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Effects of pH and Nitrite on Potassium Uptake and Growth of Rice Seedlings
J. Plant Physiol. Vol.
144. pp. 358-361 {1994}
Effects of pH and Nitrite on Potassium Uptake and Growth of Rice Seedlings* FERENC ZSOLDOS, AGNES VASHEGYI,
and ATTILA PECSVARADI
Department of Plant Physiology, Attila J6zsef University, P.O. Box 654, H-6701 Szeged, Hungary Received January 21, 1994 . Accepted March 30, 1994
Summary
The effects of increasing concentrations of NaN02 on the K+ uptake of rice seedlings (Oryza sativa 1. cv. Oryzella) were studied at different pH values. Increasing concentrations of NaN02 caused decreases in the K+ uptake of the roots. When NaN0 3 was present in the uptake solution, marked changes were not observed in the K + uptake. The inhibitory effect of N0 2 - was strongly influenced by the H+ concentration of the outer medium. A decrease of the pH of the external solution led to an increased inhibitory effect of N02 - on both the ion uptake and the growth of the seedlings. However, the reverse was true in the case of Na+ uptake. The inhibitory effect of N02 - was different for roots and shoots. Marked changes in the K + efflux properties were not observed at pH 6. A decrease of the pH of the efflux medium, however, led to an increased loss of K+ from the roots. The results strongly suggest a distinct role of N0 2 - in membrane damage at lower pH.
Key words: ion uptake, ion efflux, nitrite toxicity, Oryza sativa, pH, potassium, shoot-root growth, sodium.
Introduction
In most soils, the level of nitrite (N02 -) is usually negligible, but certain environmental stress conditions, e.g. waterlogging, some soil-applied herbicides, heavy metals, etc., may lead to an accumulation of N0 2 - in the soil solution to concentrations that are toxic to plants (Chalk and Smith, 1983; Haynes, 1986; Haynes and Sherlock, 1986; Marschner, 1986). N02 - is an obligatory intermediate in the assimilation of N0 3 - and is usually present at low concentrations in N0 3 -grown plants (Beevers and Hageman, 1969; Breteler and Luczak, 1982). However, it may accumulate in plant tissues under certain stress conditions and be toxic to plants. It is known, for instance, that some herbicides strongly and selectively inhibit nitrite reductase in leaves and correspondingly increase the N02 - concentration in the tissues (Peirson and Elliot, 1981). It has also been shown that N02 - in the plant tissues may react with metabolites, thereby inhibiting the activity and the synthesis of nitrate reductase (Hisamatsu et al.,
* Dedicated to Prof. Dr. H. K. Lichtenthaler on the occasion of his 60th birthday. © 1994 by Gustav Fischer Verlag, Stuttgan
1988; Takeuchi et aI., 1985). Detailed investigations of the effect of N02 - on plants are therefore of importance. Rice seedlings were selected for our investigations because this plant is cultivated mostly under flooded-soil conditions, and in addition rice seedlings are very sensitive to different environmental stress factors (Zsoldos and Karvaly, 1979).
Materials and Methods
Rice (Oryza sativa L. cv. Oryzella) seedlings were used throughout. Seeds were washed for 6 h in running tap water and then left to germinate on wet filter paper in Petri dishes. After 2 days, the seedlings were placed on stainless screens over glass beakers, transferred to a controlled growth room and cultivated in either 0.5 mM CaS04, full nutrient or N-deficient solution. The composition of the nutrient solution was as follows: NaN0 2 or NaN0 3 from 0.01 to 10 mM, 1 mM KH 2P04, 0.5 mM Na2HP04, 0.5 mM CaCh, 0.5 mM MgS0 4 and micro nutrients as described earlier (Zsoldos et at, 1986). Each beaker contained 300 mL growth solution and 10 seedlings. All nutrient solutions were renewed every second day. The seedlings were illuminated for 16 h with 5 x 50 W Sylvania incandescent
Effects of pH on nitrite toxicity A
Fig. 1: Effects of varying NaN0 2 (A) and NaNO) (B) supply on the K(B6Rb)+ uptake of the roots of 7-day-old rice seedlings at different pH values. Plants were grown in 0.5 mM CaSO. solution at pH 6.5 in the absence of N0 2 - and NO) -. Uptake solution: 1 mM K{86Rb)Cl + 0.5 mM CaCh + NaN0 2 or NaNO} as indicated. Uptake time: 1 h, pH 4 or 6.5 as indicated. SD s 10%.
