Factors affecting macropropagation of bamboo with special reference to culm cuttings: a review update
The gap between demand and supply of bamboo is increasing daily due to the destruction of natural bamboo resources. Therefore, there is a pressing need to find suitable methods for large-scale propagation of bamboo. Currently, bamboo is propagated mainly using vegetative means since seed supplies are often variable or limited.
Current literature on available methods of bamboo propagation (seed and vegetative) were reviewed to establish the key limiting factors involved in the propagation of bamboo.
The vegetative propagation of bamboo via culm cuttings was found to be the simplest and most cost-effective method developed over the last few decades.
Additional work is required to increase the success rate of culm cuttings, and finding improved substrates for the propagules could be one area of investigation.
The demand for bamboo is increasing steadily because of its multipurpose utility. Twenty million hectares of bamboo (equivalent to 80% of total world stock) are available from India, China and Myanmar. In most of these countries, including India, 99% of bamboo products are supplied from natural forests. China is considered as the largest bamboo resource of the globe, having 500 species in 40 genera distributed in 7.6 million hectares of forest land. The current taxonomic authorities for all the species referred to in this review are available from the Plant List. Phyllostachys edulis (Moso bamboo) is the most economically important bamboo species, but other species, namely Chimonocalamus delicatus, Dendrocalamus membranaceus and Fargesia yunnanens, are prevalent in China. India is considered as the second largest bamboo resource in terms of species diversity after China and of production. In India, 18 genera and 128 species of bamboo have been reported by Seethalakshmi and Kumar (1998), among which 41 species are exotic to India. Bambusa balcooa, Bambusa bambos, Bambusa tulda, Dendrocalamus hamiltonii, and Dendrocalamus strictus are the main economically important bamboo species of India. In India, according to Yengkopam (2013), 8.96 million hectares of 63.3 million hectares of forest land is covered by bamboo. However, a forest survey of India estimated that 13 million hectares of India’s forest land are covered by bamboo growing stock, which yields around 169 million tones of ‘wood’. In India, bamboo is also considered as an important economic resource for nearly 7.5 million bamboo artisans. The total value of bamboo forest produces is expected to reach USD 48 billion within 2020 in China. The economic value of total bamboo harvest in India is USD 0.30 billion and its products have a potential export value amounting to USD 4.46 billion by 2025. Human activities such as deforestation, industrialization, and illegal extraction of bamboo, have resulted in the loss of natural bamboo resource. Therefore, it is necessary to find cost-effective methods for large-scale propagation of bamboo for planting in new bamboo forests to reduce the existing gap between demand and supply. This review discusses the advantages and disadvantages of seed and various vegetative methods for propagating bamboo and the key factors affecting success.
Seeding in bamboo is very irregular and unavailable for most of the species. Bamboo flowers gregariously or sporadically after long intervals to produce seeds. In gregarious flowering, the entire local populations of the individual species die after flowering. However, in sporadic flowering, only a minority of culms (the hollow main axes of upright shoots) in clumps produce flowers and die. Sporadic flowering is reported in B. balcooa, B. nutans, B. vulgaris and B. longispiculata while gregarious flowering is reported in B. tulda, B. bambos, B. polymorpha, Dendrocalamus longispathus, D. strictus and Oxytenanthera nigrociliata. Some species do not even produce seed (e.g. Bambusa balcooa, B. vulgaris), and those that do so often flower at long intervals varying from 30 to 70 years (e.g. Bambusa bambos, B. polymorpha, Dendrocalamus strictus, Melocanna baccifea and Phyllostachys spp.). Therefore, availability of seed may only be after long time intervals, seed viability may be poor and brief, storage facilities suitable for seed may be lacking, highly heterogeneous seedling populations may occur, and often overall poor seed set and consumption of seed by wild animals are the major problems with seed-based propagation. Seed-based propagation methods, therefore, cannot be relied for mass multiplication on a sustainable basis, so there is strong need to explore the possibility of vegetative propagation for mass multiplication of bamboo.
The potential advantage of vegetative propagation is the production of true-to-type plants on a sustainable basis, which is clearly not feasible with seeds. However, plants raised through vegetative propagation maintain a definite vegetative time period before flowering, due to the physiological age of the mother plant. There is a danger that flowering, and death, could occur soon after propagation. Therefore, before collecting the propagating material, plant age and relevant physiological details need to be ascertained for each species, noting that there may be environmental effects which can change this status. Relevant establishment records for the mother plants must be developed and maintained. Widely used vegetative propagation methods for bamboo are rhizome or offset cuttings, layering and culm cuttings.
