| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Physiology and Biochemistry |
Department of Biology, P.O. Box 331, Middle Tennessee State University, Murfreesboro, Tennessee 37132 USA
Received for publication July 26, 2002. Accepted for publication December 5, 2002.
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Key Words: growth • Leguminosae • Leucaena leucocephala • NaCl • nitrogen fixation • percentage of nitrogen • salt tolerance
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
). One possible solution to improve the productivity of modern agroecosystems is to encourage symbiotic economic and ecological interactions between woody and nonwoody components that will sustain, increase, and diversify the total land output (Steppler, 1987
). When farming systems incorporate perennial trees and shrubs, they have the advantage of producing fuelwood, fruit, fodder, and other products along with annual crops. Tropical, deep-rooted trees that can fix atmospheric nitrogen (N2) have the additional advantage of adding natural fertilizers to the soil. One such tree that perfectly fits into this modern agricultural scheme is Leucaena leucocephala L.
Leucaena leucocephala is a fast-growing, tropical leguminous tree that originated in Central America. The planting of multipurpose trees like L. leucocephala not only eliminates the slash/burn form of cultivation, but also increases the economic status of these tropical farmers (IDRC, 1982
; Steppler, 1987
; Swaminathan, 1987
). Today L. leucocephala is grown in Central and South America, Africa, Australia, Southeast Asia, and most of the tropical islands for a variety of purposes, such as soil improvement, reforestation, fire breaks, wood, animal feed, and several other minor uses (Brewbaker, 1987
).
The tree as a whole is used in alley cropping, as an overstory plant for shade-loving plants (e.g., cocoa, coffee, vanilla), and in reforestation (National Research Council, 1984
). The foliage is used as mulch or is incorporated as green manure (National Research Council, 1984
). One hectare of L. leucocephala foliage can produce about 500 kg of nitrogen, roughly equivalent to 2500 kg of ammonium sulfate (Despande, 1981
). In studies on L. leucocephala grown with maize, 1 kg of L. leucocephala foliage increased grain yield by 14.8 kg if mulched and by 21.1 kg if incorporated as green manure (Brewbaker, 1987
). When used as an overstory plant, the annual dry matter is approximately equivalent to 8400 kg/ha (Brewbaker, 1987
). Leucaena leucocephala is also a prime candidate for various reforestation programs because it not only encourages growth of hardwoods, but it can also be used to choke out economically unimportant grasses like Imperato sp. that take over barren lands left behind by slash/burn cultivation (Brewbaker, 1987
).
Leucaena leucocephala leaves are also highly valuable as fodder for ruminants. The palpable leaves of L. leucocephala contain 20–30% protein and are 70% digestible (National Research Council, 1984
). When compared with acacia and alfalfa, both N2-fixing legumes, L. leucocephala had twice the amounts of nitrogen, potassium, and calcium of acacia and twice the amounts of riboflavin, vitamin K, and betacarotene of alfalfa. All other nutrients were comparable (National Acadamy of Sciences, 1977
; National Research Council, 1984
; Toky and Singh, 1995
).
Leucaena leucocephala is known to grow in soils that are subject to salt accumulation. Extensive studies have been conducted on NaCl tolerance of leguminous plants that fix nitrogen. Salt stress can lead to stunted plant growth, reduction in available photosynthates (Brugnoli and Lauteri, 1991
; Georgiev and Atkins, 1993
), and reduced nodulation (Sprent and Zahran, 1988
; Quifu and Murray, 1993
). Studies done with alfalfa (Keck et al., 1984
), Vivia faba (Yousef and Sprent, 1983
; Cordovilla et al., 1996
), chickpeas (Elsheikh and Wood, 1990
), and soybean (Singleton and Bohlool, 1984
) have shown that NaCl concentration >0.05 mol/L reduced plant growth, depressed N2-fixation, reduced nodule numbers, and decreased percentage of tissue nitrogen.
