Effect of shading on growth and dinitrogen fixation of kudzu and tropical pasture legumes

Soil Science and Plant Nutrition ISSN: 0038-0768 (Print) 1747-0765 (Online) Journal homepage: http://www.tandfonline.com/loi/tssp20 Effect of shading on growth and dinitrogen fixation of kudzu and tropical pasture legumes Kounosuke Fujita, Katsushi Matsumoto, Godfred K. Ofosu-Budu & Shoitsu Ogata To cite this article: Kounosuke Fujita, Katsushi Matsumoto, Godfred K. Ofosu-Budu & Shoitsu Ogata (1993) Effect of shading on growth and dinitrogen fixation of kudzu and tropical pasture legumes, Soil Science and Plant Nutrition, 39:1, 43-54, DOI: 10.1080/00380768.1993.10416973 To link to this article: https://doi.org/10.1080/00380768.1993.10416973 Published online: 04 Jan 2012. Submit your article to this journal Article views: 185 View related articles Citing articles: 7 View citing articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalinformation?journalcode=tssp20 Download by: [46.3.199.146] Date: 25 November 2017, At: 16:10

Soil Sci, Plant Nutr., 39 (1), 43-54, 1993 43 Effect of Shading on Growth and Dinitrogen Fixation of Kudzu and Tropical Pasture Legumes Kounosuke Fujita, Katsushi Matsumoto, Godfred K. Ofosu-Budu, and Shoitsu Ogata Faculty of Applied Biological Science, Hiroshima University, Higashihb'oshima, 724 Japan Received November 5, 1991 Dry matter production and dinitrogen fixation characteristics in kudzu (Pueraria lobata Ohwi) and some tropical pasture legumes, centro (Centrosema pubescens) and siratro (Macroptilium atropurpureum cv siratro) were studied under full sunlight and shaded conditions. Nodulating (T202) and nonnodulating (T201) soybean (Gl!tcine max {L} Merr.) isolines, were used as reference crops. The results obtained showed that: 1) Under full sunlight conditions, the whole plant weight at final harvest was in the following order: siratro > T202 > kudzu > centro>>t201. However the root to shoot ratio was significantly higher in kudzu than in the other species, particularly at the later growth stages (105 d after planting). 2) Total plant N content and carbohydrate accumulation in the legumes followed the same order as that of the whole plant weight. Higher nitrogen (N) and starch accumulation were observed in kudzu roots at the later growth stages, with a simultaneous reduction in the shoot N amount, suggesting that shoot N was retranslocated to the roots. 3) Under full sunlight conditions, the dinitrogen fixing activity estimated by the 15N dilution method was highest in siratro and lowest in kudzu, while, under shaded conditions, the dinitrogen fixation estimated by the total N difference method decreased significantly in siratro but only slightly in kudzu. 4) Except for kudzu under mild shading conditions (55% shading) where an increase in the whole plant weight and a fairly constant N amount were observed, shading reduced the total plant weight and N amount in all the legumes. 5) Shading reduced the root weight, N and carbohydrate amounts in kudzu, while its effect on the shoot weight and dinitrogen fixation activity was negligible. Based on these results, it is suggested that kudzu was the most shadetolerant legume among the species in this study. This is mainly due to the higher activities of dinitrogen fixation and shoot growth at the expense of root growth. Key Words: legumes. dinitrogen fixation, growth, kudzu, shading, tropical pasture

