Crop Science 43:1375-1379 (2003)
© 2003 Crop Science Society of America
CROP ECOLOGY, MANAGEMENT & QUALITY
The Impact of Temperature on Leaf Appearance in Bambara Groundnut Landraces
Festo J. Massawe*,
Sayed N. Azam-Ali and
Jeremy A. Roberts
Tropical Crops Research Unit, University of Nottingham, School of Biosciences, Sutton Bonington Campus, Loughborough LE12 5RD, UK
* Corresponding author (festo.massawe{at}nottingham.ac.uk)
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ABSTRACT
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Ten bambara groundnut [Vigna subterranea (L.) Verdc] landraces of diverse origin were used to examine the effect of temperature on the rate of leaf appearance (RLA) and to determine the base temperature (Tb) among landraces. Knowledge of leaf development is required in breeding programs where crop morphological development is considered an important selection criterion. The ability to predict leaf appearance would also facilitate modeling of plant development. Plants were grown in controlled-environment growth rooms under different temperatures (20, 25, 30, 32, and 35°C ± 1). The study to examine the effect of temperature on leaf appearance in bambara groundnut landraces indicated that the base temperature (Tb) for leaf appearance ranged from 8.1 to 12.0°C. Within the range of temperatures evaluated in this study, the observation of differences in Tb values among the landraces suggests that Tb values for leaf appearance are landrace dependent. The phyllochron was also variable among landraces ranging from 40.9 to 53.0°C. The RLA ranged from 0.19 to 0.63 leaves d-1 depending on temperature and landrace. The landrace variations in relation to leaf appearance may allow selection of landraces for different environments.
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INTRODUCTION
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BAMBARA GROUNDNUT is an indigenous African legume cultivated mainly by subsistence farmers in Africa under traditional low input agricultural systems. The crop provides a rich source of protein and, along with other local sources of protein, plays a major nutritional and socio-economic role in the semiarid regions of Africa. Bambara groundnut yields are low because the production environments are characterized by various abiotic and biotic stresses. However, even under optimal conditions the yields are variable and unpredictable and this is partly due to variability in growth and development of individual plants within landraces (Squire et al., 1997).
Canopy development is an important determinant of crop radiation capture, and, in the absence of stress, is mainly driven by temperature. The ability to predict leaf appearance would facilitate modeling of plant development and the rate of leaf expansion. In crop growth models, leaf area is derived from the relationship between temperature and the RLA and the relationship between leaf number and leaf area (Bonnett, 1998). Knowledge of canopy development is also needed in breeding programs where crop morphological development is considered an important selection criterion. The effect of temperature on vegetative development of bambara groundnut such as leaf appearance and leaf expansion, has not been examined in depth (Collinson et al., 1996; 1997; Anonymous, 1997). The crop exhibits a considerable degree of phenotypic diversity in morphology, growth habit, and crop duration (Begemann, 1988; Linnemann and Azam-Ali, 1993; Collinson et al., 1996; 1997; Anonymous, 1997). It is therefore likely that the influence of temperature on vegetative development is not uniform among genotypes.
The influence of temperature on growth and development of different cowpea [Vigna unguiculata (L.) Walp.) genotypes has been reported by Craufurd et al. (1996a)(b, c) and Craufurd et al. (1997). For example, Craufurd et al. (1997) reported that the Tb for leaf appearance in cowpea ranged from 9.0 to 12.0°C, while the phyllochron was the same in all the genotypes examined. Understanding the influence of temperature on the vegetative development of bambara groundnut would provide useful information to modelers (Linnemann and Craufurd, 1994; Brink, 1997). For example, in crop simulation modeling, Tb for different developmental stages in bambara groundnut has been assumed to be the same as for germination and also the same value is used for different landraces regardless of their geographical origin (Linnemann, 1994; Brink, 1999).
The objective of the present study was to investigate the effect of temperature on leaf appearance and to determine the base temperature (Tb) and phyllochron for leaf appearance in contrasting bambara groundnut landraces.
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MATERIALS AND METHODS
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Landraces of bambara groundnut used in this study, their origin, 100-seed weight, and initial seed moisture content measured by oven-drying at 105°C for 48 h and expressed as a percentage of the oven dry weight, are listed in Table 1. Because of the possible effects of the seed source and the year of harvest on the parameters studied, landraces were grouped on the basis of their year of harvest and source. The first group comprised of seeds from Tanzania that were harvested in 1997, i.e., DodC1997 and DodR1997. In the second group, seeds were the progeny of the plants used in the Tropical Crops Research Unit glasshouse experiment at the University of Nottingham, UK, in 1995. These seeds were stored at 4°C and included DodR1995, DipC1995, and LunT1995. These landraces were originally obtained from Tanzania, Botswana, and Sierra Leone, respectively. The third group included seeds obtained from Malawi in 1998, i.e., Malawi1, Malawi2, Malawi3, Malawi4, and Malawi5.