359
8
-H
0.1
1
5
-H
10
lamps and 8 x 48 W Sylvania F48T12-CW-HO white fluorescent lamps at about 70% relative humidity and 26/21 °C day/night temperatures. The irradiance at the plant level was 60W m- 2• For technical reasons 86Rb (with a specific activity of 29.3 GBq' g-I Rb) was used to monitor the K + movement. The usefulness of this had been investigated earlier (Erdei and Zsoldos, 1977). The concentration of 86Rb in the uptake solution was 555 KBq' L -I. The pH of the absorption solution was initially adjusted to the appropriate value with 0.1 M HCl or 0.1 M NaOH, and was checked after the absorption period. After a l-h uptake time, the roots of the intact plants were rinsed 3 t imes in 400 mL distilled water for 1 min. Roots were then separated and dried and weighed for DW. 86R b activity was measured directly in the roots by -y-spectrometry. For efflux studies, intact seedlings were first allowed to stand for 1 h in 400 mL K(86 Rb )Cl + 0.5 mM CaCh labelled solution. After the uptake period, the roots were rinsed three times in 400 mL distilled water and the labelled root material (seedlings) was immersed in 400 mL 0.5 mM CaCh solution with or without a NaN0 2 supply, at different pH values. After an efflux time of 1 h, their radioactivities were measured directly by -y-spectrometry. K + and Na + contents in the roots were determined by atomic absorption spectrophotometry, as described earlier by Berczy et aI. (1982). The dry weights of the roots and shoots of all plants were determined upon harvesting. Shoots and roots were harvested separately and plant parts were dried at 70°C to constant weight. A series of results from three independent experiments is presented in the Figures. All experiments were carried out with three parallel samples, and the data given are averages.
Results
Increasing concentrations of NaN02 caused decreases in the K + uptake of rice roots (Fig. 1A). The retarding effect of N02 - on the K + uptake of the roots was markedly influenced by the H+ concentration of the uptake solution. A decrease of the pH of the outer medium led to an increased inhibitory effect of N0 2 - on the ion uptake of the roots of rice seedlings. When NaN0 3 was present in the uptake solution, marked changes were not observed in the K+ uptake of rice roots (Fig. 1 B). Treatment with NaN0 2 and NaN03 for 7 days in full nutrient solution had a striking effect on the K+ and Na+ contents of rice roots (Fig. 2). Higher concentrations of NaN02 at pH 4 caused a considerable decrease in K + content, but at pH 6.5 were only slightly effective (Fig. 2 A). The reverse
0.1
1
10
Concoo1nItiDf1 of N-oouroo, mM
CoocenItaticn of /HoUn:e. mM
0.0 0.8 _ 0.7
f
Na-nltr1te
0.6
~ 0.5
~o .•
1
03
0.2
0.1
'f
~
6
=f
g
ci
~dN-eoutoM. ~
Fig. 2: Effects of varying NaN0 2 and NaN0 3 supply on the K + (A) and Na+ (B) contents of the roots of 7-day-old rice seedlings grown at different pH values in full nutrient solution in the presence of N aN02 or NaNO) as indicated. SD did not exceed ± 9 %. (n.d.: not determined)
was true in the case of Na+ content: an increase of the NaN0 2 supply of the growth solution resulted in an increase in the Na+ content too (Fig. 2B). Parallel efflux studies, illustrated in Fig. 3, clearly show that no significant change was observed in the K + efflux of the rice roots at pH 6. In contrast, at pH 4 higher concentrations of NaN0 2 caused a significant increase in the loss of K + ofthe roots. The dry weights of rice seedlings grown in nutrient solution with different N02 - supplies and pH values are shown in Fig. 4. The dry weight of the roots at pH 4 decreased sharply; at pH 6.5, the decrease was not so steep. The shoot growth decreased only at the highest N02 - supply at pH 4. When N0 3 - was present as N-source in the growth solution, the root growth was approximately the same at the two pH values. The dry matter production of the shoots increased with increasing N0 3 - supply (Fig. 4).
360
FERENC ZSOLDOS, AGNES VASHEGY!, and ATIILA PkSVARADI
100 :j)
90
\i
;p
;
90
:::~
m ~t ~;j~~ ~}
i
20
I,
'jl :!!!t
i:
;:~~:
10 .
