I. Stulen

Nitrogen in the Environment

When leafy vegetables such as spinach and lettuce are grown in greenhouses during winter and early spring, i.e. at low light intensity and short day length, they may accumulate a high amount of nitrate in the leaves (Corre and Breimer 1979). A high nitrate content in vegetables is undesirable, because it may be harmful for the consumer. Nitrate itself is not toxic, but it is easily reduced to the toxic compound nitrite. Reduction to nitrite can occur during postharvest storage of vegetables (Aworth et al. 1980), as well as after ingestion as food in saliva and in the gastrointestinal tract (Maynard et al. 1976; Walters and Walker 1979). Acute nitrite toxicity causes a respiratory dysfunction called methaemoglobinaemia. By the oxidation of the ferrous iron of haemoglobin to the ferric form, methaemoglobin is formed which cannot transport oxygen, thereby causing tissue asphyxia. Chronic nitrite poisoning may result in the formation of carcinogenic nitrosamines. These N-nitroso compounds can be formed from nitrite and secondary amine compounds, which often occur in food (Walters and Walker 1979; Vermeer et al. 1998). As yet, the occurrence of (gastric) cancer has not been directly related to the consumption of nitrate, but it is generally accepted that a high nitrate intake should be prevented (Forman et al. 1985; Westgeest 1989).

Atmospheric H2S as sulphur source for Brassica oleracea: consequences for the activity of the enzymes of the assimilatory sulphate reduction pathway

Short-term exposure of Brassica oleracea L. (curly kale) to atmospheric H2S levels (0.2–0.8 μL·L–1), which are sufficient to meet the plants sulphur requirement, resulted in a decrease in the activity of adenosine 5’-phosphosulphate reductase (APR) in the shoot. The reduction in APR activity was maximally 80 % and was already substantial after 1 d exposure to 0.2 μL·L–1 H2S. The activity of APR in the roots remained unaffected upon exposure to all levels of H2S. The activities of ATP-sulphurylase (ATPS), serine acetyltransferase (SAT) and O-acetylserine(thiol)lyase (OAS-TL), in both shoot and roots were not affected upon exposure to H2S levels ranging from 0.2–0.8 μL·L–1. There was a rapid increase in the shoot thiol content, including cysteine, upon H2S exposure and a maximal 3-fold increase in thiol content occurred after 5 h exposure. In the roots, the thiol content was only slightly increased after 2 d H2S exposure. The relationship between the pattern of thiol accumulation and changes in sulphate assimilation upon H2S exposure is discussed.

Effect of N fertilizer top-dressing at various reproductive stages on growth, N2 fixation and yield of three soybean (Glycine max (L.) Merr.) genotypes

Soybean (Glycine max (L.) Merr.) is one of the most important food and cash crops in China and a key protein source for the farmers in northern China. Previous experiments in both the field and greenhouse have shown that N2 fixation alone cannot meet the N requirement for maximizing soybean yield, and that N top-dressing at the flowering stage was more efficient than N top-dressing at the vegetative stages. However, the effect of N fertilizer application at other reproductive stages of soybean is unknown. Thus, a field experiment was conducted to study the effects of N applications at various reproductive stages on growth, N2 fixation and yield of three soybean genotypes. The results showed that starter N at 25 kg ha−1 resulted in minimum yield, total N accumulation and total amount of N2 fixed in all three genotypes. N top-dressing at 50 kg ha−1 at either the V2 or R1 stages, significantly increased N accumulation, yield and total amount of N2 fixed in all three genotypes. However, N top-dressing at the same rate at either the R3 or R5 stage did not show this positive effect in any of the three genotypes. Thus, the best timing for N top-dressing during reproduction is at the flowering stage, which increased seed yield by 21% for Wuyin 9, 27% for You 91-19, and 26% for Jufeng, respectively, compared to the treatment without N top-dressing.

Effects of NaCl salinity on 15N-nitrate fluxes and specific root length in the halophyte Plantago maritima L.

The effect of salinity on nitrate influx, efflux, nitrate net uptake rate and net nitrogen translocation to the shoot was assessed in a 15N steady state labelling experiment in the halophyte Plantago maritima L. raised for 14 days on solution supplied with 50, 100 and 200 mol m−3 sodium chloride or without sodium chloride. Additionally, salinity induced changes in root morphology were determined. Specific root length increased upon exposure to elevated sodium chloride concentrations due to variations in biomass allocation and length growth of the tap root. Changes in root morphology, however, had a minor effect on nitrate fluxes when expressed on a root fresh weight basis. The decreased rate of nitrate net uptake in plants grown on elevated levels of sodium chloride was almost entirely due to a decrease in nitrate influx. Expressed as a proportion of influx, nitrate efflux remained unchanged and was even lower at the highest salinity level. At all sodium chloride concentrations applied the initial rate of nitrogen net translocation to the shoot decreased relative to the rate of nitrate net uptake. It is concluded that under steady state conditions the negative effect of sodium chloride on the rate of nitrate net uptake at non growth-limiting salinity levels was due to the interaction between sodium chloride and nitrate transporters in the root plasma membrane and/or processes mediating the translocation of nitrogen compounds, possibly nitrate, to the shoot.

