Absorption and accumulation of nitrate in plants: influence of environmental factors.

Plants adopt various strategies to fulfill 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 affinity transport systems. The net uptake of the anion depends upon both influx as well as on its passive efflux. 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.

Response of higher plants to lead contaminated environment.

Lead concentration is increasing rapidly in the environment due to increased use of its sources by human society. Alarming concentrations of the metal have been reported in dust of densely populated urban areas and, water and land of various areas near the industrial waste disposals. Plants absorb lead and accumulation of the metal have been reported in roots, stems, leaves, root nodules and seeds etc. which increases with the increase in the exogenous lead level. Lead affects plant growth and productivity and the magnitude of the effects depend upon the plant species. Photosynthesis has been found to be one of the most sensitive plant processes and the effect of the metal is multifacial. Nitrate reduction is inhibited drastically in roots by the metal but in the leaves a differential effect is observed in various cultivars. Lead also inhibits nodulation, N-fixation and ammonium assimilation in the root nodules. It appears that the toxic effect of the metal is primarily at physiological level and provision of certain inorganic salts can antagonize the toxic effects to some extent. Further responses of plants to the metal depend on various endogenous, environmental and nutritional factors. Some plants are able to tolerate excess of Pb+2 by involving processes like exclusion, compartmentalization or synthesizing metal detoxifying peptides-the phytochelatins.

Responses of greening bean seedling leaves to nitrogen dioxide and nutrient nitrate supply.

Phaseolus vulgaris cv. Kinghorn Wax seedlings grown in darkness at 25 degrees C for 7 days with half strength Hoagland's nutrient solution containing no nitrogen, were transferred to lit continuous stirred tank reactors (CSTRs) in atmospheres containing 0 or 0.3 ppm NO(2) and irrigated with a nutrient solution containing 0 or 5 mm nitrate as sole nitrogen source and allowed to grow for a period of up to 5 days in a 14 h photoperiod. Exposure to NO(2) increased total Kjeldahl nitrogen in the leaves. Further, the exposure to NO(2) increased chlorophyll content from day 3 onwards and inhibited the leaf dry weight substantially on days 4 and 5. The primary leaves of the seedlings exposed to 0.3 ppm NO(2) and supplied with nitrate accumulated some nitrite after 5 days of exposure. Some of the seedlings were returned from CSTRs to growth chambers and allowed to grow for a further period of 5 days in a 14 h photoperiod without NO(2). The growth which developed after the NO(2) exposure growth period, as measured by fresh and dry weights of the leaves, was significantly less in NO(2)-exposed plants than in nitrate-grown plants. The experiments demonstrate that the leaves of greening seedlings are able to assimilate NO(2) and that a reduction in leaf dry weight by prolonged NO(2) exposure in the presence of nutrient nitrate can be associated with nitrite accumulation, and that NO(2) has a carry-over effect beyond the duration of NO(2) exposure. It is apparent that NO(2) induces some durable biochemical or cytological aberration in the presence of nutrient nitrate, which adversely affects subsequent leaf growth.

Effects of nitrogen dioxide and nitrate nutrition on nodulation, nitrogenase activity, growth, and nitrogen content of bean.

The influence of nutrient nitrate level (0-20 millimolar) on the effects of NO(2) (0-0.5 parts per million) on nodulation and in vivo acetylene reduction activity of the roots and on growth and nitrate and Kjeldahl N concentration in shoots was studied in bean (Phaseolus vulgaris L. cv Kinghorn Wax) plants. Exposing 8-day old seedlings for 6 hours each day, for 15 days, to 0.02 to 0.5 parts per million NO(2) decreased total nodule weight at 0 and 1 millimolar nitrate, and nitrogenase (acetylene reduction) activity at all concentrations of nitrate. The pollutant had little effect on root fresh or dry weights. Shoot growth was inhibited by NO(2). The NO(2) exposure increased nitrate concentration in roots only at 20 millimolar nutrient nitrate. Exposure to NO(2) markedly increased Kjeldahl N concentration in roots but generally decreased that in shoots. The experiments demonstrated that nutrient N level and NO(2) concentration act jointly in affecting nodulation and N fixing capability, plant growth and composition, and root/shoot relationships of bean plants.

Effects of nitrogen dioxide and nitrate nutrition on growth and nitrate assimilation in bean leaves.

The influence of nutrient nitrate level (0-20 millimolar) on the effects of NO(2) (0-0.5 parts per million) on growth, K, photosynthetic pigment, N contents, and the activities of enzymes of N assimilation was studied in bean (Phaseolus vulgaris L. cv Kinghorn Wax) leaves. Exposing 7-day old bean seedlings for 5 days continuously to 0.02 to 0.5 parts per million NO(2) increased plant height, fresh weight, chlorophyll, carotenoid, organic N and nitrate contents, and nitrate reductase and glutamate synthase activities in the leaves of seedlings supplied with no external N. At 20 millimolar nitrate, most of the parameters examined were inhibited except for organic N and nitrate contents and glutamate synthase activity which increased in most cases. Generally, with an increase in NO(2) concentration, the stimulatory effect declined and/or the inhibitory effect increased. A 3-hour exposure of 12-day-old bean seedlings to 0.1 to 2.0 parts per million NO(2) increased nitrate content and nitrate reductase activity at each nutrient nitrate level except for a slight inhibition of enzyme activity during exposure to 2.0 parts per million NO(2) at 20 millimolar nitrate. The experiments demonstrated that the effect of NO(2) is strongly influenced by nutrient N level and that NO(2) is assimilated into organic nitrogenous compounds to serve as a source of N, only to a limited extent.

Measurement of nitrite reductase in leaf tissue of Vigna mungo : A new method.

The enzyme nitrite reductase (EC 1.6.6.4) is generally assayed in terms of disappearance of nitrite from the assay medium. We describe a technique which allowed estimation of the enzyme level in leaf tissues of Vigna mungo (L). Hepper in terms of the release of the product (NH3) of the enzyme reaction. The technique is offered as an alternative, possibly more convenient method for assay of nitrite reductase in plant tissue in vivo.

A fungitoxic principle from the leaves of lawsonia inermis lam.

During antifungal screening of higher plants, the leaves of Lawsonia inermis were found to exhibit strong fungitoxicity. On chemical investigation, the antifungal factor was found to be 2-hydroxy-1,4-naphthoquinone (Lawsone). The minimum effective dose against test organism was found to be 1000 ppm. Lawsone was found to exhibit fungicidal activity, wide fungitoxic spectrum and nonphytotoxicity.