Enhancement of Tuberous Root Production and Adenosine 5'-Diphosphate Pyrophosphorylase (AGPase) Activity in Sweet Potato (Ipomoea batatas Lam.) by Exogenous Injection of Sucrose Solution

Adenosine 5'-diphosphate pyrophosphorylase (AGPase) is a key enzyme governing starch synthesis and is regarded as an important determinant of the sink activity of the sweet potato root. In this study, assuming that the expression of AGPase is under the direct or indirect control of sucrose, we investigated the effect of exogenous injection of sucrose solution into a plant on the activity of AGPase and tuberous root production. Sucrose solutions of 6 and 12 %, and distilled water as the control, were injected into the top of the shoot. The application of sucrose solution was effective in increasing tuberous root production and increasing the ratio of tuberous root weight to total root weight in a plant. AGPase activity in roots was enhanced by about 25 % by injecting sucrose solution. These results suggest that AGPase and sink activities are controlled by sucrose transported from the leaves. To increase sweet potato production effectively, AGPase activity and sink function must be enhanced, and so a genetic and physiological improvement in photosynthetic function or sucrose productivity in the leaves is necessary to increase AGPase activity in sink organs.     

北方区试中甘薯农艺和品质性状的相关性及主成分分析

为了了解影响甘薯产量和品质各性状间的相关性及主次关系,应用多元统计方法,分析了北方区试中农艺和品质性状的相关性及主成分关系。结果表明：在对农艺性状与产量的相关性分析中,干率和淀粉率均与淀粉产量呈显著正相关,茎粗和大中薯率与薯干产量均呈显著正相关,大中薯率与鲜薯产量呈显著正相关;主成分分析表明,对淀粉产量和薯干产量贡献最高的主成分中的性状为大中薯率、茎粗和黑斑病抗病指数,贡献率分别为29.67%和30.13%;而对鲜薯产量影响最大的主成分中的性状为大中薯率和根腐病病情指数,贡献率为30.71%。通过对品质性状间的相关性分析表明,淀粉含量与还原糖含量、可溶性糖含量、甜度之间呈极显著负相关关系,粗蛋白含量与可溶性糖和还原糖之间存在弱负相关关系。而品质性状间的主成分分析表明,对甘薯品质贡献最大的主成分中的性状为还原糖、可溶性糖和蒸煮后的甜度,贡献率为59.12%;第二主成分为粗蛋白含量和蒸煮后粘度性状,贡献率为23.75%。总之,农艺性状对影响不同产量目标形成的侧重点不同,甜、糯性状是影响甘薯品质的主要性状。     

Relationship of Root and Shoot Characters in Parent, F1 and F2 Populations of Rice

The direct and indirect contributions of root characters — root length, roots/plant, fresh and dry root weight on grain yield/plant were worked out from a 7 × 7 diallel set of rice hybrids. The materials were grown in pots with four replications. Path analysis was done at genotypic level of correlation.
The grain yield/plant showed positive correlation with all the root characters in parent, F₁ and F₂ except with roots/plant in F₁ population. Fresh root weight demonstrated positive direct effect on grain yield/plant in all the three generations. Roots/plant had highly positive direct effect in F₂. Direct effects were negative in respect of root length and dry root weight in F₂ generation.     

套作条件下种植密度对紫色甘薯干物质生产的影响

为了研究川中丘陵区旱地套作条件下种植密度对紫色甘薯干物质生产的影响,在与玉米套作条件下,以鲜食型紫色甘薯品种‘南紫薯008’为材料,设置5个种植密度,测定紫色甘薯叶面积指数、群体生长率、群体光合势、干物质积累、产量等指标。结果表明：在套作条件下,随着种植密度的增加,‘南紫薯008’群体叶面积指数有升高的趋势,群体生长率、群体光合势、商品薯率、干物质率、鲜薯产量、淀粉产量均呈先升后降的趋势,而单株结薯数、单株薯鲜重则下降,群体干物质积累量和T/R则随生育期的推进不同密度间表现有所差异。公顷种植密度在3.5×10⁴~4.0×10⁴株时,群体干物质积累量、鲜薯产量、淀粉产量和商品薯率均较高。套作条件下‘南紫薯008’最高鲜薯产量为11816.9 kg/hm²,约为其净作产量的42.6%~53.1%。川中丘陵区旱地套作条件下紫色甘薯适宜的种植密度为3.5×10⁴~4.0×10⁴株/hm²。     