Despite extensive studies on NaCl tolerance of leguminous plants that fix nitrogen, very few long-term studies have been conducted on L. leucocephala. A 110-d study on L. leucocephala (var. K-8 and var. K-63) showed that both have a low tolerance for NaCl when grown in 0.1 and 0.2 mol/L NaCl with var. K-8 being more tolerant (Hansen and Munn, 1984
). Another study (Hansen and Munns, 1988
) on L. leucocephala var. K-8 showed the effect of CaSO4 and NaCl on plant growth and total percentage of tissue nitrogen, but N2 fixation was not assessed. In both these studies the plants were subjected to NaCl only after they were well-nodulated; thus, experiments on seedling growth and nodulation in varying concentrations were not conducted. The purpose of this study was to evaluate the effect of various concentrations of NaCl on nodule number and mass, dry tissue mass, percentage of tissue nitrogen, nitrogenase activity (N2 fixation), and plant height measurements in L. leucocephala var. K-8.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
). Nitrogen-free nutrient solution was used to promote proper nodule development. Moisture content was monitored three times a week and plants were watered when necessary. The plants were randomly repositioned on the greenhouse bench to prevent edge effects in growth.
Salt treatment
The plants were divided into four groups. Group one was treated for 7 wk, group two for 14 wk, group three for 21 wk, and group four for 28 wk. Each group had 48 plants. The 7-wk difference between the groups was chosen to study the effect of NaCl on percentage of nitrogen in various plant organs (leaf, stem, roots, and nodules) as the plant progressed in age. Each group was further subdivided into six subgroups (eight plants/subgroup) based on the molarity of NaCl: control (NaCl-free), 0.1 mol/L NaCl, 0.05 mol/L NaCl, 0.025 mol/L NaCl, 0.0125 mol/L NaCl, and 0.00625 mol/L NaCl. NaCl treatment was initiated when the plants were 10 wk old. The 28-wk plants were subjected to NaCl treatments when the plants were 12 wk old to ensure adequate nodule formation, because these plants were used in acetylene reduction and aboveground growth measurement studies. All plants were gradually introduced to increased NaCl concentrations, starting with 0.00625 mol/L, to avoid large, initial decreases in water potential. Maintenance of plants was as explained previously. The plants and the trays holding the plants were flushed weekly with deionized water to avoid increased concentrations of NaCl in the vermiculite.
Nitrogen content
At the end of each treatment (7 wk, 14 wk, 21 wk, and 28 wk), the leaves, stem, roots, and nodules of each plant were separated. The number of nodules and fresh mass (in grams) of nodules of each plant were recorded. The separated plant tissues were placed in a 39°C oven to dry. The nodules were dried with the root tissue. The dry tissue samples were weighed and ground in a Wiley mill. Samples were subjected to Kjeldahl digestion and distillation (Burris and Wilson, 1957
). A small sample of the distillate was used to determine nitrogen content using the Nessler's reaction (Burris and Wilson, 1957
), and percentage of nitrogen was calculated.
Growth measurements
Aboveground measurements of the plants treated for 28 wk were taken every 2 wk starting at week one of NaCl treatment. Plant height (in centimeters) of each of plant was recorded. At the end of the 28-wk treatment, nodule count and fresh nodule mass (in grams) were taken to assess the effect of NaCl on nodule development.
Nitrogenase activity
Nitrogenase activity (N2 fixation) in the plants treated for 28 wk was measured using the acetylene reduction assay (ARA) (Balandreau and Dommergues, 1973
). Once every 2 wk, starting at week 1 of NaCl treatment, each of the plants was enclosed in a 27 x 28 cm airtight Ziploc bag fitted with a 6-cm-long tube and rubber stopper. The tube was attached to the bag using epoxy glue. An assay bag (32 x 40 cm) was used when the plants outgrew the smaller bag. Later, when the plants outgrew both bags, the root system was sealed in the bag using vacuum grease and spring-lock clips. The plant was first placed in the assay bag with its leaves exposed. The assay bag was then zipped, leaving a small opening around the stem. A small amount of vacuum grease was applied around the stem. Clips were placed on the zipper part of the assay bag, on either side of the stem, and pulled toward the stem, thus creating an airtight seal. Acetylene, generated from CaC2 and water, was injected into the bag to yield a 10% acetylene atmosphere. A 3-cm3 gas sample was collected from one of the bags immediately after injecting acetylene to check for background ethylene. The plants were then placed away from direct sunlight for 2–3 h to prevent overheating inside the assay bags. After a 2- or 3-h assay, a 3-cm3 gas sample was drawn from the assay bag and injected into a 3-mL vacutainer. Each gas sample was analyzed using a Varian model 3700 gas chromatograph fitted with a porapak N column (80/100 mesh; 200 x 0.3175 cm) to determine ethylene content. The background ethylene sample was also quantified and subtracted from the ethylene in the experimental samples. N2 fixation was expressed as micromoles of ethylene per plant per hour.
