Growth and yield of Solanum khasianum in Pinus roxburghii forest based silvi-medicinal system in mid hills of Indian Himalaya
© The Author(s). 2016
Received: 6 February 2016
Accepted: 2 August 2016
Published: 12 August 2016
In mid hills of Western Himalayas, Himachal Pradesh India, growth, yield and economics of Solanum khasianum as a potential medicinal herb under Pinus roxburghii (Chir pine) plantation has been studied for two consecutive years to assess the performance of Solanum khasianum in undercanopy of Pinus roxburghii for developing Solanum khasianum and Pinus roxburghii based innovative silvi-medicinal system.
Growth parameters such as plant height, number of branches per plant and leaf area index followed by yield were estimated after Solanum khasianum was grown on three topographical aspects as; Northern, North - western and Western at a spacing of 45 cm × 45 cm, followed by three tillage depths as; minimum (0 cm), medium (up to 10 cm) and deep tillage (up to 15 cm), in open and below canopy conditions treatment. The study was conducted to explore the possibility of using Solanum khasianum based silvi-medicinal system to utilize the below canopy of Chir pine forest for enhancing the productivity of forests besides the conservation of the medicinal herb.
The growth parameters such as plant height, number of branches per plant and leaf area index were non-significantly affected by topographical aspects and tillage practices, both below canopy and open conditions except fresh weight and dry weight of berries during harvesting stage. The maximum yield (0.61 t∙ha−1) was observed on Western aspect in open conditions as compared to below canopy of Chir pine. The highest gross returns were observed for the crop cultivated on Western aspect under deep tillage in open conditions than other aspect and tillage combinations. However the positive net returns from the crops raised in below canopy of Chir pine indicates its possible economic viability under agroforestry system as the gross returns was higher than the cost of cultivation.
Solanum khasianum when grown in below canopy of Pinus roxburghii, its growth and yield indicated positive net returns. Solanum khasianum and Pinus roxburghii based silvi-medicinal system has the potential to enhance the overall productivity of Chir pine forest. This silvi-medicinal system gives scope for utilizing floor Chir pine forests for growth and production of Solanum khasianum beside conservation of the medicinal herb.
KeywordsBio-economic evaluation Silvi-medicinal system Inter-specific competition Topographical aspect Tillage depth Leaf area index (LAI)
Almost all the medicinal plants collected either legally or illegally all over the Himalayas or even other parts of Asia for various purposes are collected from the natural plant communities and only a few species are cultivated (Uniyal et al. 2002). The loss or degradation of forest and grasslands worldwide has led to the shrinking habitat of medicinal plants in almost every country. As a result of which number of medicinal herbs are depleting rapidly from natural plant communities and are threatened with extinction. There are numbers of studies and survey reports those have indicated that over exploitation of several of these medicinal plants for economic gain has rendered them as endangered or vulnerable species (Behrens 2001). The regeneration, protection and preservation of the medicinal plants are a serious challenge for restoring our biological heritage (Hamilton 2004). The future of hundreds of plants species used for medicinal purposes which once grew abundantly in our forest and elsewhere are at a risk, as they often are exploited indiscriminately from the wild, leaving little scope for their regeneration. Another reason for the disappearance of many species is ignorance on our part with regard to the knowledge on identification and use of such species. One of them is Solanum khasianum Clarke, a stout, much branched, under shrub with almost straight prickles which is found in the wastelands, ravines and in the North-Eastern hills of India and the upper Gangetic plains up to an altitude of 1600 m. This species is an important medicinal plant that contains solasodine as a secondary metabolite (Maiti et al. 1979). Glycoalkaloid, solasodine, is found in mature berries of the plant which has great pharmacological importance in the synthesis of steroids (Trivedi and Pundarikakshudu 2007) and used for the treatments of asthma, inflammatory disorders, sex hormones imbalance and as oral contraceptives (Maiti et al. 1979). Consumption of this herbal medicine is increasing worldwide day