Cassava (Manihot esculenta Crantz) Growth Indices, Root Yield and its Components in Upland and Inland Valley Ecologies of Sierra Leone

Cassava is predominantly an upland crop that is also cultivated in inland valley swamps (IVS). Identifying physiological traits that can withstand excess moisture stress can aid in the selection and use of stable cassava cultivars in IVS. Three cassava cultivars were evaluated for growth and yield in the upland and IVS ecologies in the 1993–94 crop seasons using a randomized complete block design. In the upland, 80/40 outyielded 87/29 and 'coco', while in the IVS, 87/29 had the highest yield. Highest yielding cultivars in each ecology also had the highest tuberous root bulking rate (TBR), dry matter (DM) production, crop growth rate (CGR), relative growth rate, net assimilation rate (NAR), leaf area index, leaf production rate and leaf life. Positive correlations between these parameters and tuberous root yield were noted. High yield of 87/29 in IVS was partly due to its low tuberous root rotting. High root rotting in 80/40 was partly as a result of the greater depth of tuberous roots in the soil (0.3–0.6 m) as compared to the other cultivars (0.15–0.3 m) where roots were in contact with stressful water levels earlier than others. The yield and yield components, growth and leaf characters were all drastically reduced when the cultivars were grown in IVS as compared to upland. For example, root yields of 'coco', 87/29 and 80/40 were reduced by 53 %, 60 % and 92 %, respectively, in IVS. Selection of cassava cultivars with longer leaf life and leaf area maintenance leading to high CGR, TBR, NAR and harvest index (in IVS and upland), coupled with a shallow tuberous root formation zone (in IVS) can give higher root and leaf yields in the two ecologies.     

Physiological Responses of Tomato Plants (Solanum lycopersicum) as Affected by Root Hypoxia

In order to investigate the effects of root hypoxia (1–2 % oxygen) on physiology and on some fructification parameters of tomato plants ( Solanum lycopersicum cv. H63-5), a range of physiological and agronomic measurements were performed on plants submitted to gradual root hypoxia at the second leaf stage. Stomatal conductance (g_s), total chlorophyll content, leaf area (LA) and leaf mass per area (LMA) were quantified at 14 days of treatment in the first leaf appeared before (L1) and after (L3) root hypoxia application. The results obtained showed that 7 days of treatment has slight effects on plant weight growth. However, 14 days of treatment caused decreased root dry weight (DW) with a concomitant increase in shoot DW. Root hypoxia significantly decreased g_s and chlorophyll content of L3, but no effects were noted for L1. LA was significantly reduced for L1 and L3, contrary to LMA which was significantly increased. At the reproductive stage, root hypoxia resulted in a decrease in flower and fruit production, accelerated the entrance in the fruit expansion phase, but did not affect the rate of fruit growth, nor the final fruit size.     

施钾对甘薯根系生长和产量的影响及其生理机制

为探讨施钾调控甘薯根系生长的生理机制,设K0(K_2O:0 kg hm~(-2))、K1(K_2O:75 kg hm~(-2))、K2(K_2O:150 kg hm~(-2))和K3(K_2O:225 kg hm~(-2))4个处理,调查施钾对甘薯生长前期和薯块膨大期根系生长、13C分配量、碳代谢酶活性、光合特性、叶绿素荧光特性、以及产量和产量构成的影响。结果表明,与CK相比,施钾处理2个生长时期光电子传递速率(ETR)提高12.7%~63.6%,净光合速率(Pn)提高7.2%~26.4%,施钾通过提高光合特性加速光合产物积累,为根系生长提供物质基础。同时,施钾有利于光合产物由地上部向地下部运转,地下部13C分配量提高10.6%~66.2%(P0.05)。其次,施钾处理提高了块根中蔗糖合酶、磷酸蔗糖合酶、腺苷二磷酸葡萄糖焦磷酸化酶活性,加速了块根中碳的同化,利于光合产物在块根中的积聚,促进甘薯根系分化与生长。生长前期,施钾处理总根长提高13.6%~22.8%,根平均直径提高11.3%~51.9%,显著提高了不定根向毛细根和块根分化量(P0.05),有利于有效薯块的早期形成,保证有效的单株结薯数。薯块膨大期,施钾处理提高块根生物量,有利于薯块的膨大,提高平均薯块重,最终显著增产。与CK相比,2014年K1、K2和K3处理分别增产15.8%、24.3%和44.7%,2015年分别增产7.9%、13.4%和22.8%。     