Statistical analysis of data
All data were analyzed by ANOVA and Student-Newman-Keuls Multiple Comparison Procedure using SigmaStat Statistical Software (SPSS, Chicago, Illinois, USA). Data did not require transformation.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
|
|
|
Effect of NaCl on percentage of nitrogen
As early as 7 wk after beginning treatment, there was a significant (P < 0.05) reduction in the percentage of nitrogen in leaf (Fig. 4) and root (Fig. 5) tissues subjected to NaCl concentrations of 0.025 mol/L and higher. However, by 28 wk after the onset of treatments, percentage of nitrogen did not differ significantly between control and 0.00625 mol/L and 0.0125 mol/L NaCl in stems (Fig. 6) and roots (Fig. 5). This effect was contrary to mean tissue mass in these organs (Figs. 1 and 3).
|
|
|
Effect of NaCl on nodule number and fresh mass
Some interesting results were obtained regarding the effect of NaCl on the number of nodules per plant (Fig. 7). As would be expected, higher NaCl concentrations (
0.025 mol/L) resulted in the loss of nodulation by 28 wk after beginning treatment. Earlier in the treatment, nodulation occurred at the highest NaCl concentration, although at a significantly lower level than in control plants. What is interesting to note, however, is that by 21 and 28 wk after the beginning of treatment, the nodulation in plants receiving the lower concentrations of NaCl was significantly (P < 0.05) greater than that of the control plants.
|
|
|
0.025 mol/L were dead. Plants treated with NaCl concentrations <0.025 mol/L did grow between weeks 6 and 10, but only minimally, from week 10 to week 18. Between week 18 and week 28, plants treated with 0.00625 and 0.0125 mol/L NaCl grew rapidly, and by week 23, the mean heights did not differ significantly (P < 0.05) from that of the control plants. An interesting note is that the number of nodes did not increase after 18 wk, but plants continued to increase in height, indicating that the growth was due primarily from an increase in internode length, rather than the formation of new apical tissues.
|
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
, 1988
). Most researchers have assessed the effect of NaCl on nodule number, mass, and structure, dry tissue mass, percentage of nitrogen in tissues, nitrogenase activity, and growth. A majority of these studies used NaCl concentrations equal to or less than 0.05 mol/L to test salinity tolerance. In this study, NaCl concentrations as low as 0.00625 mol/L were used to study the salinity tolerance of L. leucocephala. Another key feature of the present study was the duration of the treatments. Adequately nodulated L. leucocephala plants were treated with varying concentrations of NaCl for 7, 14, 21, or 28 wk to study the effects of different concentrations of NaCl on nodule number and mass, dry tissue mass, and percentage of nitrogen. This lengthy duration allowed for the assessment of the effects of NaCl during different growth periods. The most extensive portion of this study was conducted on plants treated for 28 wk. These 28-wk-old plants were used not only in the studies mentioned, but also in the study on NaCl effects on nitrogenase activity and aboveground growth.
NaCl caused some interesting trends in nodule number and mass of L. leucocephla plants. During the first 7 wk, plants treated with any concentration of NaCl were similar to control plants, but during the next 7 wk the nodule mass of plants treated at each of the different concentrations was less than the control plants. At 14 wk of treatment, distinct differences began to occur between the various treatments, with plants treated with NaCl 0.025 mol/L having the greatest decrease. A study on soybeans (Singleton and Bohlool, 1984
) corroborated this trend, showing that NaCl concentration of 0.026 mol/L decreased nodule number and mass by 50%. The depressive effect of NaCl concentrations 0.05 mol/L was also observed in chickpeas (Elsheikh and Wood, 1990
) and in peas and faba-beans (Delgado et al., 1994
). In a study on L. leucocephala (Hansen and Munns, 1988
), addition of 0.025 mol/L CaSO4 to plants treated with 0.05 mol/L NaCl increased nodular dry mass. Calcium is important in maintaining selective permeability of membranes, while sodium can increase membrane leakage rates (Hansen and Munns, 1988
).