44 K. FUJITA et al. No suitable leguminous crop in terms of high dry matter production and biological nitrogen fixation (BNF) which is adapted to the climatic conditions of the southwestern district of Japan has been identified yet. The district is characterized by a warm summer with an average temperature of about 24.8~ a total precipitation of about 460 mm distributed in two peaks, the highest falling between late June until mid July and the other in September. The two rainfall peaks are interrupted by a period of low rainfall and solar radiation after summer. These weather changes are characterized by a high temperature at the mid-vegetative stage and a low solar radiation at the later growth stages of most crops. These factors were considered in our attempt to select a legume suitable for the climatic conditions prevailing in this district. The introduction of tropical pasture legumes was considered due to their inherent tolerance to high temperature. Siratro is a perennial, prostrate tropical legume with characteristic thickened roots containing large amounts of carbohydrates. Centro is also a tropical legume but with small roots. Kudzu, however is a temperate legume with growth and morphological characteristics similar to those of siratro, especially the roots contain large amounts of carbohydrates. Although Tsugawa et al. (1989) studied the morphological and growth characteristics of the kudzu plant, the information relating to its physiological characteristics is limited. The characteristic performance of tropical legumes under the climatic conditions in southwestern Japan has not been well documented. Effective forage yield is represented by the shoot biomass. The biomass production depends on the leaf area and photosynthetic activity of the leaf. Thus the partitioning of the dry matter between the shoot and root can be used as a criterion in selection. Since the major source of nitrogen (N) of legumes is mainly through BNF, the partitioning of fixed-n between shoot and roots may also play a role in forage legume production. Variations in yield reduction caused by shading in some tropical forage legumes (Eriksen and Whitney 1982) have been reported. The yields of Desmodium cv. Greenleaf and centro were not appreciably affected while that of siratro was significantly reduced by shading. The poor growth of siratro under reduced sunlight (Ludlow et al. 1974) has also been reported. Although kudzu is mostly cultivated as a cover crop to suppress weeds, improve soil fertility under plantation trees, the response of kudzu to shading has not yet been elucidated. Decreased acetylene reduction activity following shading in white clover (Trifolium repens L.) (Moustafa et al. 1969) and some tropical pasture legumes (Eriksen and Whitney 1982) has been reported. In addition, the deleterious effects of shading on the nodulation of some tropical pasture legumes (Whiteman and Lulham 1970) have been reported. Chu and Robertson (1974) attributed the decreased acetylene reduction activity to the decrease in the nodule number rather than to the decrease in the specific activity. We report in this paper, a study on the growth and dinitrogen fixing ability of kudzu and other tropical pasture legumes under full sunlight and shaded conditions, using soybean as the reference crop. MATERIALS AND METHODS Experiment 1: Growth and dinitrogen-fixing activity under field conditions. Kudzu (Pueraria lobata Ohwi), two tropical pasture legumes, siratro (Macroptilium atropurpureum cv. siratro) and centro (Centrosema pubescens), and nodulating (T202) and nonnodulating (T201) soybean (Glycine max L.) isolines, were grown in concrete frames in the feld. The bottom-less concrete frames (90 90 cm) containing granite regosol received each

Effect of Shading on BNF and Dry Matter Production 45 of the following; 2 g N m -2 as ~SN-ammonium sulfate (10 atom % excess), 15 g P2Os m ~ as superphosphate, and 15 g K20 m -2 as potassium sulfate, prior to planting. Kudzu and the tropical pastures legumes were inoculated by mixing soil collected from fields where the respective plants had been cultivated during the previous year. The soil for inoculation purposes (about 5 kg) was mixed with a required amount of fertilizers for the respective plants and 5 kg of granite regosol using a pot mixer (Nikko Co. Ltd., Type NM 25BM) before filling the concrete frames. Soybean seeds were inoculated with a slurry of commercial preparation of Bradyrhizobium japonicum. The soil ph was adjusted with dolomitic calcium carbonate to about 6.0. The plants were spaced at 10 cm 10 cm and thinned to one plant per hill, equivalent to 100 plants per m 2. In order to restrict the growth of kudzu and the tropical pastures legumes within the specified area, the plots were partitioned with iron screens (1 m height), which also acted as a support for the legumes of the vining type. Irrigation and weeding were applied when necessary. The data on the weather conditions of the experimental site are presented in Fig. 1. The value of solar radiation which varied considerably after planting approximately exceeded 1,465 J cm -2 until the middle of September and decreased afterwards. The average air temperature was about 24.8~ during the growing period. The temperature which was lower after planting until the middle of July, increased to over 30~ during the first week of September, and thereafter decreased continuously. The total amount of rainfall throughout the growth period was approximately 460 mm. A greater amount of precipitation was however observed during the early part of plant growth for 3 weeks, starting from the end of June to mid July, and also during mid-september. Plants were harvested at 56, 105, and 122 d after planting (DAP), and separated into E u X o ~3 ca 213 15 lo- r m el, E a ( ) I! 60 50 E 40 o 3O m c~ o oi lri June July August I I September OctoBer 20 10 o Fig. 1. Solar radiation, average temperature and precipitation during the experimental period, O, solar radiation; Q, average temperature; ~; precipitation.