Effects of Tillage and Fertilization on Biomass, Roots, N-Accumulation and Nodule Bacteria of Vetch (Vicia sativa cv. Alexander)

Field experiments were conducted to determine the effects of tillage (ploughing followed by one rotary hoeing (CT), rotary hoeing (MT) and no-tillage (NT)) and type of fertilization (NP 154/75 kg ha⁻¹, digested farmyard manure 30 t ha⁻¹) on growth (biomass and roots), N-accumulation and nodulation of vetch plants (Vicia sativa cv. Alexander) at different stages. The highest dry matter yields and root weight were determined for the NT and the lowest for the CT systems. The NT system promoted nodulation of vetch plants to a higher degree than the MT and CT systems. At the time just before harvesting, the N-accumulation in the above ground plants, was 54.3 for CT, 76.7 for MT, and 109.0 kg ha⁻¹ for NT, and the corresponding values at roots were 73.3, 112.0 and 173.3 kg ha⁻¹. Growth and nodulation of vetch plants was in general much better on farmyard manure plots than it was with the NP fertilization. The closest correlation coefficients between dry matter of biomass and roots on the one hand, and number of nodules and percentage N in roots on the other, were found with NT. The highest correlation between nodulation and percentage N in roots (r = 0.91***) was observed at the 128-days-old vetch plants. The highest correlation between the number of nodules and percentage N in biomass (r = 0.82**) was found at the beginning of the plants' flowering period. *, **, *** indicate significance at P = 0.05, 0.01 and 0.001 respectively.     

Morphogenetic Effects of Different Mineral N-compounds on Roots of Maize (Zea mays L.), Spring Barley (Hordeum vulgare L.) and Broad Bean (Vicia faba L.)

Plants of maize ( Zea mays L.), spring barley ( Hordeum vulgare L.) and broad bean ( Vicia faba L.) were raised in tubes filled with a soil-sand mixture and supplied with nutrient solution. The nutrient supply was varied with regard to concentration and typ of compound of "N". Due to the relatively high concentrations of "N", dry matter of shoots and roots was reduced in comparison to the control; root morphology was affected in terms of root length and root thickness. The N-compounds, used in the experiment, exerted quantitatively different influences on dry matter production and root morphology, NH₄NO₃ being the N-form which had the weakest detrimental effects. There was no specific response to NH⁺₄ or NO⁻₃ but a strong influence could be shown for NH₄Cl and N_aNO₃. Probably not only the concentration of "N" and the form of "N" have a specific influence, but also the cations and anions associated with the N-containing ion are effective.     

不同种植模式下根系分泌物对玉米生长及产量的影响

为探究大豆和甘薯的根系分泌物对玉米生长及产量的影响,通过盆栽根系分泌物吸附试验,研究了不同活性炭处理方式对玉米/大豆(B1)、玉米/甘薯(B2)及玉米净作(B3)3种种植模式下玉米光合特性、荧光参数及生物产量的变化规律。结果表明:加活性炭处理(A1)较未加活性炭处理(A2)抑制了玉米生长,但玉米产量差异不显著。加活性炭时,由于根系分泌物被吸附,玉米的光合速率、荧光参数和产量在各种植模式之间差异不显著;未加活性炭时,大豆的根系分泌物促进玉米生长和提高玉米产量,甘薯的根系分泌物抑制玉米生长和降低玉米产量,B1处理下玉米的根干重、根系活力、净光合速率和子粒干重比B3处理分别高15.81%、53.20%、19.94%和60.84%,而B2处理则比B3处理分别低19.92%、4.33%、16.96%和24.32%。     

