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African Crop Science Journal, Vol. 9. No. 3, pp. 577-590

SHORT COMMUNICATION

THE EFFECT OF LIGHT INTENSITY ON THE GROWTH, DEVELOPMENT AND YIELD OF SOYBEAN IN SOUTHWEST NIGERIA

F. O. ODELEYE, A.O. TOGUNand T.O. TAYO¹

Department of Crop Protection and Environmental Biology, University of Ibadan, Ibadan, Nigeria
¹College of Plant Science and Crop production, University of Agriculture, Abeokuta, Nigeria

Received 10 September, 1999
Accepted 1 February, 2001

Code Number: cs01076

ABSTRACT

Soybean (Glycine max (L.) Merrill) is an important cheap source of plant protein in Nigeria. Its production is constrained by low light intensity in the southern rainforest ecology due to cloud cover during the production season. Understanding the physiological basis of performance under low light intensity is therefore central to efforts geared towards the crop's productivity improvement in the zone. Soybean plants were subjected to two weeks of 75 and 50% daylight regimes at the vegetative (V4-5); early flowering (R2); and pod filling (R5) stages of growth. Full exposure or 100% daylight served as the control. In the field and pots, the 75% and 50% light intensities (obtained by covering cages with one or two layers of 1mm mesh net, respectively) significantly enhanced the vegetative growth over plants grown under 100% light intensity. Plants grown under 50% light reduction (L2 plants) had significantly (P=0.05) higher leaf area plant^-1, stem height plant^-1 and number of branches plant^-1 than plants grown under 75% light intensity (L1 plants). Similarly, L1 plants had higher values of these vegetative parameters than plants grown under 100% light intensity (L0 plants). Generally, the results of the pot and field trials were similar. Reduced light regimes led to a higher accumulation of dry matter in the various plant parts particularly when imposed at the vegetative stage of growth. Additionally, reduced light regimes led to reduced leaf chlorophyll concentration and a general reduction in yield. The highest reduction in yield resulted from 50% light intensity imposed for two weeks at the pod filling stage of growth. In pots the seed dry weight plant^-1 of L0, L1 and L2 plants were 40.6g, 30.4g and 23.3g, respectively. While for the field trials, the seed dry weight plant^-1 for L0, L1, L2 plants were 37.3g, 34.3g and 32.5g, respectively. The seed dry weight plant^-1 for light reduction at the vegetative stage (35.5g) was similar to that at the early flowering stage (35.5g) but was significantly higher than the seed dry weight plant^-1 at the pod filling stage (33.2g). Results reveal that reduced light intensity was most damaging to soybean performance at the pod filling stage and the lower the light intensity the greater the yield reduction.

Key Words: Chlorophyll content, Glycine max, light intensities, soybeans, stage of growth, yield

