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Allelopathic potential of five agroforestry trees, Botswana

Anne Lillian Nakafeero1, Mark S. Reed2 and Nkbobi M. Moleele3*

1 Fisheries Office, Entebbe Municipal Council, Entebbe, Uganda, 2Sustainability Research Institute, School of Earth & Environment, University of Leeds, Leeds LS2 9JT, UK and 3Department of Environmental Science, University of Botswana, Private Bag 00704, Gaborone, Botswana

Abstract

Agroforestry systems have the potential to enhance agricultural production and buffer rural livelihoods against drought in semi-arid countries such as Botswana. However, allelopathy constrains the range of tree species that can be used. Concentrations of allelopathic compounds can become particularly high in semi-arid soils because of low leaching and high evaporation rates, leading to a reduction in sub-canopy vegetation biomass. In this study, allelopathic potential was evaluated for five multi-purpose trees that are currently promoted for agroforestry in Botswana: Acacia erubescens, Acacia tortilis, Combretum imberbe, Sclerocarya birrea and Terminalia sericea. It was established that leaves from each species contained a variety of phenolic compounds (flavones, flavonols, flavonones, anthocyanins, leucoanthocyanins, coumarins and tannins) and alkaloids (tertiary and quaternary alkaloids) in varying concentrations. Although the implications for crop production are still uncertain, these results suggest that agroforestry extension in Botswana should be wary about recommending the use of these species before further research. Key words: agroforestry, alkaloids, allelochemicals, allelopathy, Botswana, phenolic compounds

´ le potentiel allelopathique pour cinq arbres dont on fait divers usages et dont la culture est actuellement en´ couragee pour l'agroforesterie au Botswana : Acacia erubescens, Acacia tortilis, Combretum imberbe, Sclerocarya ´ birrea et Terminalia sericea. On a etabli que les feuilles de ` ´ ´ ´ chaque espece contenait une variete de composant phenolique (flavones, flavonols, flavonones, anthocyanine, coumarines et tanins) et des alcaloides (tertiaires et qua¨ ´ ternaires) en concentrations variees. Bien que les implications pour la production soient encore mal connues, ` ´ ces resultats suggerent que l'extension de l'agroforesterie devrait se faire avec prudence au Botswana quand il s'agit ` de recommander l'utilisation de ces especes avant que l'on ´ ´ ait realise de plus amples recherches.

Introduction

In semi-arid environments, agroforestry has the potential to enhance agricultural production and buffer rural livelihoods against drought. For example, nitrogen-fixing trees can enhance the nutrient status of typically poor semi-arid soils and reduce wind erosion (Rao et al., 1999). They can also provide shade and fodder for livestock during drought, in addition to providing a range of other goods and services such as fruit, fibre, gums, firewood and dune stabilization (Nair, 1991). Although agroforestry in Botswana is largely limited to Government research stations at present, attempts are being made to increase its uptake among farmers. However, there are concerns that many of the indigenous species currently recommended for agroforestry may reduce crop and fodder production through their allelopathic effects on sub-canopy vegetation. Allelopathy refers to the effects (both harmful and beneficial) that plants can have on one another through the release of chemicals by processes such as leaching, root exudation and volatilization. Allelopathic chemicals such as phenolic compounds and alkaloids from trees have been observed to suppress yields in a variety of food and fodder crops (Rizvi et al., 1999). Alkaloids and phenolic compounds are known to have toxic or inhibitory effects on a wide range

´ ´ Resume

` Les divers systemes d'agroforesterie peuvent augmenter la ´ production agricole et reguler les moyens d'existence en ´ milieu rural en cas de secheresse, dans des pays semi-arides ´ comme le Botswana. Cependant, l'allelopathie limite la ` ^ ´ gamme d'especes d'arbres qui peuvent etre utilisees. Des ´ concentrations de composants allelopathiques peuvent ` ´ ´ devenir particulierement elevees dans des sols semi-arides ´ ´ ´ en raison de la faible filtration et de taux eleves d'eva´ poration qui entrainent une reduction de la biomasse ^ ´ ´ ´ ´ ´ vegetale sous la canopee. Dans cette recherche, on a evalue

*Correspondence: E-mail: [email protected]

Ó 2007 The Authors. Journal compilation Ó 2007 Blackwell Publishing Ltd, Afr. J. Ecol.

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of plant species (Gallet & Pellissier, 1997; Wink, Schmeller & Latz-Bruning, 1998). Some alkaloids are known to have particularly long persistence in the soil because of their anti-microbial activity (Wink et al., 1998). Concentrations of allelopathic compounds can become particularly high in semi-arid soils because of low leaching and high evaporation rates (Breman & Kessler, 1995). To investigate these concerns and improve extension advice to farmers, we tested alkaloid and phenolic compound concentrations in the leaves of five trees commonly recommended for agroforestry in Botswana: Acacia erubescens Welw. ex Oliv., Acacia tortilis (Forsk.) Hayne, Combretum imberbe Wawra, Sclerocarya birrea (A.Rich.) Hochst. and Terminalia sericea Burch ex DC.