不同施钾量条件下甘薯块根形成的内源激素变化及其与块根数量的关系

In order to explore the changes of endogenous hormones on storage root formation and its relationship with storage root number under different potassium application rates of sweet potato root, sweet potato varieties ‘Yanshu 25’ and ‘Beijing 553’ with significant differences in storage root number were used as experimental materials, potassium oxide (K₂O) was used as fertilizer, and four potassium fertilizer gradients of 0 (K0), 120 (K2), 240 (K3), and 360 kg hm^-2 (K4) were set. The effects of different potassium application rates on the contents of endogenous hormones, the activity of related metabolic enzymes, the number of storage root and root evenness in swelling roots of sweet potato during root formation and harvest stage were studied. Compared with the treatment without potassium fertilizer (K0), the application of potassium fertilizer decreased the enzyme activities of indoleacetic acid oxidase (IAAO) and peroxidase (POD), increased the content of indole-3-acetic acid (IAA), increased the content of zeatin riboside (ZR) and decreased the content of gibberellins (GA₃). Potassium application enhanced the activity of primary cambium and promoted the differentiation from adventitious root to storage root by regulating the content of endogenous hormones in swelling roots of sweet potato. Compared with the control, the application of potash fertilizer significantly increased the number and weight of storage roots per plant and root yield of ‘Yanshu 25’ and ‘Beijing 553’. The number of storage roots per plant of ‘Yanshu 25’ and ‘Beijing 553’ increased by 3.16%-25.40% and 3.85%-33.11%, respectively, and the yield increased by 4.22%-17.31% and 3.94%-18.45%, respectively. Compared with the potassium application treatments of the two varieties, the K2 treatment had the highest storage roots number per plant, the highest weight and yield of storage roots, with the best root evenness.     

结薯数差异显著的甘薯品种生长前期根系特性及根叶糖组分比较

选用结薯数差异显著的甘薯品种商薯19和济徐23,于2013年和2014年分别在大田和盆栽条件下调查生长前期根系生长发育情况,测定根叶糖组分,分析其变化规律及其与单株有效薯块数形成的关系。结果表明,与济徐23相比较,商薯19发根缓苗迅速,整个生长前期均较为侧重吸收根系建成,且秧苗栽后15 d和30 d地上部和整个根系生物量比值(T/TR)相似;而济徐23较早完成不定根的发生(秧苗栽后15 d)和不定根向块根的分化建成(秧苗栽后30 d),封垄期（秧苗栽后45 d）具有较小的地上部和块根生物量比值(T/SR)。2个品种在生长前期根叶中蔗糖、己糖代谢和贮藏糖类多聚物的形成具有显著差异,其中商薯19整个生长前期比济徐23的根系蔗糖/己糖比率显著低,在块根分化建成过程中根叶间具有较高的蔗糖浓度梯度,且根系中存在2种低果聚糖(蔗果三糖和蔗果四糖),秧苗栽后30、45 d叶片淀粉含量显著低;而济徐23在块根分化建成过程中根系仅有蔗果四糖,且与商薯19比,封垄期根系蔗糖/可溶性总糖比率相似而根系蔗糖/淀粉比率显著低。2013年和2014年的大田试验均表明,封垄期商薯19具有显著多的单株有效薯块数和显著高的单株有效薯块鲜重(F=10.71,P=0.0170;F=13.97,P=0.0212),而收获时商薯19具有显著多的单株有效薯块数和显著高的块根产量(F=353.89,P<0.0001;F=88.94,P<0.0001),济徐23具有显著高的平均单薯鲜重(F=10.32,P=0.0124)。     

Effects of Root-Zone Temperature on Growth and Development of Roots of Two Potato (Solanum tuberosum L.) Clones as Influenced by Plant Age, Nutrient Supply, and Light Intensity

The effects of supraoptimal root-zone temperature (30 °C) on growth and development of roots were investigated in water and soil culture with cuttings of two potato clones DTO-2 (heat tolerant) and LT-1 (heat sensitive) at different plant ages, with different compositions of the nutrient solution, and different light intensities. It is suggested that the negative relation between cutting age and heat susceptibility observed in our experiments, is a reflection of the decreasing shoot/root dry weight ratio rather than of age. Heat damage of root systems increased with the concentration of the nutrient solution, nitrate being the main factor responsible. The influence of light intensity on temperature mediated restriction of root growth was relatively small. However the roots of plants grown under low light intensity (60 W m⁻² PAR) appeared to be less stressed by supraoptimal temperature, i.e. roots appeared brighter, thinner and more ramified than plants growing at high light intensity (190 W m⁻²).     