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Table 1. List of bambara groundnut landraces used in this investigation. Values are mean and standard deviation for five seed samples of each landrace.
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Seeds were sown in the growth rooms at 20, 25, 30, 32 and 35°C ± 1 with a 12-h daylength supplied by fluorescent tubes and tungsten lamps [65 W; white light providing approximately 320 µmol m-2 s-1 PAR (photosynthetically active radiation) at plant height). Table 2 shows the PAR measured with a quantum sensor (Skye Instruments, UK) recorded at randomly selected pot positions in each of the growth rooms. NOTE: 12-h daylength was used in this experiment on the basis of results from previous experiments conducted at the Universities of Nottingham and Wageningen in UK and the Netherlands, respectively.
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Table 2. Photosynthetically Active Radiation (PAR) (x10 µmol m-2 s-1) measured with a quantum sensor recorded at randomly selected pot positions in each of the growth rooms.
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Seeds were sterilized for 15 min in 10% sodium hypochlorite (v/v) and were rinsed several times with distilled water before sowing in plastic pots (21-cm diameter). The pots were perforated at the bottom to allow free drainage of water and filled with loam-based compost (John Innes Compost No. 2, UK) mixed with silver sand (William Sinclair Horticulture Ltd, UK) in the ratio of 3:1, respectively. The soil was initially wetted to field capacity and was kept moist by adding water when required. The experiment was arranged in a complete randomized design with three replicate pots (each pot with five seeds) for each treatment (temperature regime) for each landrace. Temperature treatments were not replicated but the entire experiment was repeated three times. Seeds were sown at a 3-cm depth and seedling emergence was determined each day after sowing (DAS). A seed was considered to have emerged when the epicotyl hook appeared above the soil surface. Pots were redistributed within the growth room each day to ensure a random arrangement of the pots because the measurements of PAR indicated some variability within the growth room (Table 2). Each pot was irrigated with an equal amount of water when necessary to avoid stressing the plants. Irrigation was more frequent in the growth room set at 35°C than in the growth rooms set at lower temperatures.
Leaf numbers (a bambara groundnut leaf consists of three leaflets) were recorded at 3-d intervals for five plants of each landrace per treatment from 10 to 60 DAS. The RLA (leaves d-1) in all individual landrace x temperature treatments was estimated by regressing the mean (of replicates) cumulative leaf number against DAS. These values of RLA were regressed against mean temperature over the period when the RLA was estimated to determine the value of Tb and phyllochron. The Tb value was the point of interception of the regression line established by extrapolation of the fitted line and the phyllochron values were the degree days accumulated above Tb values (thermal time) between the appearances of successive leaves. Linear regression equations were fitted by means of either Microsoft Excel 97 or Genstat Statistical Packages.
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RESULTS AND DISCUSSION
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There was a significant (P < 0.001) linear relationship between RLA and temperature (Fig. 1A) in all landraces. Temperature had a highly significant effect (P < 0.001) on RLA among the landraces and RLA increased with elevated mean temperature from 20 to 32.5°C but decreased at a constant temperature of 35°C (Fig. 1B). For example, in DodR1997, the RLA at 25 and 32.5°C was 0.31 and 0.54 leaves d-1, respectively. On average, the interval between the appearances of successive leaves ranged from 3.8 to 6.0 d (SE 0.2) at 20°C and 1.6 to 2.3 d (SE 0.1) at 32.5°C (Table 3).
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Fig. 1. Relationship between the rate of leaf appearance (RLA) and mean temperature for 10 bambara groundnut landraces. (A) Combined data for all 10 landraces showing linear relationship between RLA and temperature (data at 35°C are not included) (B) Scatter diagram indicates observation for each landrace. Points are observed data and solid line indicates the fitted line. Data are means of three replicates.
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Table 3. Interval in days between the appearances of successive leaves at different temperatures. Data are mean of three replicates.
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The ability to predict the timing of successive leaf appearance is important because of its influences on the timing of developmental stages of the plant and on the rate of leaf expansion. The RLA in bambara groundnut landraces, as shown in this study, is linearly related to temperature. Similar results have been shown in other tropical legumes (Craufurd et al., 1997; Qi et al., 1999). However, there are studies that have shown the temperature response for the RLA to be non-linear, at least in cereals (Jame et al., 1999). Although outside the scope of the present study, it has been shown in other legumes that photoperiod does not affect the RLA (Craufurd et al., 1997 and Qi et al., 1999). In bambara groundnut, there is evidence to suggest that photoperiod affects the production of leaves such that when plants are grown under long photoperiods they produce more leaves than those grown under short photoperiods (Linnemann, 1994; Brink, 1999). However, this may be related to the delay in flowering and pod set under long photoperiods.
