- . It appears therefore, that either N03- per se, or some secondary metabolites derived from N03- assimilation exert some regulatory effects on plants. Several plant biologists have suggested that N03- regulated some plant processes, besides acting as a nutrient nitrogen source3. Schlieble et al.4 used a tobacco mutant with very low N03- reductase activity to demonstrate that N03- ion acted as a signal molecule in carbohydrate metabolism. In soybean embryonic axis, the supply of N03- to the seedlings increases both NR activity and

102 INDIAN J EXP BIOL, FEBRUARY 200 1

nitric oxide synthesis5. Nitric oxide is known to act as a regulator and as a signal molecule in animals, and such a function in plants is also hi ghly probable. Depending upon its concentration, NO appears to act as a stress coping or inhibitory effect on leaf growth

at Ieast6. Further, N03- accumulation in plants, specially in the vegetables7 is also a subject of

concern for human and animal health . Dietary N03-has been linked with the incidence of methemoglobinemia and gastric cancer. Factors

affecting uptake and accumulation of N03- in pl ant system, therefore, need a careful and critical consideration as they might affec t plant growth and development through various mechani sms and may also affect the nutritional value of the plant derived foods and feeds.

Nitrate uptake Nitrate concentration in the soi l fluctuates and is

affected by several fac tors, which are both abiotic and

biotic. An usual range of soil N03- is 0 .1 to 5.0 mM8

•

However, the N03- concentration in the cytoplasm of root cells exposed to N03- has been esti mated in the

range of 5 to 30 mM 9. Obviously, the uptake of N03-by the roots is an energy dependent active process and is responsive to the soluble carbohydrate content of the roots. Roots absorb N03- select ively from the

soil through different types of N03- transporters present in the plasma membrane. On the bas is of various types of physiological and biochemical studies, it has been concluded that pl ants have a very

elaborate system of N03- uptake, which involves both a hi gh affinity transport system (HATS) and low affi nity transport system (LATS) 10• Further, on the basis of the kinetic experiments involving a wide range of N03- concentrations, it has been demonstrated that the high affinity transport system is partly inducible and partly constitutive. Thus, there are three types of N03- transport systems :

1. Constitutive low affinity transport system

(cLATS), that operates at high N03-concentrations ( above - 0.5 mM)

2. Constitutive high affinity transport system (cHATS) which operates be low - 0.5 mM N03-concentration and

3. Inducible high affinity transport system (iHATS) which also operates at lower concentrations of N03- and is inducible by N03-

It has been further demonstrated that at low

external N03- concentration (i.e. less than 1 mM), the uptake kinetics is typical Michaelis-Menten type, while at concentrations above 1 mM the kinetics may be either saturable or linear 11 • The Km for N03-uptake by these two types of transport systems (HATS and LA TS) varies according to the species, but is usually in the range of about 0 .005 to 0.1 mM for low affinity transport system and in millimol ar ranges for high affinity transport system 12• The

capacity for N03- absorption is higher by LATS than that by the HATS . For example, uptake rate in Arabidopsis at 10 mM N03- (by LATS) is about 24

mmol h- 1 g-1 fresh weight as compared to only about

1 mmol hr- 1 g-1 fresh weight by the HATS 13• Thus,

while HATS might be important in N03- absorption

at low soil N03-, LATS might be responsible for the

bulk of the N03- acquired by the plants. In fertile

soils, the N03- concentration is usually in the range of 0 .2 to 5 mM but at times may reach 30 mM after the application of nitrogenous fertili ser and then the low affinity transport system seems to be of . 14. tmportance

Both of the N03- transport systems, seem to

operate via a 2H+: 1 N03- symport15 and thus uptake

of N03- causes alkalisation of the medium. Recently Pouliquin et a/. 16 have described the presence of a N03- uniporter system also in plasma membrane vesicles from maize root cells. The system has an acidic (6.5) optimum pH and is si mil ar to H+-ATPase in its properties. Attempts have been made to characterise the inducible N03- transport proteins in a few systems. A 45 kDa protei n has been detected in the plasma membrane of the cyanobacterium