by day and harvesting from the wild as a raw material causing loss of genetic diversity and habit destructions (Canter et al. 2005; Julsing et al. 2007). Therefore need was felt to regenerate and conserve it in the wild (Sanwal et al. 2011). Hence considering the socio-economic importance of Solanum khasianum for human health and well beings, a field trial was conducted for two consecutive years (2006–07, 2007–08) to assess its regeneration and conservation potential under the Chir pine plantations in its local habitat. Chir pine (Pinus roxburghii), which is closely related to Canary Island pine, Turkish pine and Maritime pine, is one of the most important timber species in the mountain subtropical region of India. It is typically gregarious, indigenous to India, forming extensive natural pure stands, though occur mixed with some broad leaf species at its upper and lower limits (Troup 1921), covering an area of 3853 km2 in Himachal Pradesh (IFS, 2002). The species has a wide adaptability, less demanding of nutrient rich soil than other conifer. Chir pine, like other pine also subject to influence of biotic and abiotic factors, and changing climatic conditions (Kumar et al. 2013; Khaki et al. 2015; Kumar et al. 2016a; Kumar et al. 2016b). But the Chir pine forests have very less or no undercanopy vegetation and hence the productivity of these forests is very low. So keeping in view the vast area under Pinus roxburghii in India and also the greater demand for this medicinal herb, the present study were undertaken to assess the possibility of regenerating Solanum khasianum in undercanopy of Pinus roxburghii for developing innovative Solanum khasianum and Pinus roxburghii based silvi-medicinal system in mid hills of western Himalayas. Thus the experiment was conducted to sustainably utilize the unutilized floor of Chir pine forest while assessing the growth, yield and economics of Solanum khasinaum as a potential medicinal herb in below canopy of Chir pine for its commercial exploitation and conservation.
Experiment technique and statistical analysis
Seedlings of Solanum khasianum procured from nursery, were used for the transplantation in the planting site at the spacing of 45 cm × 45 cm. Generally it does not require any fertilizer and irrigation practices as the herb is well adapted to the waste lands. Weeding as one of the cultural operations carried out for better growth and yield of the Solanum khasianum. The seedlings were transplanted during monsoons (June–July) and berries harvested at the end of winter season. Solanum khasianum grown under Chir pine canopy and in open conditions on different aspects and under different tillage practices were studied separately to assess former’s growth and yield. Hence, studies involved three factors i.e. Aspects, tillage practices and systems. Solanum khasianum was grown on three aspects such as; Northern, North-Western and Western at a spacing of 45 cm × 45 cm adopting recommended cultural practices, followed by three tillage depths such as; minimum (T1: 0 cm), medium (T2: up to 10 cm) and deep tillage (T3: up to 15 cm) in open conditions and under canopy of Chir pine plantations. The 18 treatments combinations, including all possible combination of three aspects, three tillage depths and two systems were evaluated for growth, yield and economics of Solanum khasianum using three replicates in factorial randomized block design. The entire data generated from the present investigation were analyzed statistically using the technique of analysis of variance for factorial randomized block design in accordance with the procedure outlined by Gomez and Gomez (1984).
Field preparation and transplanting
The forest floor was cleared from needle by manual laborers who collected the needles heaped on the forest floor and removed before transplanting the seedlings at the site. After removing the pine needles, tillage practices were adopted just before the onset of monsoon. The whole experiment was conducted under rainfed conditions entirely depending on the monsoon rains. Keeping in view the forest site conditions, no irrigation and fertilizer was applied because crop has minimum input requirement. For transplanting of seedlings, nursery was prepared, and at the commencement of monsoon, healthy seedlings of Solanum khasianum were lifted from nursery and transplanted in the experimental field after recording sufficient moisture in the soil during first fortnight of July. Planting was done using a wooden stick with good penetration in the experimental area including in minimum tillage plots area. The crop was harvested in the month of December.