Résumé

Le soja (Glycine max (L.) Merrill) est une source importante et bon marché de proteines végetales au Nigeria. Sa production est contrainte par la faible intensité lumineuse dans l'écologié de forêt pluvieusedu sud due à la couverture des nuages pendant la saison de production. Comprendre les bases physiologiques de la performance sous la faible intensite lumineuse est par consequent centrale pour l'amélioration des efforts dans la zone. Les plantes de soja ont été soumises à deux semaines de 75 et 50% de régime de lumierè du jour a l'étape de croissance végétative (V4-5); defloraison precoce (R2) et de remplissement de gousses( R5), 100% de la lumiere journaliere a servi de contrôle. Sur le champs et dans les pots, les 75% et 50% d'intensité lumineuse(obtenue par cages couverte avec une ou deux couches de 1mm mailles de filet, respectivement) d'une manière significative ont accrue la croissance végétale par rapportux plantes poussant sous 100% d'intensité lumineuse. Les plantes poussant sous 50% de lumière réduite (plantes L2) ont eu de manière significative (P=0.05) le nombre élevé de feuilles par plante, surface de feuilles par plante, hauteur de tiges par plantes et le nombre de branches par plante que les plantes cultivées sous 75% d'intensit' lumineuse (plantes L1). Similairement, les plantes L1 ont eu des valeurs élevées de ces paramètres végétaux que les plantes poussant sous 100% d'intensite lumineuse (plantes L0). Généralement, les résultats des essais des pots et du champs étaient les mêmes. Les régimes lumineux réduits ont entrainé une grande accumulation de la matière séche dans les differentes parties des plantes particulièrement quand où impose l'etape végétative de dévelopement. Les régimes lumineux réduits ont conduit à la réduction de la concentration de la chlorophylle par feuille et en générale, la réduction en production. La plus grande réduction en rendement a résulté de 50% de lumière imposée en deux semaines à l'tape du development du remplissage des gousses. Dans les pots, le poids des graines par plant de plantes L0, L1 et L2 ont été de 40.6g, 34.4get 23.3g, respectivement. Alors que sur le champs, la matière séche des graines par plante pour la réduction lumineuse à l'etape végétative (35.5g) était similaire à celle des plantes à l'etape de floraison précose (35.5g) mais significativement élevée que les poids de graines par plante de l'étape de remplissement des gousses (33.2g). Les résultats ont montré que l'intensité lumineuse réduite pour deux semaines étaient plus destructive à la performance du soja à l'étape de remplissement de la gousse et plus faible était l'intensité lumineuse, plus grande était la reduction de rendement.

Mots Clés: Le contenu en hlorophylle, Glycine max, intensités lumière, soja, stode croissance

INTRODUCTION

Soybean,(Glycine max (L.) Merrill) is a food legume of considerable nutritional potential in Nigeria where animal protein is inadequate in human diets. Soybean provides oil for domestic cooking and cakes for animal feeds (Knipscheer and Ay, 1992) as well as snaks for human consumption (Uwaegbute, 1999). Milk extracted from soybean is a cheaper alternative to cow's milk. Through N₂-fixation, soybean is able to contribute N to the soil through the mineralisation of its residue left in the field thereby building up the N status of the soil (Muyinda et al., 1998). It is therefore being considered for soil fertility improvement (Carsky et al., 1997). In spite of the vast potentials of soybean, its production is constrained by a number of problems which include limitations posed by soil and other environmental factors. The low light intensity prevalent in the forest zone of south west Nigeria in the rainy season has been identified as responsible for reducing crop productivity. Crop yields for instance in the forest zone are consistently lower than in the savanna zone (Ezedinma,1973; Kassam and Kowal, 1973). Most soybean in Nigeria is produced in the savanna zone where higher light intensity abound compared to the southern part, yet total production is still relatively low. The level of soybean production in the country can be improved by extending soybean production to the southern forest zone. Before this can be successfully done, however, the response of soybean to reduced light intensity occasioned by shading by taller intercrops in this area, needs to be studied.

Light is an important resource in crop production because of its roles in photosynthesis and morphogenesis. It is therefore important to grow crops under light intensity that will maximise growth, development and yield. More often than not however, plants do not receive optimum light intensity during cultivation as a result of factors such as dense cloud covers and shading by taller intercrops (Evans,1972). Field grown soybean in Nigeria is generally intercropped with maize (Zea mays) and cassava (Manihot spp.). This particular cropping system has been reported to reduce the yields of soybean by up to 60% in Indonesia (Surmano,1987). This study therefore was undertaken to determine the response of three soybean cultivars to varying light intensities at various stages of growth and development, in south west Nigeria.

MATERIALS and METHODS

A study was conducted in the field and in pots. Both experiments (field and pot) were conducted under caged conditions. The three soybean varieties used for this study were obtained from the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria. The soybean varieties and their characteristics are as follows:

TGx 1485-1D: Early maturing (95 days after planting) erect, determinate type,

TGx 849-313D: Medium maturing (103-105 days after planting), semi-determinate type, and

Malayan: Late maturing (110-115 days after planting), semi-determinate type.

