Khi mình đọc sách phân loại thì có 1 chỗ làm mình thắc mắc rằng tại sao khi cây tre nó nở hoa thì lại chết,trường hợp này mình chưa từng gặp,vậy có ai biết về chuyện này không xin giải đáp cho mình biết với?cám ơn vô cùng những ai giải đáp thắc mắc của mình.

FLOWERING IN BAMBOO: AN ENIGMA!
S. M. S. D. Ramanayake
Plant Biotechnology Project, Institute of Fundamental Studies, Hantana Road, Kandy, Sri Lanka.
Accepted: 19 October 2006
…when the roots and culms are about to rot,
blossoms and seeds then appear.
They are certain to die when sixty…
Dai Kai Zhi (467 AD)
ABSTRACT
The switch to flowering is the most important event in the life cycle of a plant, signaling its commitment to set seed ensuring survival of the species. Effective and efficient pollination leading to seed set requires that all individuals in a population flower together. Environmental cues are the most effective in bringing about simultaneous flowering in populations growing over wide extents of land.
However, flowering in certain species of bamboo is intriguing, in that the cues that trigger flowering still remain a mystery. These manifest a cyclic pattern of flowering after long, and sometimes regular, vegetative periods that extend up to even 120 years. All individuals growing over vast expanses of land flower in synchrony along with individuals growing elsewhere far away. This phenomenon of mast flowering or mast seeding is the synchronized flowering and production of seed at long intervals by a large population. In bamboo, the population produces wind-pollinated flowers, sets seed in large quantity and perishes. The seeds regenerate to repeat the life cycle. Thus flowering is related to its life span, similar to that of annuals or ephemerals that flower and set seed only once before perishing. Mast flowering is uncommon in the plant kingdom and even in bamboo, only a few species exhibit this phenomenon. The unpredictable nature of this event has brought about devastations to people whose livelihood depends on bamboo. The growing economic importance of bamboo thus requires the understanding of flowering in this group of plants. Induction of flowering in vitro would be an approach to this solution in the absence of any other mechanism that can be used to trigger flowering in nature.
Key words: Bamboo, Flowering in bamboo, mast flowering, gregarious flowering, sporadic flowering.
INTRODUCTION
The switch to flowering is the most important event in the life cycle of a plant, signaling its commitment to set seed ensuring survival of the species. Effective and efficient pollination leading to seed set requires that all individuals in a population flower together. How is this achieved? Since early this century it has been experimentally demonstrated that flowering in plants can be triggered by environmental cues (Garner and Allard, 1920, Thomas and Vince-Prue, 1997). Changes in the photoperiod (long or short days), temperature, light quality, vernalization, nutrient deprivation, release from stress, etc are effective in bringing about simultaneous flowering in populations growing over wide extents of land. Thus the reproductive success of a plant depends on its flowering time
being adapted to the environment in which it grows. Once the cues that bring about flowering are known, species of economic importance could be stimulated to flower or introduced into new environments conducive to flowering for seed set and thereby agricultural expansion.
In certain species of bamboo, factors that trigger flowering remain a botanical enigma and there is no scientific method yet developed for predicting flowering. These species suddenly flower gregariously and die at the end of it (Janzen, 1976). Death in large populations is a cause of concern due to ecological, social and economic crises that set forth (John and Nadgauda, 2002).