Another interesting trend was seen in the 21- and 28-wk L. leucocephala plants treated with 0.0125 mol/L NaCl. These plants produced more nodules than the control or 0.00625 mol/L NaCl-treated plants, but most of the nodules were tiny. A study on the NaCl tolerance of L. leucocephala-specific Rhizobium (Rafiq, 1997
) has shown that Rhizobium species are tolerant to NaCl concentrations as high as 0.05 mol/L with a maximum growth rate in 0.0125 mol/L NaCl. At 0.0125 mol/L NaCl, Rhizobium apparently is actively infecting roots, and nodules develop, but conditions within the nodules prevent them from enlarging. In studies on white lupin (Fernandez-Pascual et al., 1996
) growing in NaCl concentrations of 0.15 mol/L, potassium was substituted with sodium within the cortex of the nodules. Sodium thus replaced a major plant mineral that could interfere with the proper functioning of plant metabolism. A similar problem may be arising in the nodules of L. leucocephala treated with NaCl concentrations 0.0125 mol/L.
Results of the acetylene reduction assay closely relate to the results of nodule number and mass. In the 28-wk study, NaCl concentrations 0.025 mol/L severely depressed nitrogenase activity in L. leucocephala plants. By week 11, plants treated with NaCl concentrations 0.025 mol/L were either dead or showed minimal activity. Acetylene reduction decreased after 24 wk in the 0.0125 mol/L NaCl-treated plants. This decrease probably relates to the size of the nodules in the 21- and 28-wk treatments.
One possible reason for small nodules and decreased acetylene reduction in the 0.0125 mol/L NaCl-treated plants could be due to a drop in bacteroid respiration and a reduction in leghemoglobin content. A study on the effects of salinity on growth, nodulation, acetylene reduction, nodule leghemoglobin content, and respiratory capacity of bacteroids in peas (Pisum sativum), faba-beans (Vicia faba), bean (Phaseolus vulgaris), and soybeans (Glycine max) have shown some interesting results (Delgado et al., 1994
). In peas and faba-beans (both transport nitrogen from nodules via amides), NaCl concentration 0.05 mol/L greatly reduced the respiratory capacity of the bacteroids resulting in reduced oxygen uptake for N2-fixation. In contrast, respiratory capacity did not decrease in beans and soybeans (both transport nitrogen from nodules via ureides). Delgado et al. (1994)
showed reduced leghemoglobin in peas and faba-beans, moderate reduction in beans, but no reduction in soybeans. Peas and faba-beans also had low acetylene reduction activity when compared to soybeans. A similar interrelationship between reduced acetylene reduction activity and low leghemoglobin content was observed in studies on Medicago sativa (Becana et al., 1986
) and Vicia faba (Guerin et al., 1990
). The depressive effects of NaCl concentration 0.05 mol/L on nodulation and N2 fixation were also observed in Vicia faba, Glycine max, Medicago sativa, and Phaseolus vulgaris (Bernstein and Ogata, 1966
; Lakshmi et al., 1974
; Abdil-Ghaffar et al., 1982
; and Cordovilla et al., 1996).
Results of the dry tissue mass in L. leucocephala plants showed major fluctuations in NaCl tolerance in plants treated for 14 wk. The stem, root, and leaf tissues had different levels of tolerance to NaCl at different growth periods. During the first 7 wk of treatment, the stem dry tissue mass of plants treated with NaCl concentrations 0.0125 mol/L was less than in the control plants, while root and leaf tissues of the same plants treated with varying concentrations of NaCl had the same dry tissue mass as the control plants. Between 7 and 14 wk of treatment, the dry tissue mass of the leaves subjected to NaCl had less mass than control plants. At this growth stage, the tolerance level of the roots increased and stem tissue maintained the same tolerance levels. After 14 wk of treatment, the plants began to stabilize, and the root, stem, and leaf tissue within a particular growth period had the same level of NaCl tolerance. In the 14- and 21-wk treatment periods, NaCl concentrations 0.05 mol/L had the most negative effect on dry tissue mass. Results obtained from the 14- and 21-wk treatment periods were similar to studies done on other salt-sensitive legumes in which NaCl concentrations 0.05 mol/L reduced shoot dry mass in chickpeas (Elsheikh and Wood, 1990
) and in peas and faba-beans (Delgado et al., 1994
). Shoot dry mass of soybeans was not reduced by 0.05 mol/L NaCl (Singleton and Bohlool, 1984
; Delgado et al., 1994
).