A brief account on the different methods of vegetative propagation is discussed here under the following subheadings.
Bamboo seedlings possess the capacity to proliferate. In this method, the rhizome system of a seedling is cut into pieces, each with roots and shoots. Then each piece with a shoot and root is used as propagating material. By this method, each plant can be multiplied three to seven times, depending on the species. However, seed dependency is the major drawback of this method.
‘Rhizome’- or ‘offset’-based propagation
‘Offsets’ or ‘rhizomes’ are conventionally used for propagating the sympodial bamboos. In this method, either whole rhizomes or portions each with single culm having three to five basal nodes (called offsets) are used as propagating material. Offsets, however, are difficult to handle and transport because of their bulkiness, and are not cost-effective due to limited availability and a low regeneration capacity, i.e. slow growth and/or poor multiplication rates. They are also prone to desiccation, are labor intensive, season dependent and have a low survival rate, and therefore are unsuitable for large-scale plantations.
In this method either a whole culm or a branch bearing part of the culm is bent down to the ground so that the nodes are pegged into the soil. Roots develop from the nodal regions are separated and planted in polybags for hardening. Different types of layering include:
A.) Ground layering, where a culm is bent down to ground and covered with rooting medium.
B.) Air layering/marcotting, in which the root-promoting medium (garden soil, leaf mold and commercial plant growth regulators, etc.) is applied around the nodes of the culm and often wrapped longitudinally with coconut fiber and/or water hyacinth roots.
C.) Stump layering, one to two node stumps of severed culms are covered with a suitable propagating medium. The low success rate of this layering method limits its use. Through stump layering, only 10% survival has been reported in B. vulgaris and D. giganteus. This method is also difficult to follow if clumps of bamboo are tightly bunched or culms have thin walls which limits its application for other species.
Vegetative propagation using ‘cuttings’ has advantages over ‘rhizome’/‘offsets’. Cutting-based propagation does not damage the mother plant and cuttings are also easily available for large-scale propagation. This could be achieved by either ‘branch cuttings’ or ‘culm cuttings’.
A.) Branch cuttings
The primary branches are easily separated from the hollow main stem of an upright shoot (culm) and reduced to two- to three-node cuttings and buried in growth medium. The basic advantages of branch cuttings are abundant availability and ease of handling. However, the method is restricted to thick-walled bamboo species only. Branch cuttings require 6–12 months for rooting and 12–30 months for rhizome development and thus are time-consuming to produce indicating that plants produced by this method have slower growth.
B.) Culm cuttings
In this method, the culm segments with two–three nodes from the mother plants are taken to propagate bamboo species. This method is applicable for both thick- and thin-walled bamboo, but the success rate is quite low in thin-walled bamboo. Rooting can be possible within 90 days of setting of cuttings in rooting media.
The lack of success of vegetative propagation methods mentioned earlier (macroproliferation, ‘rhizome’- or ‘offset’-based propagation and layering-based propagation) other than cuttings is notorious. It is evident from the literature that culm/branch cuttings are suitable for propagation because of easy handling and abundant availability. Ahlawat et al. (2002) reported that culm cuttings are 40–80% more successful than the offset method. The chief advantage of culm cuttings is that they can be collected from the stock material without affecting the rhizome system. Besides wider applicability, culm cuttings are advantageous in circumventing the limitations (discussed earlier) of branch cuttings. Culm cuttings were found to be better than branch cuttings; in terms of rooting, 88.3% rooting in culm cuttings was observed which was more than double in comparison to that in branch cuttings (40%) in B. nutans, while in B. tulda it was 23.3%, compared with no rooting in branch cutting material (Singh et al. 2011). Culm cuttings have performed better with respect to rooting response percentage (~70.6%), number of roots/node (~15.3) and number of shoots/node (~9.4), than branch cuttings which showed ~60.3% response percentage, ~2.6 shoots/node and ~6.4 roots/node (Deb et al. 2016). The success of the culm cuttings is the end-result of multiple factors that we need to address properly.