Characteristics of Chir pine stand
Height, diameter, crown area and crown density of chir pine stands on different aspects
Crown area (m2)
Effect of topographical aspect on available nutrient in below canopy and open conditions
Organic carbon (%)
SEm ± (S)
Effect of tillage practices on available nutrients in below canopy and open conditions
Organic carbon (%)
SEm ± (S)
Effect of tree canopy
Effect of topographical aspect
Effect of tillage
Effect of tillage practices on plant height and number of branches per plant of Solanum khasianum
Plant height (cm)
Number of branches per plant
13.02c ± 2.54
19.15a ± 6.87
30.61a ± 6.49
5.83a ± 0.85
16.67b ± 3.65
23.16a ± 5.98
38.53a ± 7.59
6.87a ± 1.15
19.34a ± 3.42
26.36a ± 6.59
43.54a ± 8.67
8.37a ± 1.35
Effect of tillage practices on leaf area index and yield of Solanum khasianum
Leaf area index (LAI)
Yield (Dry weight of berries) (qha−1)
1.63a ± 0.45
0.38c ± 0.08
1.77a ± 0.56
0.43b ± 0.06
2.19a ± 0.61
0.57a ± 0.12
Economics of silvi-medicinal systems
Bio-economic appraisal of Solanum khasianum ($ha−1)
Gross return ($)
Cost of cultivation ($)
Net return ($)
The production potential of Solanum khasianum can be judged by the effect of different aspect, tillage and systems on growth parameters and final yield.
Effect of tree canopy
Plant growth and yield parameter of Solanum khasianum was generally higher outside tree canopy than below tree canopy (Fig. 2). However the higher values for all growth parameters and yield attributes of Solanum khasianum outside tree canopy suggests that the plants grown outside tree canopy as sole crop has better opportunities to reap more solar energy for photosynthetic activity, less intra-specific competition for critical resources like water, nutrients, and photo synthetically active radiation. These favorable factors seem to result in higher values of growth and yield parameters of Solanum khasianum outside tree canopy conditions. Chauhan (2000), Karikalan et al. (2002), Lott et al. (2000), Okorio (2000) and Singh et al. (2012a) have earlier made similar observations for different agricultural crop and grasses under agroforestry systems. Apart from above the lower values of growth parameters and yield attributes of Solanum khasianum in below canopy of Chir pine might be because of allelopathic effect of tree on crops. Some of the previous study had shown allelopathic effect of Chir pine on different plants (Singh and Verma 1988; Gupta et al. 2007; Aliloo et al. 2012; Sharma 2013a; Sharma 2013b). The findings of this investigation are also in agreement with the earlier findings of many researchers (William and Gordon 1995; Jose et al. 2000; Singh et al. 2012b) who have reported higher production of dry matter in the outside tree canopy than in the intercropped field.
Effect of topographical aspect
Our result showed the growth and yield parameters observed on Western aspect was greater than North-west and Northern aspect (Fig. 3). On the other hand, Nevo et al. (1999) found that plant cover may reach 150 % on the Northern aspect. Nevo et al. (2000) also confirmed that species inhabit on different aspect display genetic, morphologic, physiological and behavioral adaptive complexes in relation to each of the aspect. The maximum growth and yield of Solanum khasianum on Western aspect was attributed to higher intensity of light during forenoon when the temperature is more favourable and leaves remain turgid which limit rate of photosynthesis. On the other hand Northern aspect receiving lower intensity of light in afternoon when the temperature is less favourable and leaves remain less turgid thereby reducing photosynthetic efficiency of the crop on this aspect. Similar findings were observed when Mucuna pruriens and Andrographis paniculata showed highest yield when cultivated on Western aspect (Sanwal et al. 2013, 2015 and Chandra et al. 2016). Nevo (1997) also proved that microclimatic conditions on the aspects vary dramatically, affecting the biology of plants at all levels.
Effect of tillage
In our study, plant growth and yield parameter in deep tillage was recorded more than medium and minimum tillage (Table 5). Higher values of growth and yield in deep tillage were due to better soil permeability, soil aeration, root penetration and weed control. Similar findings were observed by Sanwal et al. (2013, 2015), when medicinal plants like Mucuna pruriens and Andrographis paniculata showed highest yield when cultivated on deep tillage. These results are in agreement with those of Khan et al. (1999), Iqbal et al. (2005), Keshavarzpour and Rashidi (2008), Rashidi and Keshavarzpour (2008) and Rashidi et al. (2008). Khurshid et al. (2006) also reported that development of roots was better in less compacted soil whereas dense soil markedly reduced root growth. This was attributed to the favorable effect on plant height, number of branches per plant and shoot and biomass yield. Thus the greater value of growth parameter and yield in deep tillage is attributed to the higher infiltration and increased soil depth for moisture storage (Moreno et al. 1997). While the lower yield under minimum tillage is attributed to less favorable condition for shoot and root growth, and less moisture storage and poor soil aeration. Lampurlanes et al. (2002) also reported the reduced shoot growth in compact soil because of the poor root development. The other reason for lower value of growth parameters and yield attributes might be because of poor control on weed growth and less nutrient availability under minimum tillage. Unger and Baumhardt (1999) also reported reduction in yield under no tillage as compared to conventional tillage occurred due to lack of control over the weed population.