.~f o
~
d -
~
~
d -
~Q/Na-nI1r1W,
~
mM
Fig. 3: Effects of varying the NaN0 2 supply on the K(86 RbY efflux of the roots of 7-day-old rice seedlings at different pH values. Plants were grown in 0.5 mM caSo 4 solution at pH 6 in the absence of N02 -. Uptake solution: ImM K(86Rb)CI + 0.5mM CaCho Uptake time: 1 h. Efflux medium: 0.5 mM CaCh + NaN02 as indicated. Efflux time: 1 h. Control (0): K(86Rb)+ uptake during 1 h. SD ~9 .
creases in K+ uptake of the roots (Zsoldos et al., 1993). The reverse was true, however, when the seedlings were pretreated with NaN02~ The retarding effect of NaNO z on the K+ uptake and gr~wth of wheat seedlings was strongly influenced by the H+ concentration of the external solution. It is notewortl)y, that the toxic effect of NO z- is believed to be due to undissociated HN02 rather than to NO z- (Breteler and Luczak, 1982; Lee, 1979; Zsoldos et al., 1993). The present experiments show that with decreasing pH the reactivity of N02- increases, leading to physiological disorders and toxicity. It is noteworthy that at pH 4 and in contrast with Na+, the K+ content of the roots was lower in seedlings pretreated with a higher level of NaNO z, which indicates the direct influence of NO z- on the root membrane permeability and the leakage of K + (Fig. 1). Our results show that the decrease in the K + uptake of the roots at higher NaNOz concentration is due to the toxic effect of NO z- and only partly to growing Na+ concentration of the uptake solution. The effect of NaNO z appeared as a combined action of Na+ and NO z-. This range of Na+ concentration exerted a slight reducing effect on K + content, which was intensified by N02-. Using NaNO) instead of NaNO z in the uptake solution did not decrease the ion uptake of roots in the case of rice seedlings (Fig. 2). It seems that high Na+ /K+ ratios may impair the selectivity of root plasma membrane and result in passive accumulation of Na+ in the seedlings (Kramer et al., 1977). In all the studies conducted, a negative effect of NO z- on the ion uptake by rice roots was observed; the growth of the roots was also impaired, even at higher pH values. It is remarkable that, similarly as in wheat seedlings, in rice seedlings the inhibitory effect of NO z- was also different for roots and shoots (Fig. 4). The roots and especially the root hairs (data not shown) were extremely sensitive to N0 2treatment. Acknowledgements
Fig. 4: Effects of varying the NaN0 2 and NaNO) supply on the dry weight of 14-day-old rice seedlings grown in complete nutrient solution at different pH values. SD ~ 1.
Statistical analysis (ANOVA) of data revealed that NaN02 treatment was effective; samples exposed to this agent (except Fig. 3, pH 6) show significant difference compared with the control and with each other. Results of regression analysis in each case suggest that a connection between concentration of nitrite and the resulting effects there of is not linear. SD did not exceed 10%. Discussion
Opinions differ as to whether N02- can serve as a nitrogen source for plants or not (Black, 1985; Breteler and Luczak, 1982; Lee, 1979; Marschner, 1986; Mevius and Dikussar, 1930). We earlier demonstrated that the treatment of wheat seedlings with higher concentrations of NaN02 caused de-
This work was supported in part by a grant from the PHARE ACCORD PROGRAMME (Contract No. H 9112-0124) and by the Hungarian National Science Research Foundation (Project No. 462).
References BEEVEllS, L. and R. H. HAGEMAN: Nitrate reduction in higher plants. Annu. Rev. Plant PhysioL 20, 495-522 (1969). BERCZY, A., Z. OUH, and L. ERDEI: Potassium transport in wheat seedlings grown with different potassium supplies. I. Ion contents and potassium influx. PhysioL Plant. 55, 371- 376 (1982). BLACK Jr., c. C.: Effects of herbicides on photosynthesis. In: DUKE, S. O. (ed.): Weed Physiology, Vol. II. pp. 2-36, CRC Press (1985). BRETELER, H. and W. LUCZAK: Utilization of nitrate and nitrate by dwarf bean. Planta 156, 226-232 (1982). CHALK, P. M. and C. J. SMITH: Chemodenitrification. In: FRENEY, J. R. and SIMPSON (eds.): Gaseous Loss of Nitrogen from Plant-Soil Systems, pp. 65-89, Martinus Nijhoff, Dr. W. Junk, The Hague (1983).