Synechococcus when fed with N03- , but this protei n is absent in the NH/ fed bacterium 17• In maize root cell's plasma membrane, an induced synthesis of 30-3 1 kDa protein has been demonstrated by Me Clure et a/. 18, while Ni and Beevers 19, in the same system demonstrated the N03- induced synthesis of 33,38,49 and 50 kDa prote ins, of which 33 and 49 kDa proteins were integrated in the plasma membrane. The

recognition of N03- might involve a guanidine group of arginine residue in the transport protein, because ketones and carbonyl compounds that bind to

guanidium groups inhibit N03- uptake in maize roots20.

The molecular genetics of N03- uptake has been examined in severa l rece nt studies. Genes coding for

BOSE & SRIVASTAVA: ABSORPTION AND ACCUMULATION OF NITRATE IN PLANTS 103

N03- transporter in root cells have been isolated and characterised 21 '22 . Two families of genes encoding N03- uptake systems- the Nrtl and Nrt2 (NRT2:1) -have been identified. Both these genes code for carrier proteins that drive N03- uptake by co-transporting at least two protons for every one N03-(ref. 23). Experiments involving expression and functional studies indicate that NRTJ is a low affinity and NRT2 is a high affinity transporter, although the CHLJ (AtNRTJ) is a double affinity N03-transporter24. Huang et a/?5 have isolated a AtNRT1:2 gene for constitutive low affinity N03- transporter from Arabidopsis. This is in addition to the inducible transporter gene CHLJ (NRTJ) reported earlier26. When the gene was injected to Xenopus oocytes, it yielded a Km for N03- of 5.9 mM. A N03- transpoter gene called OsNRTJ has been cloned from rice also, which displayed a low affinity transport activity with a Km value of 9.0 mM in Xenopus oocytes27. Such molecular studies might be helpful in genetic engineering of the crop plants for regulated N03-uptake with a view to increase nitrogen use efficiency and/or to restrict the N03- content of the edible parts of the plant.

The net uptake of N03- in plants is equal to the difference between N03- influx and N03- efflux. Therefore, the uptake of N03- is regulated either by N03- influx or by N03- efflux, both of which are substantial and probably regulated independently. As mentioned earlier influx is a carrier mediated active process. However, the efflux appears to be a channel

d. d . 28 me tate passive process .

Nitrate accumulation

The accumulation of N03- in the plant tissues results from the difference in the absorption and the assimilation of N03- . The amount of N03-accumulated depends upon several factors such as (a) the pl ant species (b) soil N03- content (c) N03-assimilating potential of the plant (d) presence of other nutrients and (e) environmental factors influencing the uptake and assimilation of N03-(Table 1). Species variation in N03- accumulation has been demonstrated by Nambiar et a/.29 who compared the N03- content in the leaves of groundnut, cowpea, soybean, maize, and Sorghum at 66 to 94 days after sowing the seeds and at 0, 100 or 200 kg ha- 1

externally applied nitrogen as urea. At almost all sampling dates , groundnut (non-nodulating) and cow

pea had high N03- content and maize had lower N03-content. On percent basis, tomato may contain about 20% of its total nitrogen in the form of N03-. Usually there is a positive correlation between the soil N03-content and the plant N03- content as has been observed in a variety of species including tomato and beans30·31 even though the relationship may not be exactly linear. However, many reports have shown a negative relationship between N03- uptake rate and No - I . . I 3? 33 Th ' b . I 3 accumu at1on 111 p ants -· . IS o vtous y reflects a complex mechanism of N03- uptake which may not be related to the external N03-concentrations.

Usually roots accumulate more N03- than the shoots (Table 1). Cardenas-Navarra et a/ .34 have demonstrated that in tomato and lettuce, the N03-content is directly related with the water content on g-1 dry weight basis under different light intensities and day/night conditions. They have suggested that changes in tissue N03- content are due to changes in N03- content of a water reservoir of variable size.