The major fluctuations in percentage of nitrogen in plants of L. leucocephala treated with varying concentrations of NaCl showed trends similar to that of results of dry tissue mass during the first 14 wk of treatment. During this growth phase, NaCl concentrations 0.025 mol/L caused the greatest reduction in percentage of nitrogen in leaf and root tissue. Between 14 and 21 wk, these plants overcame their intolerance to 0.025 mol/L NaCl, and only NaCl concentrations 0.05 mol/L depressed percentage of nitrogen in leaves and roots. A 1-yr study on L. leucocephala grown without NaCl showed that maximum nitrogen was found in the leaves and nodules (DuBois et al., 1990). In the present study, the percentage of nitrogen in stem tissue was depressed by only 0.1 mol/L NaCl. As the plant matures, the tolerance of the stem tissue to NaCl increases, while that of the leaves decreases. The present study found NaCl concentrations as low as 0.0125 mol/L decreased acetylene reduction activity. This decline in N2 fixation probably caused a decrease in the flow of nitrogen from the nodules to the leaves. Here, NaCl is probably causing some metabolic or structural change that is reducing the flow of nitrogen into the leaves, but not interfering in the stem tissue. Concentrations of NaCl 0.05 mol/L also markedly reduced percentage nitrogen in chickpeas (Elsheikh and Wood, 1990
), soybeans (Elsheikh and Wood, 1995
), and peas and faba-beans (Delgado et al., 1994
). Tolerance differences seen in the dry tissue mass of the 21- and 28-wk treatment were also observed in the percentage of nitrogen results.
In general, NaCl concentrations 0.025 mol/L drastically reduced both plant height and leaf growth. Hansen and Munns (1984)
stated that 0.05 mol/L and 0.1 mol/L NaCl reduced plant height in L. leucocephala, but did not show data. A greenhouse study on 36 species of Acacias in which a stepwise increase of NaCl (addition of 0.025 mol/L NaCl every 2 d until plants died) showed that the salt tolerance ranged from 0.04 to 1.3 mol/L NaCl (Aswathappa et al., 1986
). Because Acacia species appear more salt tolerant and grow in habitats where L. leucocephala performs poorly (Aswathappa et al., 1986
; Brewbaker, 1986
), the two studies are not equivalent; therefore, comparisons cannot be made. Studies on soybeans (Elsheikh and Wood, 1995
), chickpeas (Elsheikh and Wood, 1990
), and peas and faba-bean (Delgado et al., 1994
) showed reduced shoot growth in these plants when they were treated with NaCl concentrations of 0.05 mol/L and 0.1 mol/L. This reduced shoot growth was based on reduction in nodule number and mass, percentage of nitrogen, and dry tissue mass. Plant height was not measured in any study. In Phaseolus vulgaris, concentrations of 0.05 mol/L NaCl caused stunted growth from a salt-induced reduction in photosynthates (Brugnoli and Lauteri, 1991
).
Unlike the results for growth, acetylene reduction, and percentage of total nitrogen in L. leucocephala, NaCl concentrations of 0.05 mol/L and 0.1 mol/L only reduced seed germination by 10%. Seventy percent of the seeds treated with 0.025 mol/L NaCl germinated compared with 50% of the control seeds (soaked in deionized H2O). Thus, NaCl concentrations 
0.1 mol/L probably do not interfere with seed germination.
Overall, the results of nodule number and mass, acetylene reduction, dry tissue mass, percentage of nitrogen and aboveground growth indicate that NaCl concentrations as low as 0.025 mol/L can cause severe damage to L. leucocephala plants. Two trends were observed in this study. The first trend, seen in the 7-, 14-, and 21-wk treatments, showed that as plants matured, NaCl tolerance increased. Second, seasonal changes caused differences in NaCl tolerance. Plants grown and harvested during the cooler season (the 14- and 21-wk treatments) had a major reduction in dry tissue mass and percentage of nitrogen when treated with NaCl concentrations of 0.05 mol/L and 0.1 mol/L. In the 28-wk treatment, plants were started in the cooler months, but were harvested in summer. In these plants, NaCl concentrations as low as 0.025 mol/L reduced dry tissue mass and percentage of nitrogen.