All leaves were harvested on the same day to control for environmental factors, and combined for each species to form representative samples. They were harvested at midday to take into account diurnal fluctuations in nutrient content (Allen et al., 1974). Harvested tree branchlets (A. erubescens and A. tortilis) were taken to the laboratory for leaf detachment. Samples were air-dried for 14 days at 25­30°C and ground. Colorimetric tests for alkaloid and phenolic compounds were performed for each species and the degree of solution coloration and amount of precipitation was scored from 1 to 5. Comparisons were then made to determine the relative concentration of alkaloids and/or phenolic compounds between species (it was not possible to derive absolute concentrations using this technique).

Methods

The study area (University of Botswana Nature Reserve, Gaborone, 24°40¢S, 25°55¢E) has a mean annual temperature of 20.7°C and a mean annual precipitation of 538 mm (Jonsson, 2000). The soil is dominated by arenosols, leptosols, regosols, cambisols and acrisols, derived from fine and coarse grain, acid and basic rocks. The site has a uniform topography and is fenced. The vegetation naturally consists of deciduous trees and bush savanna (Silitshena & Mcleod, 1994). Acacia erubescens, T. sericea, S. birrea, C. imberbe, and A. tortilis are nitrogen-fixing trees that are promoted by extension services in Botswana. Field work was carried out between January and February 2003. All trees above 2 m were numbered and tagged. Leaves were harvested from 36 randomly selected trees: eleven A. erubescens, six T. sericea, five S. birrea, six C. imberbe and eight A. tortilis.

Results

Table 1 shows that all the trees contained alkaloids, particularly quaternary alkaloids. Tertiary alkaloids were only present in the leaves of A. erubescens, T. sericea and S. birrea. Polyphenols were present in the leaves of all trees (Table 2). Tannins were present only in T. sericea, S. birrea and C. imberbe leaves, whereas coumarins were present in the leaves of all five species. Flavones, flavonols and flavonones were present in some trees and absent in others whereas anthocyanins were present in all trees. Leaves of all trees except C. imberbe contained leucoanthocyanins. The concentration of quaternary alkaloids (based on total scores in Table 1) differed as follows in: C. imberbe > T. sericea > S. birrea > A. erubescens " A. tortilis. Concentration of tertiary alkaloids varied as follows in: A. erubescens > T. sericea > S. birrea. Total concentration of

Occurrence and concentration in: Test Alkaloids collectively Quaternary alkaloids Tertiary alkaloids Total scores A. erubescens 5 2+ 5+ 12+

+

T. sericea X 4+ 4+ 8+

S. birrea X 3+ 3+ 6+

C. imberbe X 5+ X 5+

A. tortilis X 2+ X 2+

Table 1 Occurrence and concentration of alkaloids in leaves of Acacia erubescens, Acacia tortilis, Combretum imberbe, Sclerocarya birrea and Terminalia sericea woody species

+ Presence of the compound under investigation. X, Absence of the compound under investigation. 5+, 4+, 3+, 2+, 1+ ­ the concentration of the compound under investigation; the highest concentration scoring 5+ > 4+ > 3+ > 2+ > 1+. Total score represents the grand score from all tests regarding the compounds in question for a particular woody species.

Ó 2007 The Authors. Journal compilation Ó 2007 Blackwell Publishing Ltd, Afr. J. Ecol.

Allelopathic potential of five agroforestry trees

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Table 2 Occurrence and concentration of phenolic compounds in leaves of Acacia erubescens, Acacia tortilis, Combretum imberbe, Sclerocarya birrea and Terminalia sericea woody species

Concentration in: Test Polyphenols collectively Tannins Coumarins Flavones Flavonols Flavonones Anthocyanins Leucoanthocyanins

+

A. erubescens 3+ X 5+ 4+ X X 1+ 3+

T. sericea 4+ 4+ 3+ 5+ 3+ X 4+ 5+

S. birrea 2+ 5+ 1+ X 5+ 5+ 2+ 3+

C. imberbe 5+ 3+ 4+ 3+ X X 3+ X

A. tortilis 1+ X 2+ X 4+ 5+ 5+ 4+

Presence of the compound under investigation. X, Absence of the compound under investigation. 5+, 4+, 3+, 2+, 1+ represent the concentration of compound under investigation; the highest concentration scoring 5+ > 4+ > 3+ > 2+ > 1+.

alkaloids in all woody species varied as follows: A. erubescens > T. sericea > S. birrea > C. imberbe > A. tortilis leaves. The following phenolic compounds were found with decreasing concentration in the leaves of the trees that were tested (Table 2): · Polyphenols: C. imberbe > T. sericea > A. erubescens > S. birrea > A. tortilis; · Tannin: S. birrea > T. sericea > C. imberbe; · Coumarins: A. erubescens > C. imberbe > T. sericea > A. tortilis > S. birrea; · Flavones: T. sericea > A. erubescens > C. imberbe; · Flavonols: S. birrea > A. tortilis > T. sericea; · Flavonones: S. birrea " A. tortilis; · Leucoanthocyanins: T. sericea > A. tortilis > S. birrea A. erubescens; and · Anthocyanins: A. tortilis > T. sericea > C. imberbe > S. birrea > A. erubescens.