Author’s e-mail: [email protected]
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Bamboo belongs to the sub-family Bambusoideae of the grass family-Poaceae. These comprise the herbaceous and the woody bamboos. Woody bamboos that belong to the tribe Bambusoidae, have played an integral role in the social, cultural and economic development in Asia, throughout history (INBAR, 1999). Bamboo contributes to the livelihood of the rural poor in these countries. Its industrial use in the manufacture of paper pulp dates back to over 2000 years while the use of bamboo strips for writing, dates back to the Han dynasty of China (206 BC – 220AD) (Farrelli, 1938; Marden, 1980). In the recent past a number of bamboo based wood substitutes has been developed (Farrelli, 1938; Jayanetti and Follet, 1998; Cruzet, 2004; Rao et al., 2004; Vasudev, 2004). At present this natural resource is overexploited and diminished requiring large-scale replanting (Dransfeild and Widjaja, 1995). Rare seeding in bamboo requires propagation by vegetative methods. Conventional methods of vegetative propagation cannot cater to the present scale of demand for propagules and the alternative is the use of tissue culture techniques. This is now applied in commercial propagation of a few bamboo species (Shirgurka et al., 1996; Arya et al., 1999; Ramanayake et al., 2006). Selection of propagules for plantation establishment and management of plantations would require understanding the causes that trigger gregarious flowering in bamboo.
Flowering in bamboo
In bamboo, the duration of the vegetative phase and switch to flowering varies according to the species. In some species such as Bambusa vulgaris, the vegetative phase is persistent. This species flower sporadically when only a few plants flower but set no seed (Ramanayake and Yakandawala, 1995; Koshy, 1997). McClure (1966) reported that although the species has never been “rejuvenated” by sexual reproduction, it is one of the most vigorous of the bamboos. In contrast, there are other species with a persistent tendency to flower but set no seed. Bambusa atra and Schizostachyum brachycladum (introduced to Sri Lanka from Indonesia) constantly flower but with no seed set and continue to grow vigorously year after year. There are yet others such as the endemic species, Ochlandra stridula (bata) that flower and set seed annually.
The species that are the most intriguing are those that fall between a state of constant sterility and constant flowering to manifest a cyclic pattern of flowering after long periods of vegetative growth that last even up to 120 years. All individuals growing over vast expanses of land flower in synchrony along with individuals growing elsewhere even far away. A recent example is the gregarious flowering of a temperate bamboo, Fargesia murielae, which was introduced to Europe and the US from China in the early 1800’s (Palen, 2006; Oprins and Trier, 2006). The species flowered and set seed during the 1990’s throughout Europe, US and China although environmental conditions, pests and disease incidences and various other factors that the plants were exposed to varied. The phenomenon, also termed mast flowering or mast seeding, is the synchronized flowering or production of seed at long intervals by a population of plants. Synchronized mast seeding in iteroparous plants such as oaks, beeches, conifers and dipterocarps is triggered by environmental cues (Janzen, 1974 and 1976; Koening and Knops1998). But bamboo is semelparous or monocarpic. The population produces wind-pollinated flowers, sets seed in large quantity and perishes. Death is attributed to reproductive exhaustion caused by the movement of food reserves from the vegetative parts. If however, the rhizome is left with sufficient resources, the clump is able to recover although all aerial culms that flower die. The seeds germinate with the first rains and this new generation repeats the life cycle. The thalipot palm (Corypha umbraculifera) also shows some semblance of gregarious flowering (Petch, 1925). But in this species death is a consequence of cessation of growth due to the transition of the apical meristem into a massive inflorescence followed by seeding.
Sometimes during gregarious flowering in bamboo, a few clumps may remain in a state of vegetative growth for no known reason. At the same time these species may also show sporadic flowering in some clumps within populations during the intermast period and also set seeds. Goraya et al., (2003) reported that although Arundinaria falcata is a gregariously flowering species, it also flowers sporadically every year. Rao (1998) observed that sporadic flowering is very regular in certain areas. In Kanchanaburi area of Thailand many species of bamboo in the bambusetum and the nearby forests, flower sporadically. Clump after clump in a population takes turn to flower and seeds are collected once or twice a year. It is becoming increasingly clear that sporadic flowering is very common in many species as observed in different countries (Rao, 1998). It is not known what triggers a few clumps to set flower in this manner.