Die Wurzelentwicklung von Zuckerrübenpflanzen auf einem Sandlöß-Standort und ihre Bedeutung für die Stickstoffernährung

Root development of sugar beet plants on a sandy loess site with regard to nitrogen nutrition.
Root development of sugar beet plants in a sandy loess soil (Haplic Phaeozem) was observed from the early seedling stage up to harvest by measuring at first the greatest vertical and lateral extension of the root systems of single plants and later the rooting density of the whole plant stands (auger method, profile wall method).
During the seedling stage not only the subsoil, but also large parts of the topsoil between the plants remained unoccupied by the root systems. In this phase the greatest lateral extension of single roots reaches nearly the length of the greatest leaf of the plant. With the closure of the canopy the rooting density in the topsoil accounts to 1–2 cm cm⁻³.
In summer roots penetrate to a depth of 100–150 cm with rooting densities of 0.1 to 1 cm - cm⁻³. Thus, the plants gain not only access to water reserves, but sometimes meet remarkable amounts of nitrate which under the relatively dry conditions of the region tends to accumulate in 60–120 cm depth and – when taken up by the beet plants in the late stage of growth – affects crop quality negatively.     

Effect of the Organic Residues of Miscanthus × giganteus on the Soil Organic Matter Level of Arable Soils

During a complex investigation programme on the evaluation of the long-term ecological impact of cropping Miscanthus , the substrate composition of residues in 4–8-year-old stands of Miscanthus × giganteus and the contribution of Miscanthus residues to soil organic matter (SOM) were investigated. Only about 50 % of the above-ground biomass produced yearly was suitable for harvest because of pre-harvest losses and harvesting residues. The potential supply to SOM was therefore 3.1 t ha⁻¹ carbon annually accumulated by the litter, and 9.1 t ha⁻¹ carbon accumulated by rhizomes and roots in the long term. These organic residues mineralized differently in laboratory experiments at generally high rates of decomposition. Analysis of soluble portions of the organic substance of these biomass components by hydrolysis with hot water, 2 % HCl and 80 % H₂SO₄ confirmed the results. Setting the SOM effect of an equivalent farmyard-manure (FYM) supply to 100 %, Miscanthus residues reached 60 % (stubble, rhizomes), 80–90 % (pre-harvest losses) and 100 % (roots). Established Miscanthus stands are able to produce about 8.2 t ha⁻¹ organic substance, which is comparable with FYM in terms of SOM impact. This kind of calculation showed higher values for Miscanthus than for the agricultural crops investigated to date. An SOM increase of about 0.5 % on sandy soils and 0.2 % on silt soil was determined after 6–8 years of cropping Miscanthus × giganteus .     

Untersuchungen zum Wurzelwachstum von Jungpflanzen alter und neuer Winterweizensorten sowie eines Spelzweizens bei variierter N-Versorgung

Root growth of seedlings of old and new winter wheat cultivars and a spelt wheat at varying levels of nitrogen
Root growth of five old (time period 1882–1920) and five new (time period 1975–1982) winter wheal cultivars and a spelt winter wheat cultivar was investigated in two greenhouse trials. Nitrogen (NH₄NO₃) was supplied at six (Exp. I) and two (Exp. II) levels (sub- to supraoptimal for shoot growth). The seedlings were grown in sand-filled polyethylene tubes of 4.5 cm diameter and 50 cm length at 20 ^oC. The plants were harvested 11 (Exp. I) and 12 (Exp. II) days after emergence. At this early stage of development root growth o: old and new cultivars differed only slightly. The old T. aestivum cultivars had more seminal roots. The longest roots of the modern cultivars penetrated into deeper layers of the substrate. For some traits, cultivar: seemed to respond specifically to the varying N-supply. However, there were no significant difference: between old and new cultivars. Neither were old cultivars better adapted to low N-fertilization nor were modern cultivars superior in tolerating toxic concentrations of NH₄NO₃. Increasing N supply changes root growth as follows: decline in root dry matter, increase in shoot/root ratio of dry weight, decrease in number of seminal roots, shortening of axes and more laterals, increase in diameter of the laterals and less significantly of the main roots, increase in density of laterals, decline in root surface area. The number of apices was affected only to a very small degree.     