The positive net returns received from Solanum khasianum when recorded in below canopy of Chir pine which indicates the great possibility of economic viability of Solanum khasianum and Chir pine based innovative silvi-medicinal system. Solanum khasianum and Pinus roxburghii based innovative silvi-medicinal system also endeavour to use the unutilized floor of Chir pine forest by associating naturally growing Solanum khasianum to improve the productivity of these forests along with conserving medicinal plants. This study can have potential implications in the Chir pine growing belt of the continent if this innovative silvi-medicinal system is implemented by the local farming community or the state forest department. The combination of Solanum khasianum with Pinus roxburghii will not only ensure regular supplies of medicinal herbs but also their conservation. The findings of the present investigation indicate that raising with Chir pine is a viable option for enhancing the diversification and rise in income from Chir pine forest.
The authors acknowledge the help and support rendered by forest range officer Mr. Tilak Raj and his team members, Lab staff and Field staff of Department of Silviculture and Agroforestry and Forest products, Dr. Y. S. Parmar University of Horticulture and Forestry, Nauni, Solan (H.P.)-173 230, India.
CSS and SDB conducted research work, while RK and RA, VK and SK were involved in Data analysis and writing manuscript, respectively. All authors read and approved the final manuscript.
1) Chandra Shekher Sanwal participated in this research as a doctoral student at Dr. Y.S. Parmar University of Horiculture and Forestry, Solan, Himachal Pradesh, India. Currently he is a member of Indian Forest Service and working as Divisional forest officer, Haldwani forest division, Haldwani Uttrakhand, India.
2) Raj Kumar was a doctoral student at Dr. Y.S. Parmar University of Horiculture and Forestry, Solan, Himachal Pradesh, India. Currently he is working as Scientist, ICAR-IISWC, RC, Vasad, Anand, India.
3) Raheel Anwar was a student perusing his Masters at Punjab Agricultural University Ludhiana India.
4) Vijaysinha Kakade was perusing his Masters at Indian agriculture research Institute, New Delhi, India. Currently he is working as Scientist, ICAR-IISWC, RC, Vasad, Anand, India
5) Sushma Kerketta was doctoral student at College of forestry at Dr. Y.S. Parmar University of Horiculture and Forestry, Solan, Himachal Pradesh, India.
6) S. D. Bhardwaj was former Dean, College of forestry at Dr. Y.S. Parmar University of Horiculture and Forestry, Solan, Himachal Pradesh, India.
The author(s) declare that they have no competing interests.
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- Aliloo AA, Shahabivand S, Farjam L, Heravi S (2012) Allelopathic effects of Pine needle extracts on germination and seedling growth of Ryegrass and Kentucky Bluegrass. Adv Environm Biol 6(9):2513–2518Google Scholar
- Behrens J (2001) Can the utilisation and conservation of medicinal plants coexist? Eur J Herbal Med 5(3):18–26Google Scholar
- Canter PH, Thomas H, Ernst E (2005) Bringing medicinal plants in to cultivation: opportunities and challenges for biotechnology. Trend Biotechnol 23:180–185View ArticleGoogle Scholar
- Chandra SS, Raj K, Raheel A, Bhardwaj SD (2016) Performance of Mucuna prurience under Chirpine (Pinus roxburghii) Plantation of Mid Hills of Western Himalayas. Agri Res Tech: Open Access J 1(2):555560Google Scholar
- Chatterjiee SK, Craker LE, Simon JE, Jatisatiener A, Lewinsohn E (2004) The future of aromatic and medicinal plants, a proceeding of the 26th International Horticultural congress, Toranto, Canada 11–17 Aug.2002. Acta horticulturae 629:23–29Google Scholar