Effects of pH on nitrite toxicity ERDEI, L. and F. ZSOLDOS: Potassium absorption by rice at different levels of organization. 1. Effects of temperature and calcium on K+ fluxes and content. Physio!. Plant. 41, 99-104 (1977). HAYNES, R. ].: Nitrification. In: HAYNES, R.]. (ed.): Mineral Nitrogen in the Plant-Soil System, pp. 127 -165, Acad. Press, New York (1986). HAYNES, R.]. and R. R. SHERLOCK: Gaseous losses of nitrogen. In: HAYNES, R. J. (ed.): Mineral Nitrogen in the Plant-Soil System, pp. 242-302, Acad. Press, New York (1986). HISAMATSU, S.,]. NIHIRA, Y. TAKEUCHI, S. SATOH, and N. KONDO: N0 2 suppression of light-induced nitrate reductase in squash cotyledons. Plant Cell Physio!. 29, 395-401 (1988). KRAMER, D., A. LXUCHLI, A. R. YEO, and]. GULLASCH: Transfer cells in roots of Phaseolus coccineus: Ultrastructure and possible function in exclusion of sodium from the shoot. Ann. Bot. 41, 1031-1040 (1977). LEE, R. B.: The effect of nitrite on root growth of barley and maize. NewPhyto!' 83, 615-622(1979). MARSCHNER, H.: Mineral Nutrition of Higher Plants. Acad. Press, New York (1986).
361
MEVIUS, W. and 1. DIKUSSAR: Nitrite als Stickstoffquellen fur h6here Pflanzen. Ein Beitrag zur Frage nach der Assimilation des Nitratstickstoffs. Jahrb. f. wiss. Bot. 73, 633-703 (1930). PEIRSON, D. R. and J. R. ELLIOT: In vivo nitrite reduction in leaf tissue of Phaseolus vulgaris. Plant Physio!' 68, 1068-1072 (1981). TAKEUCHI, Y., J. NIHIRA, N. KONDO, and T. TEZUKA: Change in nitrate-reducing activity in squash seedlings with N02 fumigation. Plant Cell Physio!. 26, 1027 -1036 (1985). ZSOLDOS, F. and B. KARVALY: Cold-shock injury and its relation to ion transport by roots. In: LYONS,]. M., D. GRAHAM, and]. K. RAISON (eds.): Low Temperature Stress in Crop Plants, pp. 123139, Acad. Press, New York (1979). ZSOLDOS, F., E. HAUNOLD, and A. VASHEGYI: The effects of phosphate supply on uptake of potassium ions, 2,4-D and atrazine by wheat and maize. Physio!. Plant. 68, 154-158 (1986). ZSOLDOS, F., E. HAUNoLD, A. VASHEGYI, and P. HERGER: Nitrite in root zone and its effects on ion uptake and growth of wheat seedlings. Physio!. Plant. 89, 626-631 (1993).
144. pp. 358-361 {1994}
Effects of pH and Nitrite on Potassium Uptake and Growth of Rice Seedlings* FERENC ZSOLDOS, AGNES VASHEGYI,
and ATTILA PECSVARADI
Department of Plant Physiology, Attila J6zsef University, P.O. Box 654, H-6701 Szeged, Hungary Received January 21, 1994 . Accepted March 30, 1994
Summary
The effects of increasing concentrations of NaN02 on the K+ uptake of rice seedlings (Oryza sativa 1. cv. Oryzella) were studied at different pH values. Increasing concentrations of NaN02 caused decreases in the K+ uptake of the roots. When NaN0 3 was present in the uptake solution, marked changes were not observed in the K + uptake. The inhibitory effect of N0 2 - was strongly influenced by the H+ concentration of the outer medium. A decrease of the pH of the external solution led to an increased inhibitory effect of N02 - on both the ion uptake and the growth of the seedlings. However, the reverse was true in the case of Na+ uptake. The inhibitory effect of N02 - was different for roots and shoots. Marked changes in the K + efflux properties were not observed at pH 6. A decrease of the pH of the efflux medium, however, led to an increased loss of K+ from the roots. The results strongly suggest a distinct role of N0 2 - in membrane damage at lower pH.
Key words: ion uptake, ion efflux, nitrite toxicity, Oryza sativa, pH, potassium, shoot-root growth, sodium.