46 K. FUJITA et al. leaves, stem and petioles, roots and nodules. The leaf area was measured immediately with an auto-area meter (Hayashi Co. Ltd.). Data on the dry weight of the plant parts, dried at 800C in an air-forced oven for more than 3 d were collected. Ground samples were used for the determination of the total N contents (Kjeldahl method) and the ~SN atom % (mass spectrometric method, Hitachi RMI-2), sugar and starch contents (sulfuric acid anthrone method). The total nodule activity (TNA) of the nodules for three plants at 60 and I05 DAP was determined by the acetylene reduction method described by Hardy et al. (1968). Since the N content of the soil used in this experiment was extremely low 1.3 10-4 kg kg-' and the supply of soil N for plant growth was limited, the ~SN dilution method was found to be useful for the estimation of the N economy of the system. The amount of fixed-n throughout the growth period of a given legume species was estimated by the lsn dilution method using the formula described by Peoples and Herridge (1990), P=I I~N atom % excess legume N XSN atom % excess soil N and fertilizer N where P is the proportion of N derived from N2 fixation to the total N in plants. The total N difference method was also used in estimating the fixed-n amount using T201 as the non-fixing plant. Experiment 2: Effect of shading on growth and dinitrogen fixing activity. Legumes species except for T202 and other cultivation methods used were similar to those described in Experiment 1. Two shading levels, 55 and 25% of the incoming solar radiation were applied at 76 DAP when the reference soybean plants were in the pod elongation phase. Shading levels were achieved by using black screens stretched horizontally at a height of about one metre above the plants. Plants were harvested 34 d after the application of the treatment. Data on teal area, and dry weights were collected. The total N, sugar and starch contents of various plant parts were determined using the methods described in Experiment 1. All the plants in individual plots were sampled in each sampling period with two to four replications in both experiments. RESULTS Experiment 1: Growth and dinitrogen-fixing activity under field conditions Dry weight. Except for the nonnodulating soybean isoline T201 where dry matter accumulation was not observed after 56 DAP, the whole plant weight of all the species increased considerably until 102 DAP, after which a fairly stable dry weight persisted (Fig. 2). Dry matter accumulated at the final harvest was in the order of siratro> T202~_kudzu> centro>)t201. Shoot weight accounted for about 90% of the whole plant weight of the tropical pasture legumes and soybean isoline T202. However in kudzu, the root weight accounted for about 40% of the whole plant weight. While the kudzu root weight increased rapidly at 102 DAP and after, there was a simultaneous decrease in the shoot weight. Centro exhibited a greater nodule to root weight ratio than the other legumes (data not shown). N concentration. The leaves showed the highest and the root the lowest N concentration in all the species (Fig. 3). Generally, the leaf N concentration of the tropical pasture legumes and the root N concentration of kudzu at 105 DAP were higher than those of the other species.