Although environmental factors influence the growth of plants, and in the majority of plants they
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trigger flowering, factors that trigger flowering in bamboo still remain a mystery. It is not possible to observe the cyclic nature of flowering due to their long life spans necessitating dependence on past records of flowering. Many causes have been attributed to trigger gregarious flowering in bamboo but it has not been possible to establish their consistency. Drought has been thought to trigger mast flowering. Seifriz (1950) points out that although these two phenomena, when they occur together are noticed, the appearance of one without the other is rarely noticed. His studies on flowering of Chusquea, a new world bamboo in the mountains of Jamaica, showed that gregarious flowering occurred in times of drought as well as when there was no drought, and there also have been years of drought without flowering. If bamboo flowering is affected by environmental or climatic factors, it only requires climatic tracking for verification.
Depletion of nutrients is another cause attributed to trigger bamboo flowering, but again there is no evidence to support this. A clump does not develop any new vegetative culms in the few years before and during flowering. During flowering, there is a switching of resources from vegetative to reproductive parts when stored nutrients are used up in the production of vast quantities of flowers and seeds.
According to Janzen (1976) who studied natural stands of bamboo and their records of past flowering, there are at least 137 Indian-Asian species that have populations that flower and set seed synchronously at regular and supra-annual intervals. He suggested the presence of an internal mechanism of a biological clock that controls flowering in bamboo. In fact, the life cycle of bamboo is similar to that of annuals or ephemerals that flower and set seed only once before perishing. But unlike annuals and ephemerals, bamboos attain tree-like proportions during a long period of vegetative growth before flowering and dying. Thus the monocarpic flowering habit of the herbaceous grasses has persisted as a surviving evolutionary trace in these woody forms (Rao, 1998). Mast flowering is uncommon in the plant kingdom and even in bamboo, only a few species exhibit this phenomenon.
Yet another factor under speculation is that an unknown rhythmic climatic cycle of long duration may be the causal factor like in boreal coniferous trees, beech, oak and dipterocarps that manifest mast seeding (Janzen, 1974 and 1976; Koening and Knops, 1998). If this is so, all bamboo species that flower gregariously must respond together. This has not been observed. In bamboo forests of Mizoram, India, two gregariously flowering species, Melocanna baccifera and Bambusa tulda occur together. Both are reported to flower every 48 years but not at the same time. B. tulda flowers 18 years after M. baccifera. According to Indian forest records B. tulda gregariously flowered in 1880 through 1884 and in 1928 through 1929. Mohan Ram and Hari Gopal (1981) had observed flowering in this species in 1976 when gregarious flowering lasted until 1979. They reported that gregarious flowering was first observed in the southern parts of Mizoram, spread to the northern parts and progressed to the Assam plains. They also studied gregarious flowering in M. bambusoides (M. baccifera) and according to Indian forest records, this has taken place in 1910-1912 and 1958-1959 in northern India. It flowered in synchrony in Chitagong at the same periods (Janzen, 1976). The species is expected to flower in 2007, as it is believed to have an intermast period of 48 years (Tripathi et al., 2002). This expectation has been realized as the species has started to flower gregariously in northeastern India in about 1.7 million ha (Sethi, 2004). Interestingly, M. baccifera (introduced to Sri Lanka from India in 1910), in the Royal Botanic Gardens (RBG), Peradeniya is also flowering in synchrony (Ramanayake and Weerawardene, 2003).
Dendrocalamus strictus is another bamboo that flowers gregariously in Indian bamboo forests. It is the most widely distributed species in India and used extensively in paper pulp manufacture. In this species there are cohorts that have different intermast periods of 25 years in South India, 40 to 45 years in North East and Central India, 45 years in Bangladesh and 65 years in West India (Banik, 1994). The species also flowers sporadically and seeds are available annually.
Arundinaria falcata and A. spathiflora are two montane bamboos in the Western Himalayan State of Himachal Pradesh, of great ecological and social significance. A falcata shows an intermast period of 28 to 30 years (Goraya et al., 2003; Vaitham and Chandra, 2003). They flowered gregariously across the state in 2000 and 2001, respectively. Some clumps remained in a vegetative condition during this time, whereas one or other odd clumps flower sporadically almost every year during the intermast period. A. falcata produced abundant seed resulting in profuse regeneration during gregarious flowering. This species is reported to have a 28 to 30 year cycle. Information about flowering in A. spathiflora is limited. However, it is said to have a 60-year cycle
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of gregarious flowering while a few instances of sporadic flowering is also reported (Goraya et al., 2003).