AMMONIUM AND NITRATE NUTRITION IN PLANTAGO-LANCEOLATA AND PLANTAGO-MAJOR L SSP MAJOR .1. ASPECTS OF GROWTH, CHEMICAL-COMPOSITION AND ROOT RESPIRATION

P. lanceolata andP. major were grown in culture solutions with nitrate or ammonium as the nitrogen source. Dry matter accumulation in the shoot was faster with nitrate than with ammonium, whilst that of the roots was not affected by the nitrogen source. As a consequence, the shoot-to-root ratio was lower with ammonium than with nitrate. InP. lanceolata, dry matter percentage of shoot and root tissue was lower with nitrate nutrition, suggesting better elongation growth than with ammonium. However, in shoot tissue ofP. major the opposite was found. The rate of root respiration declined with time, and this was almost completely due to a declining activity of the alternative path, which amounted to about 30–60% of total root respiration. Respiration via the cytochrome path was for a part of time slightly increased by ammonium, whereas the activity of the alternative path was strongly enhanced. The concentration of ethanol-soluble carbohydrates (SC) in the roots of both species was higher when nitrate was used, but no difference in the concentration of starch was found. When the plants were transferred from one nitrogen source to the other, many parameters, including the concentration of nitrate and chloride, and the shoot to root ratio, adjusted to the new situation in both species.P. lanceolata andP. major were grown in culture solutions with nitrate or ammonium as the nitrogen source. Dry matter accumulation in the shoot was faster with nitrate than with ammonium, whilst that of the roots was not affected by the nitrogen source. As a consequence, the shoot-to-root ratio was lower with ammonium than with nitrate. InP. lanceolata, dry matter percentage of shoot and root tissue was lower with nitrate nutrition, suggesting better elongation growth than with ammonium. However, in shoot tissue ofP. major the opposite was found. The rate of root respiration declined with time, and this was almost completely due to a declining activity of the alternative path, which amounted to about 30–60% of total root respiration. Respiration via the cytochrome path was for a part of time slightly increased by ammonium, whereas the activity of the alternative path was strongly enhanced. The concentration of ethanol-soluble carbohydrates (SC) in the roots of both species was higher when nitrate was used, but no difference in the concentration of starch was found. When the plants were transferred from one nitrogen source to the other, many parameters, including the concentration of nitrate and chloride, and the shoot to root ratio, adjusted to the new situation in both species.

NaCl salinity affects lateral root development in Plantago maritima

Root growth and morphology were assessed weekly in hydroponically-grown seedlings of the halophyte Plantago maritima L. during exposure to 0, 50, 100 and 200 mm NaCl for 21 d. Relative growth rate was reduced by 25% at 200 mm NaCl. The lower NaCl treatments did not affect relative growth rates. Primary and lateral roots responded differently to NaCl. While primary-root length increased at all NaCl concentrations, total lateral-root length increased at 50 and was not affected at 100 mm but was considerably reduced at 200 mm NaCl. NaCl concentrations of 50 and 100 mm, which had no effect on relative growth rate or total lateral-root length, severely affected root branching pattern in that the number of first, second and third order laterals was reduced. At 200 mm NaCl third order laterals were not formed at all. However, mean lateral-root length was increased at all NaCl concentrations and was highest at 200 mm NaCl. We conclude that the increase in total lateral-root length in plants at 50 and 100 mm NaCl was mainly caused by increased length growth, while the decrease in total lateral-root length at 200 mm was the consequence of inhibition of lateral root primordia and / or the activation of apical meristems rather than reduced length growth.