- Chauhan VK (2000) Evaluation and wheat and maize varieties under poplar based agroforestry systems in Paonta Doon Valley. Ph.D. Thesis, Forest Research Institute, Dehradun, IndiaGoogle Scholar
- Gomez KA, Gomez AA (1984) Statistical procedures for agricultural research, 2nd edn. John Wiley and Sons Inc, New York, p 680Google Scholar
- Gupta B, Thakur NS, Dass B (2007) Allelopathic effect of leaf leachates of Pinus roxburghii Sargent on seeds of some grasses. Indian Forester 133(7):997–1000Google Scholar
- Hamilton AC (2004) Medicinal plants, conservation and livelihoods. Biodiv Conserv 13:1477–1517View ArticleGoogle Scholar
- Harrington TB, Dagley CM, Edwards MB (2003) Above and belowground competition from longleaf pine plantations limits performance of reintroduced herbaceous species. Forest Sci 49(5):681–695Google Scholar
- Indian Forestry Statistics (2002) Ministry of environmental and forests., Government of IndiaGoogle Scholar
- Iqbal M, Hassan AU, Ali A, Rizwanullah M (2005) Residual effect of tillage and farm manure on some soil physical properties and growth of wheat (Triticum aestivum L.). Intl J Agr Biol 7:54–57Google Scholar
- Jose S, Gillespe AR, Sseifert JR (2000) Defining competition vectors in a temperate alley cropping system in the mid Western U S A: 2. Competition for water. Agroforestry Syst 48:41–59View ArticleGoogle Scholar
- Julsing MK, Quax WJ, Kysar O (2007) The engineering of medicinal plants: prospects and limitation of medicinal biotechnology. Med Plant Biotechnol 3:6Google Scholar
- Karikalan TV, Yassin MM, Duvya MP, Gopi D (2002) Effect of intercropping and nitrogen management on growth and yield of medicinal plants under Kapok. Ind J Agroforestry 4:88–93Google Scholar
- Keshavarzpour F, Rashidi M (2008) Effect of different tillage methods on soil physical properties and crop yield of watermelon (Citrullus vulgaris). World Appl Sci J 3:359–364Google Scholar
- Khaki BA, Singh VRR, Wani AA, Thakur RK (2015) Effect of forest fire on soil nutrients in blue pine (Pinus wallichiana A.B. JACKSON) ecosystems. Ind For 141(4):355–360Google Scholar
- Khan FUH, Tahir AR, Yule IJ (1999) Impact of different tillage practices and temporal factor on soil moisture content and soil bulk density. Intl J Agr Biol 1:163–166Google Scholar
- Khurshid K, Iqbal M, Arif MS, Nawaz A (2006) Effect of tillage and mulch on soil physical properties and growth of maize. Intl J Agr Biol 8:593–596Google Scholar
- Kumar R, Shamet GS, Mehta H, Alam NM, Kaushal R, Chaturvedi OP, Sharma N, Khaki BA, Gupta D (2016a) Regeneration complexities of Pinus gerardiana in dry temperate forests of Indian Himalaya. Environm Sci Poll Res 23(8):7732–7743View ArticleGoogle Scholar
- Kumar R, Shamet GS, Alam NM, Jana C (2016b) Influence of growing medium and seed size on germination and seedling growth of Pinus gerardiana Wall. Compost Sci Utiliz 24(2):94–104View ArticleGoogle Scholar
- Kumar R, Shamet GS, Avasthe RK, Singh C (2013) Ecology of chilgoza pine (Pinus gerardiana Wall) in dry temperate forests of North West Himalaya. Ecol Environm Conserv 19(4):1063–1066Google Scholar
- Lampurlanes J, Angas P, Martinez C (2002) Tillage effects on water storage during fallow, and on barley root growth and yield in two contrasting soils of the semiarid Segarra region in Spain. Soil Till Res 65:207–220View ArticleGoogle Scholar
- Lott JE, Ong CK, Black CR (2000) Long-term productivity of a Grevillea robusta-based overstorey agroforestry system in semi-arid Kenya. II. Crop growth. Forest Ecol Manage 139:187–201View ArticleGoogle Scholar
- Maiti PC, Mookherja S, Mathew R, Dan SS (1979) Studies on Indian Solanum alkaloid content and detection of solasodine. Econ Bot 33(1):75–77View ArticleGoogle Scholar