Cage construction. The cages used for the pot and field trials were made of 5cm x 5cm wood. The internal dimensions of each cage were 1.8m x 1.2m x 1.3m. The wooden frames were covered on all sides with single or double layers of synthetic,green,1mm mesh net to reduce light intensity by 25% or 50%, respectively. The light intensities within and outside the screens were measured using a light meter Model 4555 type C (Megatron, England).

Pot experiment. Seeds of the soybean varieties were sown in 675 plastic pots (5 litre capacity having 20cm rim diameter),each containing 3.95kg soil obtained from the plot where the field trial was to be carried out. Planting was done on 6 September, 1992.Three days before this, N P K (15:15:15) fertiliser was applied at the rate of 50 kg NPK per hectare, and thus 1.47g of fertiliser were added to each pot. Sufficient numbers of pots were planted to take care of unforeseen events during experimentation and for extraction of chlorophyll. Five seeds were sown per pot at a depth of 3cm. The resulting seedlings were thinned to one per pot two weeks after sowing.

The pots were arranged in a split - split plot layout on the roof top garden of the Department of Crop Protection and Environmental Biology, University of Ibadan. The soybean varieties served as the main plots; stage of growth as sub-plots; and light regimes as sub - sub plots, with five replicates. Randomisation was done according to the procedures of Gomez and Gomez (1984). Two light regimes (75 and 50% daylight) were imposed for two weeks on the soybean varieties at the vegetative (V4-5, when the plants had four to five nodes on the main stem beginning with the unifoliolate node);early flowering stage (R2,when the plants flowered at the node immediately below the uppermost node with completely unrolled leaf);and the pod filling stage (R5,when the seeds were beginning to develop and can be felt when squeezed at one of the four uppermost nodes with a completely unrolled leaf) (Fehr and Caviness,1977).

Water supply and weeding were carried out as required. Sixty five pots of the early maturing variety and eighty pots each of the medium and late maturing varieties were removed at the vegetative stage of growth and arranged inside the cages. At the flowering and pod filling stages, fifty pots of early maturing variety and 65 pots each of the medium and late maturing varieties were transferred into the cages. The plants remained inside the cages for two weeks at each stage of growth.

Sampling and measurement. The treated plants were removed from the cages after two weeks and returned to the open roof top. Sampling commenced a day after the expiration of treatment and continued at fortnightly intervals thereafter. On each sampling occassion, five plants per treatment were evaluated for leaf area, stem height, number of branches and number of pods. The dry weights of leaves, stem, roots, pods and the total dry weight plant^-1 were also determined.
At final harvest (plant maturity),data were also taken on the following yield characters: number of seed-bearing pods plant^-1 number of empty pods plant^-1 total seeds plant^-1 and seed dry weight plant^-1. The height of the shoot apex from the ground was measured and recorded as the plant height while leaf area was determined by the graph paper method. The dry weights of the various plant parts were measured using the mettler balance P1210 after oven drying the samples at 80°C for 48 hr.

Leaf chlorophyll. Chlorophyll was extracted from the second and fourth leaves obtained from the plants under the various light regimes. The standard procedures of Arnon as used by Hang et al. (1984) were used for the chlorophyll extraction. The absorbances of chlorophyll extracts were measured against 80% acetone blanks using Pye-Unicam SP6-250 visible spectrophotometer. The amounts of chlorophyll a and b in the leaves of the plants were determined using Arnon formula (Hang et al., 1984) described below:

C= (20.2 x D645+ 8.02 x D663)x 50/1000x100/5x1/2

Where C = chlorophyll concentration (mg g^-1)
D645= absorbance at 645nm (chlorophyll a)
D663= Absorbance at 663 nm (Chlorophyll b)

Soil analysis. Pre-cultivation soil analysis was done according to the procedures of Udo and Ogunkunle (1986).