Effect of Shading on BNF and Dry Matter Production 47 Fig. 2. Dry weight of plant parts of kudzu and tropical pasture legumes at different growth stages. [Z~, leaves and stem; ~[gg~od, pods; [~]], fallen parts; ~, roots and nodules. N amount. N accumulation and distribution showed a similar pattern to that of the whole plant weight and its distribution among plant parts (Fig. 4). A larger amount of N was accumulated in the roots of kudzu compared to the other species. Except for kudzu, above 90% of the total plant N was found in the shoot. The N amount of fallen plant parts accounted for about 8% of the whole plant N in siratro. Carbohydrate concentration. The sugar concentration ranged between 1.8 and 10.5 10.2 kg kg -1. Although no definite trend among plant parts or between species was observed (Fig. 3), the starch concentration of the siratro and kudzu roots increased at the later growth stages. Except for the soybean isoline, the starch concentration was considerably higher in the roots than in the other plant parts. The root starch concentration of kudzu and siratro was significantly higher than that of the other species. The starch concentration increased by about 43 10 -z kg kg -~ for kudzu and 31 10.2 kg kg -~ for siratro roots at the final harvest. Carbohydrate amount. The whole plant carbohydrate, starch plus sugar amount increased with growth except for a slight decrease in T201 (Fig. 5). Kudzu accumulated a larger carbohydrate amount than the other species, mainly due to the considerable accumulation in the roots. The accumulation of carbohydrates in the roots was observed around 105 DAP in kudzu and tropical pasture legumes, and the rate of accumulation was greater in kudzu. Dinitrogen-fixing activity (TNA). The TNA of centro was the highest and of kudzu the lowest for the first 105 d, among the legumes examined (Table I). The higher TNA of centro was due to its higher specific nodule activity (SNA) and larger nodule weight. Siratro showed a relatively lower TNA due to its smaller nodule weight. The TNA of kudzu was consistently low throughout the growth period apparently due to its extremely small nodule weight. A decrease in TNA in all the legumes was observed at 60 DAP and after, particularly in T202. The amount of fixed-n estimated by the ~SN dilution method was

48 K. FUJITA et al. Leaves & 5 tom Roots &Nodules 2 o U3..ffiffi g~ C 0 I I E I I I L 56 102 122 56 1(~ 122 Days after planting Fig. 3. Nitrogen, sugar and starch concentrations of plant parts of kudzu and tropical pasture legumes at different growth stages. siratro;,% kudzu; ~, centro; C), T202. highly correlated with the value estimated from the total N difference method (r=0.999"*), using T201 as the non-fixing plant (Table 1). Based on the XSN-dilution method, over 95% of the N accumulated by the legumes was derived from the dinitrogen fixing activity of the root nodules. Siratro showed the largest and kudzu, the smallest amount of N fixed among the legumes, respectively. Experiment 2: Effect of shading on growth and dinitrogen-fixing activity Dry weight. Shading decreased the leaf area of all the species and the degree of reduction was lowest in kudzu (Fig. 6). Under full solar radiation, siratro recorded the largest, and kudzu the smallest whole plant weight among the legumes, respectively (Fig. 6). Shading reduced the whole plant weight. The magnitude of the decrease was small in kudzu compared to the other two tropical pasture legumes. While the whole plant weight of the tropical pasture legumes decreased by shading, medium shading (55%) unexpectedly increased the whole plant weight of kudzu. Differential response of the plant parts to shading was observed in kudzu; compared to the roots, the shoot weight showed a marginal decrease under 25% shading conditions. N concentration. N concentration in shoot was higher than that in roots in all the species except for kudzu, where the root N concentration was higher (Table 2). Shading

Effect of Shading on BNF and Dry Matter Production 49 Fig. 4. Nitrogen amount of plant parts of kudzu, tropical pasture legumes and soybean isolines T201 and T202 at different growth stages. [--], leaves and stem; [og-g~, pods; [~~, fallen parts; EEl, roots and nodules. Fig. 5. Carbohydrate (sugars+starch) amount in shoot and roots of kudzu, tropical pasture legumes and soybean isolines T201 and T202 at different growth stages. [ I, leaves and stem; ~, roots and nodules.