Factors affecting culm cutting-based propagation
Several internal and external factors are responsible for successful macropropagation with culm cuttings: age of mother plant; type of cutting ; method of setting; type(s) and concentration of plant growth regulators; season and substrate used for macropropagation. In addition, environmental factors such as temperature, humidity, water balance, rooting medium, light intensity, wind speed, and pests and diseases, are also limiting factors for the macropropagation. A summary of the findings of the previous literature on macropropagation through culm cuttings in different species of bamboo is provided in Table 1. The most important controllable critical factors are briefly discussed below.
Age of stock plants/cutting materials
Limited literature is available on the age of stock plants to be used for culm cutting. Culms of 1–5 years are better for many species, as reported by several authors. The use of culms of 1 year in B. balcooa, 1.5 years in B. vulgaris, 2 years in B. vulgaris, Araundinaria alpine, Oxytenanthera abyssinica, and 5 years in D. asper as well as in B. vulgaris, have been reported as successful propagating materials for cutting-based vegetative propagation. Despite reports of success with culm cutting material of different ages in various bamboo species, juvenile culms are more suitable than older ones because of highly active buds that are critical for efficient propagation. In general, 1-year-old culms are sufficient for culm cutting propagation but Saharia and Sen reported higher survival rates with 2-year-old culms than with 1- or 3-year-old culms in bamboo species like B. balcooa, B. pallida, B. bambos and Melocanna bambusoides.
Characteristics of culm cuttings
Cutting-based propagation is suitable for many types of bamboo, but not cane bamboo species nor bamboo species that have a small or no hollow in the middle of the stem. The best culm cutting materials are generally obtained from the middle parts of the culm because of sufficient vigour for generation of roots and shoots. Lower sprouting from cuttings from the top region compared to those from the middle region has been reported by Bwanali et al. Ntirugulirwa et al. reported cuttings of the middle region had the highest number of sprouts (21.73) in comparison to the basal part (13.83) in Arundinaria alpine, and the figures in Bambusa vulgaris were 25.70 (middle part) and 17.37 (basal part). But the basal part of the culm, particularly for larger-diameter aerial culm-bearing nodes or buds, had the higher rooting capacity. Number of nodes in each culm cutting is again important. Two- or one-nodal cutting was found better than whole-culm cuttings for high rooting and survival of B. blumeana. Culm cuttings with one to three nodes have been reported in Bambusa balcooa, B. bambos, B. nutans, B. tulda, B. vulgaris, Dendrocalamus giganteus, D. hamiltonii, D. strictus and Araundinaria alpine by several workers. Cutting-based propagation using culms with six nodes has been reported by Chhetri and Kumar for B. ventricosa. However, Bhol and Nayak reported high survival percentage for one- (93.33%) and two- (90%) nodal culm cuttings in B. vulgaris. The one-node cutting method is the most promising for propagation because it is the most economical and easiest to handle. This method is recommended for raising planting stocks of the genera Bambusa, Dendrocalamus and Gigantochloa.
During propagation, the orientation of cutting materials in bedding material is considered a critical factor to get optimum response. Horizontal setting (98.89%) instead of vertical setting (81.11%) was found more effective in terms of survival percentage for B. vulgaris. Two-nodal culm cuttings from the basal and middle parts in Oxytenanthera abyssinica under horizontal setting gave high survival rate. High survival rates with horizontal setting may be due to increase of surface area which is responsible for high water uptake from the surrounding medium. Higher sprouting with horizontal setting was also reported by Chhetri and Kumar ) and Elbasheer and Raddad respectively for B. ventricosa and Oxytenanthera abyssinica.
Most studies on bamboo macropropagation have been restricted to the summer season for several bamboo species including Bambusa balcooa, B. pallida, B. bambos and Melocanna bambusoides, B. vulgaris, D. asper and B. nutans. Singh et al. reported without addition of plant growth regulators, 98% rooting in culm cuttings of D. asper during summer. Better adventitious rooting (88%) in summer was reported than winter (53.4%) for B. nutans . Deb et al. reported that ~70.6% of culm segments set during April responded positively, with ~9.4 sprouts/node with ~15.3 roots/node within 6 months of setting; while setting during January gave ~31% response of cutting materials with ~3.1 sprouts/node and ~2.2 roots/node for B. nagalandiana. Gulabrao et al. observed that culm cuttings of B. bambos, B. vulgaris, B. balcooa, D. hamiltonii, B. tulda, D. strictus, D. giganteus, and B. nutans rooted best in spring (56.67%), closely followed by summer (54.58%), but less during winter (36.67%).Thus, it is evident that the interaction between species and season is the most critical and summer is as the most suitable season for the majority of the species.