Field experiment. The experiment was carried out at the Teaching and Research Farm, University of Ibadan. The layout consisted of three main plots, nine sub-plots and twenty-seven sub-sub plots, replicated five times. Each main plot measured 4.2m x 6.0m,each sub-plot measured 1.4m x 6.0m while each sub-sub plot measured 1.4m x 2.0m. The main plots were separated by 1m rows on all sides; sub-plots were demarcated by pegs while sub-sub plots were separated by gaps created along the rows of soybean plants after emergence. The varieties were randomly allocated to main plots, stage of growth to sub plots and light regimes to sub sub- plots. The experimental design was therefore a split-split plot.
Planting was done on 19 July,1994. The seeds were drilled along each row. Spacing was 60cm x 5cm which is the recommended spacing for soybean growing farmers in south west Nigeria. Thinning was done 10 days after sowing (DAS). The plots were weeded at 3, 6 and 10 weeks after sowing. Fertiliser (NPK 15:15:15) was applied at a rate of 50kg ha^-1, two weeks after sowing. Light reducing cages were reassembled on the field and treatment imposition on the soybean plants as well as the sampling methods and data collection were as in the pot experiment.

Statistical analysis. The data gathered from the pot and field trials were separately subjected to split-split plot analysis of variance using the statistical analysis system (SAS) computer software. Comparisons of the various treatment means were done using the least significant difference (L.S.D)at the 5% level of significance.

RESULTS

Pot experiment. The late maturing variety (V3) had a larger leaf area than the medium (V2) and early (V1) maturing varieties one day after treatment. However, six weeks after treatment (WAT) V1 and V2 had a greater leaf area than V3. The number of branches of V3 plants was significantly higher than those on V1 and V2 after treatment. The stem heights of V3 and V2 were similar but significantly higher than that of V1plants. The number of pods was highest in V2 and lowest in V1 plants at maturity (Fig. 1).

The leaf dry weight of V3 plants was initially higher than those of V1 and V2 plants but at 6 WAT, the leaf dry weight of V2 plants was the highest and that of V1 lowest. The stem and root dry weights of V2 plants were significantly higher than those of V3 and V1with V1 having the lowest values throughout the sampling period (Fig.1).

Initially, the soybean plants treated at the flowering stage (S2) had significantly larger leaf area than plants treated at the vegetative (S1) and pod filling (S3) stages of growth. Later S1 plants had larger leaf area than S2 and S3 plants. Stem height and branch number were similar in S2 and S3 plants although the plants had significantly more branches than S1 plants. The S1 plants had the lowest number of pods while S2 plants had the highest from 2-6 WAT. However, pod number was lowest in S3 plants and highest in S1 plants at maturity (Fig. 2).

The dry weights of the various plant parts were initially lower in S1 plants than S2 and S3 plants. The dry weight later increased in the various parts of S1 plants such that they had the highest total per plant at final harvest (Fig. 2).

Plants grown under reduced light regimes had significantly larger leaf area than plants grown in the open. Throughout, the tallest plants were those grown under 50% light intensity (L2 plants) while the shortest were those grown under 100% light intensity (L0 plants). Plants grown for two weeks under 75% light intensity (L1 plants) had intermediate values. At maturity L0 plants had significantly more pods than L1 and L2 plants. The number of pods per plant exposed to L0, L1and L2 treatments were 34.9g, 26.9g 23.3g, respectively. The differences were significant (Fig. 3).

Plants subjected to L2 treatment had significantly higher leaf, stem and root dry weights than L0 and L1 plants, for most of the growth period after treatment. Pod dry weight and total dry weight plant^-1 for L0 treated plants were significantly higher than under L2 at plant maturity (Fig. 3).

Medium maturity variety had significantly higher values of seed-bearing pods plant^-1, total seeds plant^-1 and seed dry weight plant^-1,while V1 had the lowest values for these parameters. All these differences were significant. Plants subjected to light intensity treatments at pod filling (S3) had the lowest values of these yield parameters. These parameters were also significantly higher in L0 plants than in L1 and L2 plants, with L2 plants having the lowest values. The greatest number of empty pods were in S3 and L0 plants (Tables 1 and 2).