Pseudoxytenanthera ritcheyi (Munro) Naithani flowered gregariously in Western Ghats in 1999 (Bhangre, 2001). Kulkarni et al. (2001) also reported that this species flowered gregariously at the same time in Pune, Maharashtra.
Bamboos of the New World too, have species that exhibit mast flowering. Seifriz (1950) observed mast flowering and seeding of Chusquea abeitifolia in Jamaica. His studies indicated an intermast period of 32 years in this species. Records showed that it flowered in 1880 through 1886 in the mountains of Jamaica. A living specimen from Jamaica planted in the Kew gardens in the early 1880 also flowered at the same time. Seifriz (1950) in his visit to Jamaica in 1919, observed the species only as an entangled dead mat of plants together with a foot-high seedlings, covering the mountains. Thus the offspring was successfully replacing the parents and these flowered in 1948- 1949 repeating the 32-year life cycle.
Gregarious flowering is observed in bamboos under cultivation as well as in the wild. This is contrary to Janzen’s (1976) belief that mast flowering and seeding occurred only in wild stands of bamboo. According to Seifriz (1950) gregarious flowering is due to gregarious planting either by nature or man. The industrial utilization of bamboo has led to the replanting of bamboo on a large scale. Large-scale planting will require clonally raised propagules due to scarcity of seeds. To prevent occurrence of gregarious flowering in plantations, precautionary measures such as ensuring a wide genetic base and good management practices are necessary. However, in the absence of any reliable methods to prevent flowering in bamboo, it is essential to understand the flowering behaviour and the genetic mechanisms involved.
Facts and Myths about bamboo flowering
Flowering in bamboo is believed to be an ill omen, leading to famine, death and natural disasters in cultures associated with bamboo (Mohan Ram and Hari Gopal, 1981; John and Nadgauda, 2002). Factually, when bamboo flowers gregariously, it leads to economic, social and ecological havoc. During gregarious flowering, large quantities of seeds, sometimes a layer foot or so high, are formed on the ground. Rodents and many other animals relish this nutritious source of food and reproduce fast to increase in number. People in these areas too gather the seeds for consumption (Janzen, 1976). Once the seeds are exhausted due to their germination, the rats turn to devour crops in the vicinity causing famine. In northeastern India, flowering in B. tulda (Mizo name Rawthing) is associated with the ‘Thingtam’ famine and devastation. ‘Mautam’ famine is associated with M. baccifera (Mizo name Mautak). ‘Mautam’ famine is said to be more severe as this species bear large fruits and can thus support a larger rat population. These famines have occurred in years associated with the gregarious flowering. Once the bamboo dies after flowering, the people whose livelihood depended on the bamboo resource are severely affected. The dead culms are a fire hazard. The land becomes bare as it takes time for the new generation to establish. Bare land could be disastrous in mountainous areas leading to landslides. In the late 1950s when M. baccifera in Mezoram flowered gregariously, the people formed the ‘Mizo National Famine Front’, when the Government failed to respond soon enough to assist their suffering. Their disillusionment and anger finally resulted in the formation of an ethnic political party that led to a 20-year war ending with a peace accord in 1987. At present, M. baccifera has commenced to flower and is expected to spread over a larger extent of land. The impending gregarious flowering in this species, triggered the Government of India to plan ahead in order to combat the recurrence of suffering experienced during the past (Trivedi et al., 2002). Strategies to utilize the resource in the paper pulp industry, keep close watch on the rodent population, mapping of non- flowering cohorts, fast regeneration of bamboo forest after flowering, provision of food relief and medical supplies to affected human populations, are some of the plans that are underway (Sethi, 2004; Katwal and Pal, 2004). However, most of the forest areas in Northern India are inaccessible. This indicates a measure of the problem created by incidence of mast flowering in bamboo forests.