Nitrate transport processes in Fagus-Laccaria-Mycorrhizae

The contribution of influx and efflux of NO3- on NO3- net uptake has been studied in excised mycorrhizae of 18–20 week old beech (Fagus sylvatica L.) trees. Net uptake rates of NO3- followed uniphasic Michaelis-Menten kinetics in the concentration range between 10 μM and 1.0 mM external NO3-, with an apparent Km of 88±7 μM, and a Vmax of 110±7 nmol g-1 root f.wt. h-1. The relative xylem loading of N, i.e. the portion of NO3- taken up that was loaded into the xylem vessels as NO3- plus reduced N, was constant over the concentration range tested (4.6–7.7%). NO3- influx proceeded linearly with increasing external NO3- supply. When the assumed regulators of net NO3- uptake, i.e. NH4+ or L-glutamate, were applied together with NO3-, net uptake rates of NO3- decreased. This inhibitory effect was caused by a reduction of NO3- influx rather than an enhanced efflux. The comparison of the present data with a recent study with non-mycorrhizal beech roots (Kreuzwieser et al., 1997; J. Exp. Bot. 48, 1431–1438) revealed that mycorrhization leads to reduced rates of NO3- net uptake. This effect is caused by reduced influx plus enhanced efflux of NO3- as compared with non-mycorrhizal beech roots.

Elevated Levels of Hydrogen Sulfide in the Plant Environment: Nutrient or Toxin

Hydrogen sulfide (H2S) is a malodorous gas with a typical “rotten egg” odor that can be smelt at levels of 0.02 μl l-1 and higher (for chemical and physical properties of H2S, see the review of Beauchamp et al. 1984). Normally H2S is only present in trace concentrations in the plant environment, but under specific conditions plants may have to cope with elevated levels of H2S in either the pedosphere or atmosphere in both natural vegetation and agriculture. Even though sulfide is a normal intermediate in plant metabolism, the impact of H2S on plants is paradoxical. On the one hand, it may be utilized as a sulfur nutrient, and on the other hand, above a certain threshold level it may negatively affect plant growth and functioning. In this chapter, our present knowledge on the impact of elevated levels of H2S on plants both as nutrient and toxin is reviewed.

Physiological response of soybean genotypes to plant density

Response of soybean (Glycine max (L.) Merr.) to plant density has occupied a segment of agronomic research for most of the century. Genotype differences have been noted especially in response to planting date, lodging problems and water limitation. There is limited information on the physiological growth parameters (such as relative growth rate, leaf area ratio, specific leaf area ratio and shoot/root) that corresponds to different soybean genotypes response to different plant densities and how these parameters affect plant yield. The purpose of this study was to investigate the effect of plant density on biomass production, N2 fixation and yield of different soybean genotypes by comparing the current farmer’s practice to other planting densities in the field. Further experiments in controlled climatic condition studied the physiological changes of growth parameters to different plant densities by different soybean genotypes. The result of the field experiment showed that total biomass, plant N derived from N2 fixation and seed yield of all three genotypes responded positively to increased plant density. Double density significantly increased seed yield per unit of area, by 93, 37 and 43% for the determinate Wuyin9, the semi-determinate You91-19 and the indeterminate Jufeng, respectively. The effect of plant density on individual plant characteristics varied in this experiment. Double density had no significant effect on individual plant biomass and seed weight per plant in the determinate genotype Wuyin9, but had a significant negative effect in the later maturing genotypes, with a reduction of 19% in biomass for both determinate Jufeng and semi-determinate You91-19. Detailed experiments under controlled climatic conditions in a growth chamber and a greenhouse in nutrient solution and soil were conducted to determine the physiological variations of growth parameters between the different soybean genotypes in response to plant density. The results showed that the negative response of the individual indeterminate Jufeng plant to planting at double density was mainly due to increases in leaf area ratio (LAR) and specific leaf area (SLA). The greenhouse experiments gave results similar to those obtained in the field. Double density had no significant effect on growth and yield of individual plants of the determinate genotype Luyuebao while it significantly decreased both biomass and yield of individual plants by 40% for semi-determinate You91-19 and 46–47% for indeterminate Jufeng, respectively. For all genotypes, yield per unit of area was higher at double than at single density. Seed yield reduction at single density was mainly associated with a decrease in pod or seed number per unit of area.

Interactions between nitrogen and carbon metabolism in a whole plant context

Investigations on the interactions between nitrogen and carbon metabolism are abundant both at a cellular, detached plant part and intact plant level. Reduction and assimilation of nitrate is integrated with photosynthesis, photorespiration and dark respiration in both shoot and roots in various ways. An outline of the interactions between the metabolic pathways wi1l be given, with special emphasis on the interactions between metabolic pathways involving C and N metabolism in intact plants as investigated in Dr. Hofstra’s group in recent years. Thus the availability of reductant for the functioning of nitrate reductase in the intact plant, the level of glutamate dehydrogenase in relation to carbohydrate level, and the compartmentation of nitrate and carbohydrates will receive attention. Possible regulatory mechanisms controlling the integration of nitrogen and carbon metabolism will be discussed.