50 K. FUJITA et al. Table 1. Dinitrogen-fi activities of kudzu, tropical pasture legumes and soybean isolines T202 at different growth stages. DAP" Centro Siratro Kudzu T202 LSD (0.05) TNA 60 30.11 11.47 4.21 21.71 9.13 (,umo[ C2H~ plant -1 h -1) 105 24.83 7.83 3.81 2.36 9.22 SNA 60 74.7 68.7 59.3 19.9 24.8 (,umol C2H~ g nodule dry weight -~ h -~) 105 71.8 107.3 105.8 4.6 40.2 Nodule dry weight 60 0.403 0.167 0.071 1.105 0.372 (g plant -1) 105 0.346 0.073 0.036 0.510 0.188 Amount of N fixed A b 36.0 57.8 23.5 32.7 I 1.9 (g N rn -2) B ~ 35.9 57.1 24.5 33.6 12.4 DAP: days after planting; b A and B were estimated by the lsn-dilution and N difference methods, respectively. Fig. 6. Effect of shading on dry weight of plant parts and leaf area of kudzu and tropical pasture legumes. [~], leaves; [~k-~, stem; ~[~g*~, fa[len parts; ~, roots and nodules. 0, LAI; C, control; Ci, at the start of treatment. $55, LTR at the top of plant was reduced to 55% of full sunlight by shading. $25, LTR at the top of plant was reduced to 25% of full sunlight by shading. increased the root N concentration of siratro and kudzu but decreased that of the shoot and roots of centro. N accumulation. Shading tended to decrease the N amount of whole plant in all the species but the decrease in siratro was the most significant, while a slight decrease was observed in kudzu (Fig. 7). In kudzu, the amount of root N decreased by shading, while the effect on the amount of shoot N was negligible. Carbohydrate concentration. Although the sugar concentration of the siratro plant parts decreased as a result of shading, minor changes in the sugar concentration in kudzu were observed (Table 2). Shading decreased the starch concentration of all the plant parts in every species except for the kudzu shoots and T201, and the decrease was most significant in siratro.

Effect of Shading on BNF and Dry Matter Production 51 Table 2. Effect of shading on concentrations (10--~ kg kg -1) of sugars, starch, and nitrogen in shoot and roots of kudzu and tropical pasture legumes (dry weight basis) in Experiment 2. LTR a Centro Siratro Kudzu (%) Shoot Root Shoot Root Shoot Root 100 4.10 2.17 3.61 1.95 3.66 4.35 Sugars 55 3.63 5.l 5 2.64 1.48 4.30 3.75 25 3.23 2.04 2.11 1.37 3.45 5.01 LSD (0.05) 0.85 0. I0 0.29 0.37 0.58 0.42 100 8.09 9.09 4.0l 22.41 6.15 16.55 Starch 55 5.73 5.16 3.68 11.57 5.34 15.55 25 5.32 3.82 3.88 15.56 7.03 12.95 LSD (0~05) 0.79 0.30 1.21 0.33 1.36 1.27 100 3.22 2.74 2.66 1.81 2.40 2.62 Nitrogen 55 2.86 2.16 2.60 1.91 1.97 3.1 l 25 2.56 2.19 2.51 2.09 2.52 3.07 LSD (0.05) 0.42 0.21 0.14 0.15 0.18 0.24 a Light transmission ratio at the top of the plant. Fig. 7. Effect of shading on nitrogen amount of plant parts of kudzu and tropical pasture legumes. [--], leaves and stem; [g%%], fallen parts; ~, roots and nodules. C, control; Ci, at the start of treatment. $55, LTR at the top of plant was reduced to 55% of full sunlight by shading. $25, LTR at the top of plant was reduced to 25% of full sunlight by shading. Amount of fixed-n. The high correlation between dinitrogen fixation estimated by the 15N-dilution and the total N difference methods in Experiment 1 shows that the N difference method can be applied to estimate the dinitrogen fixation under the soil condi- tions used in this study. Shading decreased the amount of fixed-n in all the legumes but the decrease in siratro was the most pronounced and that of kudzu the least pronounced (Fig. 8). For example, 55% shading had no effect on the amount of N fixed in kudzu.