The intracellular compartmentation of stored N03-has been examined in a few studies and it has been suggested that the major pool of N03- is vacuolar. With the help of nuclear magnetic resonance (NMR) spectroscopy, Belton et al. 35 demonstrated that the cytoplasmic pool of N03- was small as compared to the vacuolar pool . Many studies have indicated that the range of vacuolar pool of N03- is between 58% and 99% of total N03- (ref. 36) in the protoplast of the different species. In fact the cytoplasmic N03-pool is to be transient because of the activity of nitrate reductase (NR, E.C. 1.6.6.1-2), the enzyme reducing N03- to 111tnte. Any significant accumulation of N03- in the cytoplasm may be either due to activated uptake of the anion from the medium or due to the dimini shed ac tivity of the enzyme.

Factors affecting nitrate uptake and accumulation Plants growing in the natural environment have to

experience varied environmental conditions, which directly or indirectly may affect the uptake and accumulation of N03- also. These factors have to be identified and understood in order to regul ate the uptake, assimilation and accumulation of N03- in plants. Some prominent factors are described in the following paragraphs.

104 INDIAN J EXP BIOL, FEBRUARY 2001

Plant factors The rate of nitrate absorption differs according to

plant species, age, nutrition and growth conditions. The kinetic parameters of N03- absorption also differ according to species. For example, while Km for active absorption is 9.5 J..lmol in Lactuca sativa37 , it is

60 J..lmol in Vicia faba 38 . The uptake rate also varies according to the age of the plant and again, the period or age of maximum rate of N03- absorption differs according to species 39 . The differences in N03-absorption among species and also according to the plant age are perhaps due to differences in the nature

Table 1-Nitrate content of some representative plant species

Species & Plant parts Nutrient N03- status Tissue N03-N Reference J..l g g- 1 Dry wt

Arachis Field grown, 200 hypogaea kg ha- 1 urea -non nodulating/ Leaves 1194 29 -nodulating I Leaves 823 29 -Roots Hydroponically grown 96.2 98

for 66 days in 9 mM KN03 +50 mMNaCI

-S hoots --do- 19.6 98

Glycine max Leaves Field grown, 200. kg ha- 1 urea

-nonodulating 267 29 -nodulating 512 29

Hordeum vulgare -whole plant 15 mM No3- -4000-4200 99 -Roots 10 mMN03- 1300 100 -Shoots 10 mMN03- 1200 100

Phaseolus vulgaris -Shoots 8 mMN03- 800-3800 89 -Roots --do- 3300-11470 89 -Leaves 10 mM N03- for 24 hr 140 56

10 mM N03- for 7 days 1500 30

Pinus lambertiana -Needles Field grown 15-80 10 1

Pisum sativum 2 mM N03- ,hydroponic 1400 102 - Whole plant culture

Pteridiwn aquilinum Leaves --do- 20-100 101

Ricinus communis Nutrient solution 597 103 - Whole plant containing 5 mM N0.1-

Quercus kellogg i -Leaves Field grown 100-500 101

Sisymbrium officinale -Seeds --do- 26.6 104

Sorghum bicolor -Leaves Field grown, 200 kg ha- 1

urea 198 29 Zea mays -Roots 2 mM N03- ,hydroponic 9000- 12000 102

or s:md culture for 17 d -Shoots --do- 8000 102

BOSE & SRIVASTAVA: ABSORPTION AND ACCUMULATION OF NITRATE IN PLANTS lOS

and relative abundance of N03- transporters40

.

Nitrogenous salts and metabolites Nitrogen in the root environment has both positive

as well as negat ive effect on N03- uptake depending upon the form of nitrogenous salt. Induction of N03-

uptake by external N03- has been demonstrated in many studies41 . Several studies have demonstrated that this induction is by N03- per se rather than by any ass imilatory product of the anion 10• The time course of the induction process varies according to species, environmental conditions and nutritional history. The induction appears to be a result from either the derepress ion of N03-transporters already present in the membrane or from the induction of de novo synthesis (as is the case with iHATS) and subsequent inserti on of N03- transporter into the plasmalemma.