A fairly recent episode of bamboo flowering in Thailand caused a heavy loss in the bamboo shoot processing industry. Dendrocalamus asper, used for harvesting edible shoots, flowered gregariously in 1994 and set seed in 1995 in an area of 40,000 ha. The total annual loss is reported to be US $ 80 million (Tammincha, 1995). It is reported that the species was introduced to Thailand from China about 80 years ago and the planting area rapidly increased with the development of the shoot processing industry. Gregarious flowering was unexpected. This
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species is also found in Sri Lanka but has not been reported to flower.
In China, bamboo flowering is notably associated with the giant panda bear (Ailuropoda melanoleuca) (Taylor and Zisheng, 1997). Pandas are obligate bamboo browsers and thus have very specific eating habits. They feed on shoots of the bamboo species Bashania and Fargesia. Flowering in these species cause deprivation of food for the Pandas. In the past when gregarious flowering occurred, pandas were able to move to other locations where non-flowering cohorts were growing. However, with the recent agricultural expansions, the land has become fragmented and the pandas are now unable to move to locations of non-flowering bamboo stands as they did in the past. Thus the panda bear population has become endangered mainly due to habitat destruction rather than to bamboo flowering.
Gregarious flowering in another group of plants
Another group of plants known to exhibit mast flowering followed by death include some woody shrubs of Strobilanthus. E.g. S. ixiocephalus, S. callosus, S. kunthianus etc. of the family Acanthaceae (Santapau, 1962). S. kunthianus of the Nilgiri hills of India, which because of its profuse blue flowers has given the name to the hills, is said to flower every 12 years. Santapau (1962) described that during mast flowering extensive areas are covered with blue, pleasantly scented flowers. Species of Strobilanthus in Sri Lanka are collectively known as Nilu or Nelu. Their flowering is reported to attract attention not only because of the unusual sight of a large number of plants massed together in flower, but also because of the many jungle fowl that gather to feed on the seed after flowering, an easy prey for the local sportsman (Petch, 1925). Twenty-eight species of Strobilanthus are reported in Sri Lanka and there are probably more growing in the upcountry or low country forests. Petch (1925) reported that information concerning flowering of Nilu in Sri Lanka was derived principally from Mr T. Farr, a sportsman and a naturalist who resided up country and had made observations on this subject. Strobilanthus sexennis is the commonest and the largest Nilu of the hill forests and becomes a small tree up to 12 feet high with a stem 4 to 5 inches in diameter. According to records of flowering periods kept by Mr Farr, it has an 11 or 12-year intermast period. When this species flowers, the general Nilu flowering becomes noticeable and when it dies in consequence of flowering, the dead stems, covering areas extending for miles, are an equally conspicuous feature of the landscape. The smaller species, S. pulcherimus, S. calysinus, S. anceps and others flower at the same time on the same areas. It is also possible that they flower and die at other times but remain unnoticed as they are more or less hidden from view. In 2001, we as Sri Lankans were fortunate to observe gregarious flowering of a species we suspect to be Strobilanthus sexennis in the Horton Plains. The shrubby plants of this species on either side of the road were all in bloom with blue flowers. Investigations on flowering in species of Strobilanthus and other members of the Family Acanthaceae in Sri Lanka are lacking after those made by Mr Farr and recorded by Petch (1925). The best examples of gregarious flowering in Sri Lanka are afforded by species in this family. It is unfortunate that Nilu is not that abundant at present in Sri Lanka, due to clearing of forestland. Nevertheless, investigations on flowering, even in the remaining populations are necessary, as they will contribute to understanding this peculiar phenomenon of semelparous mast flowering limited to only a few groups of plants.