Another aspect of L. leucocephala should not be ignored; unlike L. leucocephala, most of the other legumes mentioned are annuals with a short life cycle. Leucaena leucocephala is a perennial tree and appears to have a higher tolerance to NaCl as the plant matures. These attributes may encourage the introduction of L. leucocephala into habitats once believed to be too harsh for its survival. Several long-term field studies must be conducted to fully understand the impact of this study because L. leucocephala is grown not only for N2 fixation, but also for a variety of other important purposes.
|
FOOTNOTES |
|---|
|
LITERATURE CITED |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Aswathappa N. N. E. Marcar L. A. J. Thomson 1986 Salt tolerance of Australian tropical and subtropical Acacias. In J. W. Turnbull [ed.], Australian Acacias in developing countries. Proceedings of an international workshop held at the Forestry Training Centre, 70–73. Australian Centre for International Agricultural Research (ACIAR), Gympie, Queensland, Australia
Balandreau J. Y. Dommergues 1973 Assaying nitrogenase (C2H2) activity in the field. Bulletin of the Ecological Research Committee 17: 247-254
Becana M. P. Aparicio-Tego J. Pena J. Aguirreolea M. Sanchez-Diaz 1986 N2-fixation (C2H2-reducing activity) and leghaemoglobin content during nitrate and water-stress-induced senescence of Medicago sativa root and nodules. Journal of Experimental Botany 37: 597-605
Bernstein L. G. Ogata 1966 Effects of salinity on nodulation, nitrogen fixation, and growth of soybeans and alfalfa. Agronomic Journal 58: 201-203
Brewbaker J. L. 1986 Performance of Australian Acacias in Hawaiian nitrogen-fixing tree trials. In J. W. Turnbull [ed.], Australian Acacias in developing countries. Proceedings of an international workshop held at the Forestry Training Centre, 180–184. Australian Centre for International Agricultural Research (ACIAR), Gympie, Queensland, Australia
Brewbaker J. L. 1987 Leucaena: a multipurpose tree genus for tropical agroforestry. In H. A. Steppler and P. K. R. Nair [eds.], Agroforestry, a decade of development, 289–320. International Council for Research on Agroforestry (ICRAF), Nairobi, Kenya
Brugnoli E. M. Lauteri 1991 Effects of salinity on stomatal conductance, photosynthetic capacity, and carbon isotope discrimination of salt-tolerant (Gossypium hisutum L.) and salt-sensitive (Phaseolus vulgaris L.) C3 non-halophytes. Plant Physiology 95: 628-635
Burris R. H. P. W. Wilson 1957 Methods for measurement of nitrogen fixation. Methods Enzymology 4: 355-367[ISI]
Cordovilla M. P. F. Ligero C. Lluch 1996 Growth and nitrogen assimilation in nodules in response to nitrate levels in Vicia faba under salt stress. Journal of Experimental Botany 47: 203-210
Delgado M. J. F. Ligero C. Lluch 1994 Effects of salt stress on growth and nitrogen fixation by pea, faba-bean, common bean and soybean plants. Soil Biology and Biochemistry 26: 371-376
DuBois J. D. H. C. Winter E. E. Dekker 1990 Free amino acid pools, 15N2 fixation, and fixed n2 assimilation in Leucaena leucocephala var. K-8. American Journal of Botany 77: 316-322[CrossRef][ISI]
Despande V. V. 1981 Leucaena leucocephla: a wonder tree. In R. N. Kaul, M. G. Gogate, and N. K. Mathur [eds.], Leucaena leucocephala in India. Proceedings of National Seminar at Bharatiya Agro Industries Foundation, Urulikanchan, 15-19. Vangard Press, Dehra Dun, India
Elsheikh E. A. E. M. Wood 1990 Effect of salinity on growth, nodulation and nitrogen yield of chickpea (Cicer arietinum L). Journal of Experimental Botany 41: 1263-1269
Elsheikh E. A. E. M. Wood 1995 Nodulation and N2 fixation by soybean inoculated with salt-tolerant Rhizobia or salt sensitive Bradyrhizobia in saline soil. Soil Biology and Biochemistry 27: 657-661[CrossRef]