In contrast to N03-, N02- and NH/ inhibit N03-

uptake by the pl ants. In barley, N02- appears to be a competitive inhibitor of N03- uptake . It shares the

same transporter and same binding site as N03 -. However, evidences that N03- and N02·- transport may use different transport systems have also been obtai ned. For example, Anti-NR immunoglobin G fragments purified from anti-NR serum inhibited N03- uptake but not the N02- uptake, in barley roots42· Ammonium ion inhibits the net uptake via

stimul ating the effl ux of N03- in several species43

.

However, evidences for inhi biti on via N03- infl ux have been obtained in barle/4• Aslam et al. 45 reported that NH4 +inhibits the hi gh affinity transport system of

N03- uptake. Methylamine, an analogue of NH/ also inhibits net N03- uptake by increasing N03- effl ux

46.

But, methionine sul fox imine, the structural analogue of glutamate and the inhibitor of glutamine synthesis accelerates N03- uptake

47. This is apparentl y because

under the conditions of inhibited glutamine synthesi s, more A TP is spared for the active uptake of N03 -.

Feedback inhibi tion of N03- uptake by nitrogenous metabolites which accumulate under condit ions of nitrogen sufficiency has also been demonstrated . In Arabidopsis thaliana, it has been shown that this inhibition acts at N03- transporter level, where the expression of Nrt2: 1 gene responsib le for in ward N03- transport is upregulated by N03- starvation in wild type and by nitrogenous metabolites in NR deficient mutant48. Thus , it appears that some product

of N03- assimilation rather than the N03- itse lf,

regulates the synthesis of N03- transporter. Exogenously supplied amino acids and amides

inhibit the uptake of N03- in Arabidopsis thaliana49

and dwarf bean50 • Several phloem transported amino

ac ids also reduce N03- uptake in wheat 51 and in

soybean52 . Rufty et at. 53 suggested that thi s type of inhibition of N03- uptake was due to feed back control of N03- transporter.

Chlorate and chlorite, the structural analogues of N03- and N02- also inhibit the N03- uptake and its reduction54 . The magnitude of inhibition at a given concentration of chlorate is almost species independent. For example, the inhibition of N03-uptake by 5mM chlorate is 25% in barley and about 40% in maize 55 .

Nitrate accumul ation in plant parts is also influenced by the nitrogenous salts in the root environment. In most studies, N03- accumulati on has been found to increase with the increase in nutrient N03- concentration

56. However, the amide glutamine

inhibits N03- accumulation in excised maize roots 57

,

although it has no effect when supplied to the intact seedlings58. But the other amide asparagine inhibits N03- accumulation in intact seedlings also

58. The

decreased accumulation in the presence of amides is considered to be the consequence of reduced N03-uptake from the medium.

Carbon metabolites

An increased supply of di ssolved inorganic carbon (NaHC03-) to root increases 10 fo ld incorporat ion of dissolved inorganic carbon and N03- uptake in tomato seedlings59. Fixation of inorganic carbon in roots provides skeleton for assimilation of the NH/ resulting from the reduction of N03- and this in turn

enhances the assimilation of N03- and also increases the uptake of N03- (ref. 59).

Malate appears to be an important metabolite in uptake and translocation of N03- .For example, supplying mal ate to the roots either by addition to the external solution or by increasing arti fic ially the transport from the shoot in the phloem, improves the net N03- uptake rate in soybean seedlings

60. The

uptake of N03- by the root increased when the supply of N03- to the shoot increased and decreased when the activity of nitrate reductase in the shoot was inh ibited by tungstate. It was concluded from these studies that the assimilation of N03- in the shoots

106 INDIAN J EXP BIOL, FEBRUARY 2001

controlled N03- uptake by the roots via malate translocation in the phloem 60.

Atmospheric pollutants The nitrogenous air pollutants specially the NO

and N02 (NOx) are known to affect N03- uptake from the soil as well as the accumulation of N03- in the plant, as they contribute to the N03- pool on their own after their entry to the plant cell s. In most experimental studies, it has been difficult the proportionate the N03- absorbed from the soil from that contributed by the di ssolution of NOx.