Introduction
In most soils, the level of nitrite (N02 -) is usually negligible, but certain environmental stress conditions, e.g. waterlogging, some soil-applied herbicides, heavy metals, etc., may lead to an accumulation of N0 2 - in the soil solution to concentrations that are toxic to plants (Chalk and Smith, 1983; Haynes, 1986; Haynes and Sherlock, 1986; Marschner, 1986). N02 - is an obligatory intermediate in the assimilation of N0 3 - and is usually present at low concentrations in N0 3 -grown plants (Beevers and Hageman, 1969; Breteler and Luczak, 1982). However, it may accumulate in plant tissues under certain stress conditions and be toxic to plants. It is known, for instance, that some herbicides strongly and selectively inhibit nitrite reductase in leaves and correspondingly increase the N02 - concentration in the tissues (Peirson and Elliot, 1981). It has also been shown that N02 - in the plant tissues may react with metabolites, thereby inhibiting the activity and the synthesis of nitrate reductase (Hisamatsu et al.,
* Dedicated to Prof. Dr. H. K. Lichtenthaler on the occasion of his 60th birthday. © 1994 by Gustav Fischer Verlag, Stuttgan
1988; Takeuchi et aI., 1985). Detailed investigations of the effect of N02 - on plants are therefore of importance. Rice seedlings were selected for our investigations because this plant is cultivated mostly under flooded-soil conditions, and in addition rice seedlings are very sensitive to different environmental stress factors (Zsoldos and Karvaly, 1979).
Materials and Methods
Rice (Oryza sativa L. cv. Oryzella) seedlings were used throughout. Seeds were washed for 6 h in running tap water and then left to germinate on wet filter paper in Petri dishes. After 2 days, the seedlings were placed on stainless screens over glass beakers, transferred to a controlled growth room and cultivated in either 0.5 mM CaS04, full nutrient or N-deficient solution. The composition of the nutrient solution was as follows: NaN0 2 or NaN0 3 from 0.01 to 10 mM, 1 mM KH 2P04, 0.5 mM Na2HP04, 0.5 mM CaCh, 0.5 mM MgS0 4 and micro nutrients as described earlier (Zsoldos et at, 1986). Each beaker contained 300 mL growth solution and 10 seedlings. All nutrient solutions were renewed every second day. The seedlings were illuminated for 16 h with 5 x 50 W Sylvania incandescent
Effects of pH on nitrite toxicity A
Fig. 1: Effects of varying NaN0 2 (A) and NaNO) (B) supply on the K(B6Rb)+ uptake of the roots of 7-day-old rice seedlings at different pH values. Plants were grown in 0.5 mM CaSO. solution at pH 6.5 in the absence of N0 2 - and NO) -. Uptake solution: 1 mM K{86Rb)Cl + 0.5 mM CaCh + NaN0 2 or NaNO} as indicated. Uptake time: 1 h, pH 4 or 6.5 as indicated. SD s 10%.
359
8
-H
0.1
1
5
-H
10
lamps and 8 x 48 W Sylvania F48T12-CW-HO white fluorescent lamps at about 70% relative humidity and 26/21 °C day/night temperatures. The irradiance at the plant level was 60W m- 2• For technical reasons 86Rb (with a specific activity of 29.3 GBq' g-I Rb) was used to monitor the K + movement. The usefulness of this had been investigated earlier (Erdei and Zsoldos, 1977). The concentration of 86Rb in the uptake solution was 555 KBq' L -I. The pH of the absorption solution was initially adjusted to the appropriate value with 0.1 M HCl or 0.1 M NaOH, and was checked after the absorption period. After a l-h uptake time, the roots of the intact plants were rinsed 3 t imes in 400 mL distilled water for 1 min. Roots were then separated and dried and weighed for DW. 86R b activity was measured directly in the roots by -y-spectrometry. For efflux studies, intact seedlings were first allowed to stand for 1 h in 400 mL K(86 Rb )Cl + 0.5 mM CaCh labelled solution. After the uptake period, the roots were rinsed three times in 400 mL distilled water and the labelled root material (seedlings) was immersed in 400 mL 0.5 mM CaCh solution with or without a NaN0 2 supply, at different pH values. After an efflux time of 1 h, their radioactivities were measured directly by -y-spectrometry. K + and Na + contents in the roots were determined by atomic absorption spectrophotometry, as described earlier by Berczy et aI. (1982). The dry weights of the roots and shoots of all plants were determined upon harvesting. Shoots and roots were harvested separately and plant parts were dried at 70°C to constant weight. A series of results from three independent experiments is presented in the Figures. All experiments were carried out with three parallel samples, and the data given are averages.