Fernandez-Pascual M. C. de Lorenzo M. R. de Felipe S. Rajalakshmi A. J. Gordon B. J. Thomas F. R. Minchin 1996 Possible reasons for relative salt stress tolerance in nodules of white lupin cv. Multolupa. Journal of Experimental Botany 47: 1709-1716
Georgiev G. I. C. A. Atkins 1993 Effects of salinity on N2 fixation, nitrogen metabolism and export and diffusive conductance of cowpea root rodules. Symbiosis 15: 239-255
Guerin V. J. C. Trinchant J. Rigaud 1990 Nitrogen fixation (C2H2 reduction) by broad bean (Vicia faba L.) nodules and bacteroids under water-restricted conditions. Plant Physiology 92: 595-601
Hansen E. H. D. N. Munns 1984 Screening of Leucaena leucocephala for NaCl tolerance. Leucaena Research Reports 5: 77-88
Hansen E. H. D. N. Munns 1988 Effects of CaSO4 and NaCl on growth and nitrogen fixation of Leucaena leucocephala. Plant and Soil 107: 94-99
IDRC (International Development Research Centre). 1982 Leucena research in the Asia-Pacific region. Proceedings of Workshop, IDRC and Nitrogen Fixing Tree Association (NFTA), Singapore
Keck T. J. R. J. Wagenet W. F. Campbell R. E. Knighton 1984 Effects of water and salt stress on growth and acetylene reduction in alfalfa. Soil Science Society of America Journal 48: 1310-1316
Lakshmi K. M. C. S. Singh N. S. Subba Rao 1974 Root hair infection and nodulation in lucerne (Medicago sativa) influenced by salinity and alkalinity. Plant and Soil 40: 261-268[CrossRef][ISI]
National Academy of Sciences. 1977 Leucaena: promising forage and tree crop for the tropics. National Academy of Sciences, Washington, D.C., USA
National Research Council. 1984 Leucaena: promising forage and tree crop for the tropics, 2nd ed. National Academy Press, Washington, D.C., USA
Quifu M. A. A. F. Murray 1993 Effects of SO2 and salinity on nitrogenase activity, nitrogen concentration and growth of young soybean plants. Environmental and Experimental Botany 33: 529-537[CrossRef][ISI]
Rafiq S. 1997 Effects of different salt concentrations on the growth of Rhizobium (Osmoadaptation). Scientia [online serial]. Available at http://www.mtsu.edu/
scientia
Singleton P. W. B. Bohlool 1984 Effect of salinity on nodule formation by soybean. Plant Physiology 74: 72-76
Sprent J. I. H. H. Zahran 1988 Infection, development and functioning of nodules under drought and salinity. In D. P. Beck and L. A. Materon [eds.], Nitrogen fixation by legumes in Mediterranean agriculture, 145–151. Martinus Nijhoff, Dordrecht, Netherlands
Steppler H. A. 1987 ICRAF: a decade of agroforestry development. In H. A. Steppler and P. K. R. Nair [eds.], Agroforestry, a decade of development, 13–24. International Council for Research on Agroforestry (ICRAF), Nairobi, Kenya
Summerfield R. J. P. A. Huxley F. R. Minchin 1977 Plant husbandry and management techniques for growing grain under simulated tropical conditions in controlled environment. Experimental Agriculture 13: 81-92[ISI]
Swaminathan M. S. 1987 ICRAF: a decade of agroforestry development. In H. A. Steppler and P. K. R. Nair [eds.], Agroforestry, a decade of development, 25–41. International Council for Research on Agroforestry (ICRAF), Nairobi, Kenya
Toky O. P. V. Singh 1995 Nutrient dynamics in Leucaena, Acacia, and Eucalyptus, short rotation, high density (‘energy’) plantations in arid India. Journal of Arid Environments 31: 401-413
Yousef A. N. J. I. Sprent 1983 Effects of NaCl on growth, nitrogen incorporation and chemical composition of inoculated and NH4NO3 fertilized Vicia faba (L.) plants. Journal of Experimental Botany 34: 941-950
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