- Moreno F, Pelegrin F, Fernandez J, Murillo JM (1997) Soil physical properties, water depletion and crop development under traditional and conservation tillage in southern Spain. Soil Till Res 41:25–42View ArticleGoogle Scholar
- Nevo E (1997) Evolution in action across phylogeny caused by microclimatic stresses at “Evolution Canyon”. Theoret Popul Biol 52:231–243View ArticleGoogle Scholar
- Nevo E, Bolshakova MA, Martyn GI, Musatenko LI, Sytnik KM, Baharav A (2000) Drought and light anatomical adaptive leaf strategies in three woody species by microclimatic selection at “Evolution Canyon”. Israel J Plant Sci 48:33–46Google Scholar
- Nevo E, Fragman O, Dafni A, Beiles A (1999) Biodiversity and interslope divergence of vascular plants caused by microclimatic differences at “Evolution Canyon” lower nahal Oren, Mount Carmel, Israel. Israel J Plant Sci 47:49–59View ArticleGoogle Scholar
- Okorio J (2000) Light interception and water use in boundary planting agroforestry systems. Ph.D. thesis, Reading University, UK., p 230Google Scholar
- Rashidi M, Keshavarzpour F (2008) Effect of different tillage methods on soil physical properties and crop yield of melon (Cucumis melo). Am-Eur J Agr Environm Sci 3:31–36Google Scholar
- Rashidi M, Keshavarzpour F, Gholami M (2008) Effect of different tillage methods on yield and yield components of forage corn. Am-Eur J Agr Environm Sci 3:347–351Google Scholar
- Sanwal CS, Sushma, Nilay K (2011) Introduction of medicinal and aromatic plants in Chir pine (Pinus roxburghii) forest of India; a sustainable technique. 2nd International Conference on Environmental Science and Technology VI-293-296.Google Scholar
- Sanwal CS, Sushma RAL, Khan PA, Pant KS, Bhardwaj SD (2013) Influence of topographical aspect and tillage practices on Kaunch (Mucuna pruriens). Ind J Ecol 40(1):158–160Google Scholar
- Sanwal CS, Raheel AL, Khan PA, Pant KS, Bhardwaj SD (2015) Effect of aspect and tillage on growth and yield attributes of Kalmegh (Andrographis paniculata). Ind For 141(2):198–202Google Scholar
- Sharma NK (2013a) Allelopathic effect of Pinus Roxburghii bark extract on Phalaris Minor. Intl J Sci Res 2(2):20–21VoGoogle Scholar
- Sharma NK (2013b) Allelopathic effect of chir pine needle litter on seedling growth of little seed canary grass. Global Res Analys 2(3):10–11Google Scholar
- Singh C, Dhadwal KS, Dhiman RC, Kumar R, Avasthe RK (2012a) Allelopathic effects of paulownia and poplar on wheat and maize crops under agroforestry systems in Doon Valley. Ind For 138(11):986–990Google Scholar
- Singh C, Dhadwal KS, Dhiman RC, Kumar R (2012b) Management of degraded bouldery riverbed lands through Paulownia based silvipastoral systems in Doon Valley. Ind For 138(3):243–247Google Scholar
- Singh SK, Verma KR (1988) Allelopathic effects of leachates and extracts of Pinus roxburghii on four legumes in Kumaun Himalayas. Ind J Agr Sci 58:412–413Google Scholar
- Trivedi P, Pundarikakshudu K (2007) Novel TLC Densitometric method for quantification of solasodine in various Solanum Species, market sample and formulations. Chromatographia 65(3/4):239–243View ArticleGoogle Scholar
- Troup RS (1921) The Silviculture of Indian Trees: (Oxford:calaranddon Press) Vol III., pp 785–1195Google Scholar
- Unger, P.W. and R.L. Baumhardt. 1999. Factors related to dryland sorghum yield increases: 1937-1997. Agron. J. 91:870-875.Google Scholar
- Uniyal SK, Awasthi A, Rawat GS (2002) Current status and distribution of commercially exploited medicinal and aromatic plants in upper Gori valley, Kumaon Himalaya, Uttaranchal. Curr Sci 82:1246–1252Google Scholar
- William PA, Gordon AM (1995) Microclimate and soil moisture effect of three intercrops on the rows of a newly planted intercropped plantation. Agrofor Syst 29:285–302View ArticleGoogle Scholar