Chlorophyll concentration was highest in V1 and lowest in V3. Plants treated at vegetative stage (S1) had significantly higher chlorophyll concentration than S2 plants, which in turn had higher chlorophyll concentration than S3 plants. The order of chlorophyll concentration was L0 > L1 > L2 (Tables 1 and 2).

The values of the interaction effects for variety and stage of growth involving the pod filling stage (S3) was the lowest for the three yield parameters (i.e., seed bearing pods plant^-1, total seeds plant^-1 and dry weight plant^-1) and chlorophyll considered (Table 3). Similarly, each of the varieties had the lowest values of the yield parameters at the lowest light intensity. The combination of the lowest light intensity at the pod filling stage produced the most depressing effects on the yield parameters and chlorophyll concentration (Table 3).

Field experiment. Initially, V3 plants had a bigger leaf area than V1 and V2 plants but at 6 WAT V2 had a higher leaf area than V1 and V3. The late maturing variety (V3) also produced more branches than V1 and V2 plants for most of the post treatment period. Both V2 and V3 were significantly taller than V1 throughout the post treatment period but V3 was significantly taller than V2 at maturity. The medium maturing variety (V2) had significantly more pods than V3 plants which also had significantly more pods than V1 plants at maturity (Fig. 4).

The leaf dry weight of V2 was highest at 6 WAT. The stem and root dry weights of V2 were significantly higher than those of V1 and V3 throughout the post treatment period. The pod and total dry weights of V2 were also significantly higher than those of V1 and V3 plants. The early maturing variety (V1) had the lowest pod dry weight (Fig. 4).

From 4 - 6 WAT, S1 plants had bigger leaf area compared with S2 and S3 plants. The heights of S1, S2 and S3 plants were similar at maturity. Also, the number of branches of all the treated plants were not significantly different at maturity. The lowest number of pods was produced by S3 plants (Fig. 5).

Plants that received treatment at flowering stage (S2) had the highest values of root, pod and total dry weights while S1 plants produced the highest dry weight of stem and leaves at 4 and 6 WAT. The differences were significant (Fig. 5).
Plants grown for two weeks under reduced light regimes had significantly bigger leaf area than plants grown in the open. Throughout the growing season, L2 plants were the tallest while L0 plants were the shortest. Plants exposed under 75% light intensity (L1) had intermediate heights. Branch number was similar for all treatments. At plant maturity, L0 plants had significantly more pods than L1 and L2 plants. The number of pods of L0, L1 and L2 plants averaged 166, 161 and 157, respectively (Fig. 6).

Towards maturity, L0 plants had significantly greater leaf and stem dry weights than L1 and L2 plants. Root dry weight was highest for plants under L0 at crop maturity. The L0 plants also produced the heaviest pods. The order of pods and total dry weights as influenced by light intensity treatments were L0 > L1 >L2. The yield characteristics and leaf chlorophyll concentration of field-grown plants were similar to that obtained in the pot experiment (Table 4).

For interaction effects, the observation in pots was accentuated. For instance, the seed dry weight of V2xS3 was significantly lower than those of V2xS1 and V2xS2. Similarly,the seed dry weight of V3xS3 was significantly lower than that of V3xS1. The interaction of the lower light intensities led to lower seed dry weight compared to varietal interaction with L0 (Table 4).

DISCUSSION

Reduced light intensities imposed for two weeks at the vegetative stage significantly enhanced the vegetative growth of soybean cultivars used in this study. Such plants that were grown under subdued light had more leaf area and grew taller as compared to plants grown in the open throughout the growing season. This is attributed to the tendency of such plants to grow more vigorously under better illuminated conditions (Fitter and Hay, 1977). Mohr (1972) reported possible involvement of phytochrome in the control of internodal lengthening of these plants. The increased leaf area under subdued light was probably response intended to maintain photosynthetic assimilation at a lower photon flux density coupled with the need to maximise interception of the little available light. This supports reports of Sunarlim (1985), Taiz and Zeiger (1991), Mohr and Schopfer (1995).