Results
Increasing concentrations of NaN02 caused decreases in the K + uptake of rice roots (Fig. 1A). The retarding effect of N02 - on the K + uptake of the roots was markedly influenced by the H+ concentration of the uptake solution. A decrease of the pH of the outer medium led to an increased inhibitory effect of N0 2 - on the ion uptake of the roots of rice seedlings. When NaN0 3 was present in the uptake solution, marked changes were not observed in the K+ uptake of rice roots (Fig. 1 B). Treatment with NaN0 2 and NaN03 for 7 days in full nutrient solution had a striking effect on the K+ and Na+ contents of rice roots (Fig. 2). Higher concentrations of NaN02 at pH 4 caused a considerable decrease in K + content, but at pH 6.5 were only slightly effective (Fig. 2 A). The reverse
0.1
1
10
Concoo1nItiDf1 of N-oouroo, mM
CoocenItaticn of /HoUn:e. mM
0.0 0.8 _ 0.7
f
Na-nltr1te
0.6
~ 0.5
~o .•
1
03
0.2
0.1
'f
~
6
=f
g
ci
~dN-eoutoM. ~
Fig. 2: Effects of varying NaN0 2 and NaN0 3 supply on the K + (A) and Na+ (B) contents of the roots of 7-day-old rice seedlings grown at different pH values in full nutrient solution in the presence of N aN02 or NaNO) as indicated. SD did not exceed ± 9 %. (n.d.: not determined)
was true in the case of Na+ content: an increase of the NaN0 2 supply of the growth solution resulted in an increase in the Na+ content too (Fig. 2B). Parallel efflux studies, illustrated in Fig. 3, clearly show that no significant change was observed in the K + efflux of the rice roots at pH 6. In contrast, at pH 4 higher concentrations of NaN0 2 caused a significant increase in the loss of K + ofthe roots. The dry weights of rice seedlings grown in nutrient solution with different N02 - supplies and pH values are shown in Fig. 4. The dry weight of the roots at pH 4 decreased sharply; at pH 6.5, the decrease was not so steep. The shoot growth decreased only at the highest N02 - supply at pH 4. When N0 3 - was present as N-source in the growth solution, the root growth was approximately the same at the two pH values. The dry matter production of the shoots increased with increasing N0 3 - supply (Fig. 4).
360
FERENC ZSOLDOS, AGNES VASHEGY!, and ATIILA PkSVARADI
100 :j)
90
\i
;p
;
90
:::~
m ~t ~;j~~ ~}
i
20
I,
'jl :!!!t
i:
;:~~:
10 .
.~f o
~
d -
~
~
d -
~Q/Na-nI1r1W,
~
mM
Fig. 3: Effects of varying the NaN0 2 supply on the K(86 RbY efflux of the roots of 7-day-old rice seedlings at different pH values. Plants were grown in 0.5 mM caSo 4 solution at pH 6 in the absence of N02 -. Uptake solution: ImM K(86Rb)CI + 0.5mM CaCho Uptake time: 1 h. Efflux medium: 0.5 mM CaCh + NaN02 as indicated. Efflux time: 1 h. Control (0): K(86Rb)+ uptake during 1 h. SD ~9 .