52 K. FUJITA et al. C ~ n t r el S [ r a I r ', 1110 O o r E N Z < 0 t l s55 s25 t I I C $55 $25 T r c a t m e n t s C S55 $25 Fig. 8, Effect of shading on dinitrogen fixation of kudzu and tropical pasture legumes. C, control. $55, LTR at the top of plant was reduced to 55% of full sunlight by shading. $25, LTR at the top of plant was reduced to 25% of full sunlight by shading. DISCUSSION Cultural management such as providing support for the border plants of the vining type with iron screens and the absence of plants around the concrete frame improved the light conditions of the tropical pasture legumes, centro and siratro, and kudzu. This step resulted in a higher dry matter yield than under the ordinary field conditions reported by Eriksen and Whitney (1982). Siratro exhibited a larger whole plant weight than the nodulating soybean isoline T202 (Fig. 2), probably due to its greater leaf area and longer leaf duration under full sunlight conditions (data not shown). These findings indicate that a tropical pasture legume displays a higher dry matter production ability than soybean if optimum growth conditions are provided. Although the dry matter yield of kudzu was similar to that of the nodulating soybean isoline T202, which was also lower than that of siratro, the root to shoot ratio was far higher than that of the other legumes (Fig. 2). These results suggest that although siratro and kudzu showed a similar growth habit (climbing type), the contribution of root or shoot to the total plant weight was markedly different (Fig. 2). The contribution of the shoot to the total plant weight in siratro was higher, while in kudzu the root dominated. These findings suggest that in spite of the similarity in their growth habit, the proportion of the shoot to the total plant weight, which is a reflection of their potential pasture productivity was quite different. The greater root to shoot ratio of kudzu (Fig. 2), also was responsible for the larger accumulation of N (Fig. 4) and carbohydrates (Fig. 5) compared to the other legumes and various plant parts, further emphasizing the stronger root sink activity of kudzu. At the later growth stages, at 105 DAP and after, the rate of dry matter accumulation of the kudzu roots increased significantly, with a simultaneous decrease in the shoot dry weight (Fig. 2). During the same period, sharp increases in N and carbohydrate accumula-

Effect of Shading on BNF and Dry Matter Production 53 tion in the kudzu roots and a simultaneous decrease in the shoot N amount were observed (Figs. 4 and 5). These results suggest that N from the shoot was retranslocated to the roots in response to the greater root demand. The relationship between the dinitrogen fixing activity estimated by the N difference and the ~SN dilution methods was found to be highly correlated (r=0.999"*) (Table 1). The ~SN dilution method showed that the dinitrogen-fixing activity by root nodules accounted for over 95% of the amount of N accumulated in the plant, presumably due to the low N status of the soil (4.5 10-4 kg kg -~) used in this study. The results support the earlier findings that the total N difference method can be used for estimating the dinitrogen fixation of legumes under conditions of low soil N (Rennie 1984; Vasilas and Ham 1984). Under the natural light conditions, centro exhibited the highest and kudzu the lowest TNA among the legmninous species examined at 60 DAP, respectively. The TNA values of siratro and kudzu in the present study were lower than those of the plants grown in pots or at a wider spacing under field conditions. The TNA of siratro and kudzu under potted conditions ranged between 15.3 and 30.2,umol C2H4 plant-~h -~ and 14.0,umol C2H4 plant-th -1 (Fujita, unpublished data). The difference in TNA under the two conditions was due to the lower SNA and smaller nodule weight in siratro and the extremely smaller nodule weight of kudzu. The distribution of a larger amount of nodules on the tertiary roots and the easiness with which the nodules underwent abscission, did not facilitate the collection of nodules for TNA determination under field conditions. A high nodule loss may have occurred which could account for the smaller nodule weight of kudzu in the present study. Based on the isn dilution and the N difference methods, as siratro accumulated the largest amount of N mainly through dinitrogen fixation, the dinitrogen fixation determined by the acetylene reduction method in the present study may have been underestimated. This difference in the estimation may have been caused by the loss of a greater proportion of the nodules during root excavation prior to the TNA determination. Another possible cause may be the reduced solar radiation associated with the high population density as shown by the lower SNA value. Based on these results, it is suggested that the dinitrogen fixing-activity of tropical pasture legumes such as centro and siratro under natural light conditions is equivalent to or higher than that of soybean while that of kudzu is relatively lower. In the present study the dinitrogen~fixing activity of siratro and kudzu could not be determined effectively by the acetylene reduction method due to problems related to the sampling of the nodules in the field. Shading caused a significant decrease in the whole plant weight in all the species examined except for the unexpected slight increase in kudzu under 55% shading conditions (Fig. 6). The decrease was more conspicuous for the shoot than the root weight in all the species except for kudzu, where the decrease in root weight was more significant. The marked reduction in the leaf area due to severe leaf abscission may account for the pronounced shading effect in siratro, which also supports the findings of Eriksen and Whitney (1982) who stated that siratro is highly sensitive to shading. The overall results suggest that kudzu is more tolerant to shading in terms of dry matter production than the tropical pasture legumes and soybean. The trend in the changes in the plant N amount and distribution in the various plant parts was similar to that of the changes in the dry weight, suggesting that N accumulation may be involved in the shade tolerance of kudzu (Figs. 6 and 7). This assumption was further corroborated by the tolerance of dinitrogen fixation of kudzu to shading, but drastic reduction in the other legumes (Fig. 8). These results suggest that the high tolerance of kudzu