优质紫肉甘薯新品种‘榕紫薯5号’的选育及栽培技术

旨在探讨、验证优质食用型紫肉甘薯新品种创制的最佳策略,为甘薯高效育种提供借鉴。以来自日本的优质紫肉型甘薯‘川山紫’为母本,以高产、高糖材料‘冀薯6-8’为父本,通过有性杂交、系统选育获得优质食用型紫肉甘薯新品种‘榕紫薯5号’。该品种2011—2013进行多点选拔、品比及鉴定,2014—2015年参加福建省甘薯区试。对‘榕紫薯5号’的形态特征、农艺性状、品质性状、抗病性、生产力、适应性进行了试验研究。结果表明,‘榕紫薯5号’为中晚熟品种,生育期135天,萌芽性好,分枝能力中等,全生育期群体长势强劲,结薯集中。2014—2015年福建省甘薯区域试验平均鲜薯产量30.24 t/hm ²,平均干物率30.12%,平均出粉率19.84%,食味评分81.6分,比对照高1.6分。外观评分81.3分,比对照高1.3分。花青素含量26.95 mg/100 g FW。大中薯率68.7%。省区试综合鉴定结果为：株型中蔓半直立;鲜薯产量、薯干产量和淀粉产量比对照高;干物率和淀粉率与对照相当;食味品质、外观品质均优于对照;抗蔓割病,中抗薯瘟病,耐贮性好,该品种达到食用型甘薯品种审定标准,可作为鲜食及特用薯在中国南方薯区广地域种植。     

干旱胁迫对马铃薯生理特性及解剖结构的影响

通过研究不同土壤含水量条件下马铃薯的植物学性状、生理指标和显微指标等,探讨马铃薯在干旱胁迫下的抗旱反应和抗旱生理机制。试验用‘青薯9号’和‘闽薯1号’2个品种,以T₀ (CK)适宜水分（土壤相对含水量为70%~75%）为对照、其他土壤含水量分别为55%~60% (T₁)、40%~45% (T₂)、25%~30% (T₃)和出苗后不再浇水(T₄)等水分胁迫处理下,分析其植物学性状、生理指标和显微指标。试验结果：（1）水分胁迫对2个马铃薯品种的植物学性状（株高、茎粗、地上部分鲜重、地上部分干重、叶面积、根的鲜重、根的干重和根长）均有显著影响,随着土壤含水量的降低,‘青薯9号’和‘闽薯1号’2个品种的上述性状指标均有降低趋势,其中‘青薯9号’的株高、地上部分鲜重、地上部分干重、根的鲜重、干重和根长均大于‘闽薯1号’。（2）生理指标分析表明：在T₂~T₄水分胁迫下,马铃薯的脯氨酸(Pro)、丙二醛(MDA)和超氧化物歧化酶(SOD)等含量与对照T₀相比有极显著差异(P<0.01);（3）显微结构分析表明：叶片的上表皮厚度、下表皮厚度、栅栏组织厚度、海绵组织厚度和栅海比均受到水分胁迫影响。该试验为马铃薯的抗旱栽培和育种提供了理论依据。     

Heterosis and Inbreeding Depression Estimation of Root and Shoot Characters in Rice (Oryza sativa L.)

The nature and extent of hybrid vigour and inbreeding depression were estimated from a diallel set of rice varieties. The seven parents and their 21 F₁s and 21 F₂s were grown in pots with four replications. Data were collected on four root and two shoot characters.
Maximum heterosis was recorded for root weight followed by roots/plant and root length. The inbreeding depression did not significantly affect the dry root weight in majority of the crosses. Heterosis for shoot weight was found to be associated with the heterosis for roots/plant and root weight.     

Nitrogen Fixation by Hybrids of White Clover (Trifolium repens L.) and Trifolium nigrescens

Development of hybrids between white clover ( Trifolium repens L.) and Trifolium nigrescens provides a novel route for genetically improving the reproductive capacity of white clover, provided the hybrids are agronomically viable, particularly with respect to N₂ fixation. A comparative study of growth and rates of N₂ fixation over 21 days was conducted with the parental species, F ₁ hybrids and backcross hybrids, in flowing solution culture, without a supply of mineral N to the plants. T. nigrescens was unable to fix N₂ in association with the strains of Rhizobium leguminosarum biovar. trifolii selected for inoculation. Rates of N₂ fixation per plant increased in the order T. nigrescens < F ₁ hybrid < T. repens < backcross 1. Specific rates of N₂ fixation (days 0–21) increased in the order T. nigrescens < F ₁ hybrid < backcross 1 <  T. repens . Dry matter production and nodule biomass per plant increased at a higher rate in backcross 1 hybrids than in T. repens. The results suggest that the potential for N₂ fixation by backcross 1 hybrids is at least as great as that by T. repens .