creases in K+ uptake of the roots (Zsoldos et al., 1993). The reverse was true, however, when the seedlings were pretreated with NaN02~ The retarding effect of NaNO z on the K+ uptake and gr~wth of wheat seedlings was strongly influenced by the H+ concentration of the external solution. It is notewortl)y, that the toxic effect of NO z- is believed to be due to undissociated HN02 rather than to NO z- (Breteler and Luczak, 1982; Lee, 1979; Zsoldos et al., 1993). The present experiments show that with decreasing pH the reactivity of N02- increases, leading to physiological disorders and toxicity. It is noteworthy that at pH 4 and in contrast with Na+, the K+ content of the roots was lower in seedlings pretreated with a higher level of NaNO z, which indicates the direct influence of NO z- on the root membrane permeability and the leakage of K + (Fig. 1). Our results show that the decrease in the K + uptake of the roots at higher NaNOz concentration is due to the toxic effect of NO z- and only partly to growing Na+ concentration of the uptake solution. The effect of NaNO z appeared as a combined action of Na+ and NO z-. This range of Na+ concentration exerted a slight reducing effect on K + content, which was intensified by N02-. Using NaNO) instead of NaNO z in the uptake solution did not decrease the ion uptake of roots in the case of rice seedlings (Fig. 2). It seems that high Na+ /K+ ratios may impair the selectivity of root plasma membrane and result in passive accumulation of Na+ in the seedlings (Kramer et al., 1977). In all the studies conducted, a negative effect of NO z- on the ion uptake by rice roots was observed; the growth of the roots was also impaired, even at higher pH values. It is remarkable that, similarly as in wheat seedlings, in rice seedlings the inhibitory effect of NO z- was also different for roots and shoots (Fig. 4). The roots and especially the root hairs (data not shown) were extremely sensitive to N0 2treatment. Acknowledgements
Fig. 4: Effects of varying the NaN0 2 and NaNO) supply on the dry weight of 14-day-old rice seedlings grown in complete nutrient solution at different pH values. SD ~ 1.
Statistical analysis (ANOVA) of data revealed that NaN02 treatment was effective; samples exposed to this agent (except Fig. 3, pH 6) show significant difference compared with the control and with each other. Results of regression analysis in each case suggest that a connection between concentration of nitrite and the resulting effects there of is not linear. SD did not exceed 10%. Discussion
Opinions differ as to whether N02- can serve as a nitrogen source for plants or not (Black, 1985; Breteler and Luczak, 1982; Lee, 1979; Marschner, 1986; Mevius and Dikussar, 1930). We earlier demonstrated that the treatment of wheat seedlings with higher concentrations of NaN02 caused de-
This work was supported in part by a grant from the PHARE ACCORD PROGRAMME (Contract No. H 9112-0124) and by the Hungarian National Science Research Foundation (Project No. 462).
References BEEVEllS, L. and R. H. HAGEMAN: Nitrate reduction in higher plants. Annu. Rev. Plant PhysioL 20, 495-522 (1969). BERCZY, A., Z. OUH, and L. ERDEI: Potassium transport in wheat seedlings grown with different potassium supplies. I. Ion contents and potassium influx. PhysioL Plant. 55, 371- 376 (1982). BLACK Jr., c. C.: Effects of herbicides on photosynthesis. In: DUKE, S. O. (ed.): Weed Physiology, Vol. II. pp. 2-36, CRC Press (1985). BRETELER, H. and W. LUCZAK: Utilization of nitrate and nitrate by dwarf bean. Planta 156, 226-232 (1982). CHALK, P. M. and C. J. SMITH: Chemodenitrification. In: FRENEY, J. R. and SIMPSON (eds.): Gaseous Loss of Nitrogen from Plant-Soil Systems, pp. 65-89, Martinus Nijhoff, Dr. W. Junk, The Hague (1983).
Effects of pH on nitrite toxicity ERDEI, L. and F. ZSOLDOS: Potassium absorption by rice at different levels of organization. 1. Effects of temperature and calcium on K+ fluxes and content. Physio!. Plant. 41, 99-104 (1977). HAYNES, R. ].: Nitrification. In: HAYNES, R.]. (ed.): Mineral Nitrogen in the Plant-Soil System, pp. 127 -165, Acad. Press, New York (1986). HAYNES, R.]. and R. R. SHERLOCK: Gaseous losses of nitrogen. In: HAYNES, R. J. (ed.): Mineral Nitrogen in the Plant-Soil System, pp. 242-302, Acad. Press, New York (1986). HISAMATSU, S.,]. NIHIRA, Y. TAKEUCHI, S. SATOH, and N. KONDO: N0 2 suppression of light-induced nitrate reductase in squash cotyledons. Plant Cell Physio!. 29, 395-401 (1988). KRAMER, D., A. LXUCHLI, A. R. YEO, and]. GULLASCH: Transfer cells in roots of Phaseolus coccineus: Ultrastructure and possible function in exclusion of sodium from the shoot. Ann. Bot. 41, 1031-1040 (1977). LEE, R. B.: The effect of nitrite on root growth of barley and maize. NewPhyto!' 83, 615-622(1979). MARSCHNER, H.: Mineral Nutrition of Higher Plants. Acad. Press, New York (1986).
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