54 K. FUJITA et al. to shading may be attributed partly to its ability to fix nitrogen under reduced sunlight conditions. Differential responses to shading between shoot and roots were observed. Root weight and N accumulation (Figs. 6 and 7) as well as starch concentration (Table 2) of kudzu decreased with increasing shading intensity, whereas the shading effects on the shoot weight and N amount were negligible. The differences in the partitioning of N and carbohydrates in kudzu compared to those of the other legumes (Figs. 4 and 5), that is the distribution of larger amounts of N and carbohydrate to roots, suggest that the N and carbohydrate accumulation in the roots may be one of the factors conferring the high shade tolerance of kudzu. It is surprising that in spite of the significant reductions in the root starch concentration in kudzu at 55% shading (Table 2) the dinitrogen-fixing activity was still high (Fig. 8). Currently translocated photosynthates to nodules are a major substrate for the nodule function in soybean (Kouchi and Yoneyama 1984). In the kudzu plant, it is possible that some of the reserve root carbohydrates had been remobilized for the nodule function as suggested by Schweizer and Harper (1980). Further investigations on the mechanism of the relatively high dinitrogen fixation under shaded conditions in kudzu should be carried out. REFERENCES Chu, A.C.P. and Robertson, A.G. 1974: The effect of shading and defoliation on nodulation and nitrogen fixation by white clover. Plant Soft, 41, 509-519 Eriksen, F.I. and Whitney, A.S. 1982: Growth and N fixation of some tropical forage legumes as influenced by solar radiation regimes. Agron. i, 74, 703-709 Hardy~ R.W.F., Holsten, R.D., Jackson, E.K., and Havelka, U.D. 1968: The acetylene-ethylene assay for N2 fixation. Laboratory and field evaluation. Plant Physiol., 43, 1185-1207 Koucbi, H. and Yoneyama, T. 1984: Dynamics of carbon photosynthetically assimilated in nodulated soybean plants under steady-state conditions. I. Development and application of 13CO2 assimilation system at a constant 13C abundance. Ann. Bot., 53, 875 882 Ludlow, M.M., Wilson, G.L., and Heslehurst, M.R. 1974: Studies on the productivity of tropical pasture plants. Effect of shading on growth, photosynthesis and respiration in two grasses and two legumes. Aust. J. Agric. Res., 25, 425-433 Moustafa, E., Ball, R., and Field, T.R.O. 1969: The use of acetylene reduction to study the effect of nitrogen fertilizer and defoliation on nitrogen fixation by field grown white clover. N.Z.J. Agric. Res., 12, 691-696 Peoples, M.B. and Herridge, D.F. 1990: Nitrogen fixation by legumes in tropical and subtropical agriculture. Adv. Agron., 44, 155-223 Rennie, R.J. 1984: Comparison of N balance and ~SN isotope dilution to quantify N2 fixation in field-grown legumes. Agron. i, 76, 785-790 Schweitzer, L.S. and Harper, J.E. 1980: Effect of light, dark, and temperature on root nodule activity (acetylene reduction) of soybeans. Plant Physiol., 65, 51-56 Tsugawa, H., Sasek, T.W., Komatsu, N., Tange, M., and Nishikawa, K. 1989: Seasonal changes in dry matter production and leaf area expansion of first year stands of kudzu-vine (Pueraria lobata Ohwi) differing in spacing. J. Jpn. Grassl. Sci., 35, 193-205 Vasilas, B.L. and Ham, G.E. 1984: Nitrogen fixation in soybeans: An evaluation of measurement techniques. Agron. 1, 76, 759-764 Whiteman, P.C. and Lulham, A. 1970: Seasonal changes in growth and nodulation of perennial tropical pasture legumes in the field. I. The influence of planting date and grazing and cutting on Desmodium incinatum and Phaseolus atropurpureus. Aust. J. Agrie. Res., 21, 195-206