Inhibition of soil N03- uptake by 1.1 ml 1-1 N02 for

7 days has been shown in soybean , which is more apparent when the soi l N03- concentration is 1 mM than at 5 ~1 • It has been suggested that thi s inhibition is due to increased proton concentration in the plant cell due to di ssolution of N02 in the cell sap. However, exposure to N02 causes accumulation of N03- in many spec ies

62. A signi ficant portion of thi s

N03- seems to be compartmentalized in the storage pools (vacuoles) . In a study with a wild type cv Zephyr and N02 tolerant mutants (B1 and W5) of barley, it had been found that the inducible activity of nitrate reductase in the leaves was significantly enhanced even after 3 d of the termination of 0.5 ppm N0 2 exposure

63. It was apparentl y due to gradual

mobili sation of N02 derived N03- from storage pool to the metabolic pool.

Evidences are available fo r the acqui sition and accumulati on of nitrogen in the form of N03- and NH/ from the atmospheric nitrogenous gases also. Nitrogen deposition from the nit rogenous pollutants in the atmosphere, is considered to be the major factor in forest dec line in Europe and other heavil y industri alised parts of the world64 . The N03- -N is deri ved from the gases such as NO, N02, N20 , N20 5, HN03 vapours and particul ate N03 - . The rate of N0 3--N deposi tion has been estimated to be as high as 455 mg m-2 hr- 1 by Ceonothus crassifolius65 .

Heavy metals Heavy metals, both essenti al as well as non-

essenti al, are known to affect various plant physiological and biochemical processes. In many instances they have been found to inhibit uptake, even when present in micromolar range.

The presence of 400 mM Al 3+ has no signi ficant effect on 15N03- uptake by the tea plants

66 although

m some other species similar or even lower concentrations inhibit N03- uptake

67-

69. This

inhibition may be due to the di sruption of the plasma membrane by the heavy metal and hence the di sorganisation or blockage of the N03- transport system70• Further, the net uptake may also be reduced due to accelerated efflux of the ion, as has been

d d . h . Al3+ 71 recor e Ill w eat roots Ill response to .

Inhibition of N03- uptake by the heavy metal Cd2+

has been demonstrated in wheat (Tit icum aesti vum)72 , birch (Betula pendula)73 and Helianthus annuus69 . In pea plants, the inhibition of net uptake by 50 mM Cd is complete within 24 hr of heavy metal application, although the inhibition is reversible i.e. when the plants are transferred to Cd free medium, the uptake is normali zed 74 •

Essenti al heavy metals al so inhibit N03- uptake when suppl ied above sub-optimum level. For example, Zn2+ inhibits N03- uptake in Helianthus

69 annuus .

Accumulation of N03- is known to be inhi bited by Cd in bean and in tomato, which is apparentl y due to reduced uptake 75 .

Osmotic stress Osmotic stress caused by ei ther salin ity or by

withholding water usuall y inhibits N03- uptake fro m the medium76 . The threshold level of the salinity which causes a signi ficant inhi bition of N03- uptake, varies according to the species . Inhibiti on of N0 3-uptake has been demonstrated in maize in the presence of 100 mA1 NaCI 77 and by 60 mM of the salt in wheat78 .In hydroponically grown Leucaena leucocephala, 20 mM NaCI had no effect on N0 3-uptake by the seedlings 79.

Salinity has no effec t on N0 3- accumulation ir Lolium seedlings , when the nutrient nitrogen i ~ (NH4)2S04, although it increases accumulation in th( roots in the presence of NH4N0 3 or NaN0 3 (ref. 80) On the other hand, increase in salinity up to 150 m!v causes a gradual dec line in N03- accumulation i1 wheat 81• This is apparently due to decreasing N03 uptake with the increas ing salinity. N02 accumulation in chickpea decreases when water stres is created by withholding water82. As describe earlier, this is apparently due to decreased NO uptake during water stress.