The low light levels available for shaded plants might have also caused a restriction of their genetic potential resulting in the modification of their growth pattern. The larger leaf area of plants grown in the open throughout the growing season compared with plants under reduced light regimes for two weeks at the pod filling stage (S3) was due to the rapid loss of leaves in the S3 plants because at the pod filling stage ageing had set in and thus, the rate of renewed vegetative growth after treatment could not match the rate of senescence during treatment.

Total dry matter accumulation was highest in plants grown in the open at the three stages of growth considered in this study. Even though plants grown under reduced light intensity initially (just after treatment) had higher stem, leaf and root dry weights compared with plants grown in the open, the latter plants eventually had the highest values of these parameters especially at harvest maturity. It is clear, therefore, that plants grown under subdued light for two weeks at different stages of growth were unable to recover from the effects of growth under reduced light intensity and hence the lower dry matter accumulation which invariably probably led to reduced yields in these plants.

Results of this study reveal that yield reduction was obtained when light intensity was reduced for two weeks at the vegetative, early flowering and pod filling stages of growth. However, yield reduction was lowest with light reduction at the vegetative stage. The reduced yield at all stages stemmed directly from lower number of pods plant^-1, reduced seed weight plant^-1 and lower chlorophyll concentration plant^-1. The lower leaf chlorophyll concentration observation is in agreement with reports of Sunarlim (1985) who recorded lower chlorophyll contents in leaves of plants grown under subdued light intensities. Since chlorophyll is sensitive indicator of photosynthesis that reflects photosynthetic carbon assimilation capacity(Sivak and Walker,1985), it follows that leaves with lower chlorophyll concentration will have lower assimilates available for seed growth and development and hence giving low yields.

The greatest yield reduction obtained by shading at the pod filling stage contrasts with findings of Prine,(1976) who recoreded slight reduction in seed weight with minimal changes in seed number. Differences in the varieties used in this study and those used by Prine may account for the varying observations of the two experiments. Besides, differences in the prevailing environmental conditions of Prine's experiment and that of this study could also have contributed to the observed variations. Varietal differences in maturity period as well as growth habit might have influenced source/sink relationships such that late maturing varieties having indeterminate growth habit might be less affected by reduction in light intensity at the pod filling stage. Generally, at pod filling growth stage, most assimilates produced by the plants are used for pod filling so that reduced net photosynthesis due to shading at this time must have reduced the amount of assimilates available for pod filling hence the highest reduction in yield.

The increased vegetative growth under reduced light intensity did not ultimately translate into higher yields. This was because the enhanced vegetative growth was more of reduced senescence than actual leaf production. It is known that reduced reproductive development of sinks moderate source activities such that reduced photosynthesis (in reaction to reduced demand) can lead to longer leaf area duration (LAD) later. Reduced light intensity, in effect, reduced reproductive development of induction of floral primordia (for shading in the vegetative phase) and/or number of reproductive nodes(for shading in the early reproductive phase). Soybean production in south west Nigeria is affected by low light intensity. To counteract or limit this effect, it is suggested that soybean should not be cultivated with taller intercrops particularly if they have a longer growth cycle. This is important in order to avoid shading in general and particularly at the pod filling stage. In addition, cultivation of soybeans should be such that the most critical stage (R5) does not coincide with the period of heavy over cast skies. This can be achieved by planting soybean at a period that allows the R5 stage coincide with the month of August when there is usually less rainfall and hence clearer skies in south west Nigeria. The suitability and adoption of this suggestion will, however, depend on the onset of the rains and the maturity period of the variety to be cultivated.

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