Discussion and conclusions

The presence of alkaloids and phenolic compounds in the leaves of the five woody species implies that they have the potential to inhibit seed germination, and may reduce the productivity of crops of fodder growing in their vicinity (Rice, 1984). However, relative allelopathic effects are likely to differ between the species tested. Tables 1 and 2 show that T. sericea contained nine of the ten tested allelopathic compounds. Compared with the other species, T. sericea had the highest concentrations of flavones and leucoanthocyanins, and the second highest concentrations of tertiary and quaternary alkaloids, polyphenols, tannins

and anthocyanins. In contrast, C. imberbe contained only six of the ten tested compounds. Although C. imberbe had the highest concentrations of quarternary alkaloids and polyphenols, it had the lowest concentrations of tannins and flavones. Using a combination of the number of allelopathic compounds present and their ranked concentration, T. sericea is the most allelopathic, followed in descending order by S. birrea, A. erubescens, A. tortilis and C. imberbe. However, the effect of these compounds on crops in agroforestry systems is likely to be complex, and crops have been shown to vary greatly in their response to allelochemicals from the same tree species. Nevertheless, there is evidence that A. tortilis inhibits germination and seedling growth of a number of crop species (Sundaramoorthy & Kalra, 1991). Although Sundaramoorthy & Kalra (1991) found that allelochemical concentrations were the greatest in tree leaves, concentrations in the soil were sufficient to exert a significant inhibitory effect on crops. Previous work on S. birrea has identified a number of phenolic compounds, particularly concentrated in the leaves (Braca et al., 2003), but there has been no research into allelopathic interactions between this or any of the other species and crops. Given the slow growth rates of the species that were tested, it may take some years before concentrations of allelopathic chemicals build up to toxic levels in agricultural soils. By this time, farmers may have incurred significant opportunity costs by investing in agroforestry systems. Given the presence of allelopathic chemicals in each of the species that were tested, and the uncertain consequences for crops in agroforestry systems based on

Ó 2007 The Authors. Journal compilation Ó 2007 Blackwell Publishing Ltd, Afr. J. Ecol.

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these species, extension services in Botswana should be wary about recommending the use of these species until further research has been carried out.

Acknowledgments

This project was made possible by the financial assistance of the Commonwealth Secretariat and support from the University of Botswana Department of Chemistry and Department of Environmental Science. We are indebted to Prof. R. T. Majinda for his creditable support. Thanks are due to Madam L. Motoma for her guidance in writing the research proposal.

References

Allen, S.E., Stewart, A.E., Grimshaw, H.M., Parkinson, J.A., & Quarmby, C. (1974) Chemical Analysis of Ecological Materials. Blackwell Scientific Publications, Oxford. Braca, A., Politi, M., Sanogo, R., Sanou, H., Morelli, I., Pizza, C., & De Tommasi, N. (2003) Chemical composition and antioxidant activity of phenolic compounds from wild and cultivated Sclerocarya birrea (Anacardiaceae) leaves. J. Agric. Food Chem. 51, 6689­6695. Breman, H., & Kessler, J. (1995) Woody Plants in Agro-ecosystems of Semi-arid Regions: With an Emphasis on Sahelian Countries. Springer-Verlag, Berlin, Heidelberg.

Gallet, C., & Pellissier, F. (1997) Phenolic compounds in natural solutions of a coniferous forest. J. Chem. Ecol. 23, 2401­2412. Jonsson, P. (2000) Sub-Tropical Urban Area: A Field Study of Gaborone, Botswana. Department of Physical Geography, Goteborgs ¨ Universitet, Goteborg. ¨ Nair, P.K.R. (1991) State-of-the-art of agroforestry systems. For. Ecol. Manage. 45, 5­29. Rao, M.R., Mafongoya, P.L., Kwesiga, F.R., & Maghembe, J.A. (1999) Nutrient cycling in agroforestry systems of the semi-arid tropics of Africa. Ann. Arid Zone 38, 275­307. Rice, L.E. (1984) Allelopathy, 2nd edn. Academic Press, Orlando, FL. Rizvi, S.J.H., Tahir, M., Rizvi, V., Kohli, R.K., & Ansari, A. (1999) Allelopathic interactions in agroforestry systems. Crit. Rev. Plant Sci. 18, 773­779. Silitshena, R.M.K., & Mcleod, G. (1994) Botswana ­ A Physical, Social and Economic Geography. Longman Botswana Ltd., Gaborone, Botswana. Sundaramoorthy, S., & Kalra, A. (1991) Allelopathy and vegetation in Acacia tortilis plantations in Indian desert. Ann. Arid Zone 30, 259­266. Wink, M., Schmeller, T., & Latz-Bruning, B. (1998) Modes of action of allelochemical alkaloids: Interaction with neuroreceptors, DNA, and other molecular targets. J. Chem. Ecol. 24, 1881­1937.

(Manuscript accepted 6 March 2007)

doi: 10.1111/j.1365-2028.2007.00776.x

Ó 2007 The Authors. Journal compilation Ó 2007 Blackwell Publishing Ltd, Afr. J. Ecol.

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