BOSE & SRIVASTAVA: ABSORPTION AND ACCUMULATION OF NITRATE IN PLANTS 107

External pH Nitrate uptake is usually favoured by the acidic pH

of the medium and it decreases with decreasing acidit/3. Aguera et al. 84 and Ni and Beevers20

observed that the uptake of N03- drastically decreased at a pH above 5.5 in sunflower and maize respectively. In maize roots, the total N03- uptake after 100 min of incubation in 0.5 mM Ca(N03) 2 was about 50% lower at pH 8.0 than that at pH 5.520.

In a very few studies with the effects of external pH on N03- accumulation in plants, it has been found that the accumulation in the roots of the maize seedlings is unaltered at pH 4.5 and 6.585 .

Temperature Nitrate uptake is sensitive to temperature and it

depends not only on the temperature of the root environment but also upon the temperature of shoot environment. In barley plants, the uptake is more at low temperature whereas in Pennisetum and maize the uptake is higher at high temperature86 . In maize, 30°/30°C day/night temperature appeared to be more suitable for N03- uptake and dry matter accumulation than the lower temperature86. In soybean also, the total N03- uptake is higher at 22°C than at l4°C87 . Similarly in beech (Fagus sylvatica L.) and spruce (Picea abies L.) the rate of N03- uptake by the roots at 10-l5°C is substantially lower than that at 25°C 88 . It appears therefore, that 20-30°C is the most favourable temperature for N03- uptake in most species.

In bean shoots, accumulation of N03- 1S substantially higher , when the plants are raised at 271ll°C day-night temperature than at 19/7°C (ref. 89). In Lemna fronds also, the accumulation of N03- is higher at higher (23.9°) than at lower (18.3°C) temperature90.This is apparently due to increased N03- uptake at higher temperature.

lrradiance Under experimental conditions, it has been

demonstrated that the increasing irradiance causes a decline in internal N03- concentration

9I. In bean

seedlings, N03- accumulation in roots or shoots in N02 exposed plants also dec.eases with the increase in irradiance92. This is apparently because the increase in irradiance causes an increased N03-reduction and assimilation.

Oxygen availability The requirement of oxygen for N03- uptake

indicates that the uptake of N03- depends upon the metabolic energy derived from the root respiration. From an experiment with Lolium, it has been calculated that uptake of 1 mol of N03- from a solution of 1.5 mM N03-, requires about 29 kJ of energ/3 . The energy is expended in the functioning of the N03- transporters . Therefore creating hypoxia or anoxia may cause a decline in N03- uptake. In sunflower, N03- uptake is inhibited drastically with the withdrawal of 0 2 from the medium

84.

Diurnal variations Diurnal variations in N03- uptake by the plants has

also been demonstrated by a few investigators. In b 94 • h 78 . I 95 d . soy ean , 111 w eat , 111 young tomato p ants an 111

hydroponically grown rose plants96 , it has been demonstrated that the rate of N03- uptake is substantially higher in day time than in night time. This is apparently due to higher soluble sugar content of the roots during day time than in night. However, the accumulation of N03- during light period (or day) is lower than that during dark period97, apparently because the reduction and assimilation of N03- is also higher during the day. Thus, it is always advisable to harvest the vegetables, specially the leafy ones, during the day time, because the levels of N03- will be low in such harvests.

·Conclusion and future prospects The process of uptake and accumulation of N03- in

the plant cells is better understood now than it was about a decade ago, although the processes linked to these aspects are complex and varied. The knowledge of the molecular genetics of N03- transporters , specially of the HATS may help in genetic modification of crop plants, for acquisition of N03-from even low N03- soils . The genetically modified plants may also have very little N03- accumulation in their vegetative and reproductive parts, which are often consumed ·by humans or domesticated cattles. This will not only save a substantial amount of money spent on the manufacture and application of nitrogenous fertilisers, it will also reduce the N03-contamination of soil and water, which is a maj or environmental problem in areas which have intensive cultivation of crops . However, the nature of the nutritive, osmotic or regulatory role of N03- is to be

108 INDIAN J EXP BIOL, FEBRUARY 2001

fully understood, and it has to be demonstrated that even very low levels of N03- are sufficient to perform the non-nutritive roles of the N03 - .

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