Plant growth regulators
Plant growth regulators, mainly auxins, are widely used for bamboos species where rooting is difficult. The effect of growth regulators in bamboo is largely governed by the season and species. For example, B. vulgaris is easy to root and does not require growth regulator treatment. However, B. nutans, B. tulda, and D. asper require application of growth regulators. During spring, the endogenous level of hormones is often sufficient for root induction. Singh et al. reported that indole 3-acetic acid (IAA) in summer and indole-3-butyric acid (IBA) in the rainy season were most effective for rooting in B. vulgaris and D. membranaceus. Seasonal dependency of growth regulator IBA was also reported by Singh et al.) for B. tulda and B. nutans. Among the seasons, summer (88%) was most effective in B. nutans (treated with 2 mM IBA) in comparison to spring (60%) and winter (21.8%) with respect to survival. Different concentrations of IAA, IBA and α-napthelene acetic acid (NAA) were used for culm cuttings of B. vulgaris by Bhol and Parida who suggested not using growth regulators for rooting as its endogenous hormones are sufficient for root induction. IBA is also reported as most effective for cutting-based propagation in several bamboo species. However, presoaking with a high concentration of NAA for 24 h before setting was found to be superior during summer in B. ventricosa. The effect of growth regulators also depends on setting orientation of cutting materials. Horizontal setting of IBA-treated D. asper gave higher rooting (65%) than that of under vertical position (55%) during summer.
Substrate materials for rooting
The type of substrate (rooting medium) used in propagation beds for the establishment of the macro propagules is also important. This medium supports the plant pieces by fixing the cuttings in a particular position. Different types of rooting media: viz. fine sand for eight bamboo species; sand for D. asper and Bambusa balcooa; and loamy material for B. nutans, Dendrocalamus hamiltonii and D. hookeri, have been used for macropropagation of bamboo. Mixtures of coarse sand and gravel were reported in Bambusa nutans by Islam et al., sand to soil (1:1) for Araundinaria alpine and B. vulgaris by Senyanzobe et al. Several other substrates, namely different ratios of soil, sand and farmyard manure (FYM) for B. vulgaris; soil for B. vulgaris, coarse sand mixed with fine gravel for B. vulgaris; and coarse sand, soil, and sand plus soil and vermiculite for B. balcooa have also been reported to improve rooting in terms of increasing root number and root length. Sand has multiple advantages: it is inexpensive, having good drainage and aeration because of higher porosity enables oxygen to reach the developing rooting system, and maintains a relatively uniform temperature.
From the presented knowledge, “culm cuttings” could be the best regeneration option to meet the global need for bamboo planting stock. Rooting and sprouting, as well as survival of cuttings, depend strongly on the season, species and plant growth regulators used. To maximise the success rate, in the absence of prior knowledge of the propagation characteristics of a species, we suggest the following approach. Use culms with one to two nodes from the middle of mother stock plants that are 2- to 3-years old, and set horizontally in appropriate substrate, preferably sand, during summer and maintain proper humidity (80–90%). Irrespective of age of plant, or growth regulator concentration, the best response was recorded in summer with winter being the worst. The available data are inadequate for deciding the best doses and duration of plant growth regulators (auxins) which again depend on season. So, doses and duration of auxins need to be standardised for different seasons and species. The interactions of species, auxins and seasons still need to be studied. Further research is also required on the selection of better growth-promoting substrates, to improve the potential of the culm-cutting method. It is worth mentioning that addition of biofertilizers like nitrogen fixers Azospirillum sp. or Phosphorous solubilising bacteria (PSB) to the growth medium was effective for macropropagation of banana and pomegranate respectively. Addition of compost was also found effective for propagation of another woody plant, Litchichinensis. To date, growth media were restricted to sand only in bamboo macropropagation. Others, like soil, biofertilisers, vermicompost, compost, vermiculite, and their possible combinations, are yet to be explored for improving rooting and survival rates of bamboo propagules. The cut ends of the cuttings are the entry sites of the pathogens causing several diseases. Research is needed for effective control of those infections which decrease the success rate in cutting-based propagation. Successful propagation under different agroclimatic zones is essential for wider acceptably. To date, no multi-locational trial of any cutting-based method has been reported. The immense potential of the cutting method must meet the demand, if a few more factors are addressed properly through future research.