The regression equations for individual landraces were compared to determine whether all 10 landraces had a common Tb that can be used to describe their RLA. The Tb ranged from 8.1°C (for five landraces from Malawi) to 12.0°C (for DodR1995, DipC1995 and LunT1995) (Fig. 2). Since the Tb for leaf appearance ranged from 8.1 to 12.0°C, depending on the landrace, the effect of temperature on the RLA cannot be simplified by assuming a common Tb values for different landraces. Tb values for germination (11.5–12.3°C) (Massawe, 2000) and those for leaf appearance are not different, although the range was much wider in leaf appearance. Establishing Tb values for flowering and podding was not within the scope of the present study; however, Tb values as low as 3°C for flowering and podding have been reported in bambara groundnut (Linnemann, 1994). In cowpea, Tb values for different developmental stages such as seedling emergence and time to first flowering have been reported to be similar (Craufurd et al., 1997). It should be noted, however, that Tb values are established from extrapolation of data and for tropical crops where the lowest possible temperature for growth and development of plants may be above 15°C, the calculated value of Tb of around 3°C based on extrapolation has no biological significance.
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Fig. 2. Rate of leaf appearance (RLA) for (A) DodC1997 and DodR1997, (B) DodR1995, LunT1995 and DipC1995, and (C) five landraces from Malawi against temperature (data at 35°C are not included). Points are observed data and solid–dotted lines indicate the fitted line. Regression equations describe the combined data sets.
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Within the range of temperatures used in this study, the observation of differences in Tb values among the landraces suggests that Tb for RLA is not a species characteristic but rather depends on the landrace. For modeling of leaf appearance in bambara groundnut, Tb values have to be established for different landraces and from the present study values of 8.1 to 12.0°C can be adopted. However, this should be investigated for different photoperiodic groups of bambara groundnut landraces because photoperiod has been shown to affect leaf production (Linnemann, 1994; Brink, 1999). Tb is the temperature at and below which the process of development and expansion does not proceed. The Tb should be warm enough such that when a temperature just above the Tb is applied the development process should proceed. The observed variability in Tb values implies that there is a scope for selection of landraces, in breeding programs, that will be adapted to different environmental conditions.
To produce a leaf, landraces required on average 44.9°C d (40.9–53.0°C d SE 1.2) to be accumulated above Tb values of between 8.1 to 12°C depending on the landrace (Table 4). The phyllochron, which is the thermal time between the appearances of successive leaves, was significantly different (P < 0.05) among landraces, ranging from 40.9 to 53.0°C d for Malawi5 and DipC1995, respectively. The phyllochron was calculated for each landrace on the basis of the Tb value of that particular landrace. While studies performed using other legumes have reported the phyllochron to be constant and Tb to vary among genotypes (Sinclair, 1984; Craufurd et al., 1997), this study clearly shows that the phyllochron is different for different landraces and Tb values were also variable depending on the landrace. It should be noted, however, that in cereals the phyllochron is a genetic coefficient, which is cultivar specific and in model predictions different values are used for different genotypes (Jame et al., 1999). The range of Tb values (8.1–12.0°C) reported in this study is similar to that reported for other tropical legumes such as cowpea (Craufurd et al., 1997) where Tb values ranged from 7.0 to 12.0°C among genotypes. The phyllochron (average value of 44.9°C d) obtained in the present study was slightly higher than that reported in cowpea (average value of 42.0°C d) (Craufurd et al., 1997) but lower compared to a value of 56.0°C d reported for groundnut (Leong and Ong, 1983).
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Table 4. Degree days (accumulated above Tb values of between 8.1 to 12.0°C) required to produce a leaf for different landraces.
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Relationships between temperature and canopy development (leaf number and leaf size) are difficult to determine from field data alone because temperature fluctuates daily and field environments provide a narrow range of temperatures as the season constrains when crops can be grown (Robertson et al., 1998). The use of controlled environments in the present study provides a link to field studies by defining more reliably the relationships between temperature and individual processes of growth and development. This can then be tested in the field environments, where most variables cannot be easily measured or controlled. Although there are a number of other factors that influence the RLA and leaf expansion (Jame et al., 1999), the present study was designed to establish the Tb and the phyllochron among different landraces while maintaining other environmental factors such as photoperiod, water, light, and nutrients at optimal levels. The landrace variations in relation to leaf appearance may allow selection of landraces for different environments. For example, landraces with relatively rapid leaf production (a small phyllochron) would be more suitable for producing a large biomass in a short period and for providing early ground cover, which could reduce surface evaporation and suppress weeds.
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CONCLUSION
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Using controlled environments, we identified in the present study a range of Tb values for leaf appearance and phyllochron for different bambara groundnut landraces. Tb and phyllochron values were variable reflecting landrace differences and these should be taken into account in crop simulation modeling and in bambara groundnut improvement programs. Determining the effect of differences in phyllochron for different landraces on the rate of canopy development with thermal time will require further work involving more landraces of diverse geographic origin.
Received for publication August 1, 2002.
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REFERENCES
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ABSTRACT
INTRODUCTION