Aechmanthera gossypina (Nees) is another species of the family Acanthaceae that flowers gregariously in India. Garbayal (2000) reported that the species flowered gregariously in Chaudan valley of Kumaon hills of the Uttar Pradesh in 1999. It has a 12-year flowering cycle and produces violet –blue flowers and set seed before dying. This species is neither grazed by animals nor of any commercial value to the local people, but has some cultural significance. The inhabitants of the valley believe it to be a symbol of evil and destroy it ceremoniously every 12 years to symbolize victory over evil! A scientifically interesting fact is that once the seeds germinate the plant is not seen for the next two years as it grows underground and thereafter every year, the main stem elongates producing a node each year. Once 10 nodes have developed after 10 years of growth above ground, flowering occurs gregariously in the population. Thus like in bamboo, flowering in A. gossypina is related to age. Once the plant has reached a developmental stage related to age it becomes competent to flower. This has not been studied or reported in bamboo, possibly because of practical problems associated with monitoring vegetative growth of the massive underground and above ground parts over the many years before flowering.
In vitro flowering in bamboo
This is the development of inflorescences in axillary shoots of bamboo in tissue culture. Induction of in vitro flowering in bamboo was first
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reported by Nadgauda et al. (1990). They reported that in D. strictus, D. brandisii and B. arundinacea (B. bambos) axillary shoot cultures developed viable spikelets and also produced fertile seeds in the presence of coconut water and growth regulators. Since the axillary shoots were initiated in juvenile shoots from germinated seeds, it was considered to be an expression of precocious flowering. Subsequently many others reported in vitro flowering in other species such as D. hamiltonii (Chambers et al., 1991), D. giganteus (Rajapakse, 1992) in juvenile axillary shoot cultures. Somatic embryos have also given rise to in vitro flowering in B. vulgaris, D. giganteus, D. strictus (Rout and Das, 1994). Later, in vitro flowering in axillary shoot cultures initiated from field culms in D. giganteus (Ramanayake et al., 2001a) and B. edulis (Lin et al., 2004) was also reported. Presence of cytokinins and stress has been attributed for flowering in vitro (Geilis et al., 1997; Ramanayake et al., 2001a). Reversion of flowering has also been observed under in vitro conditions. Contribution of stress and stress release in induction of in vitro flowering and its reversion respectively, are discussed in Ramanayake et al. (2001a). Viability of in vitro pollen is low. However, in vitro flowering is inconsistent and in all these reports the causes of flowering are not clearly established. Research to identify factors that bring about consistent flowering need further investigations. Induction of in vitro flowering has several other advantages. The molecular events related to flowering could be studied more readily in vitro than in the field where flowering cannot be controlled. In nature, different species rarely flower at the same time. But they may be induced to flower in synchrony by in vitro techniques and used in wide hybridization for the development of improved varieties (Nadgauda and John, 2004). In vitro flowering associated with seed set could be a perennial source of seeds for propagation.
Flowering in bamboo is taxonomically important for identification of species. Since flowering is rare in nature, only vegetative parts are available for identification. These are not always reliable as morphological features could vary with environment. D. hookeri is a species that we identified using morphological features in the absence of natural flowering. Now we have been able to induce in vitro flowering in axillary shoots of this species and studies to confirm the species identity are possible (Fig, 1 C).
Flowering of bamboo in Sri Lanka
Our observations of flowering in bamboo in
Sri Lanka for over a decade have also generated some findings. We observed flowering in the giant bamboo, D. giganteus in nature and in tissue cultures (Ramanayake and Yakandawala, 1998; Ramanayake et al., 2001a, b) (Fig. 2). Although the species is reported to have a life cycle of 75 years by Janzen (1976), our studies did not confirm it. A relatively large number of clumps flowered periodically during this time ruling out sporadic flowering. Since the entire population did not flower, this incidence was not one of gregarious flowering. Seed set was rare (Fig. 1 A & B). Only the flowering culms died and there was subsequent regeneration from the rhizome, except for two clumps, which died. Many of the flowering clumps resumed vegetative growth periodically. D. giganteus was introduced to Sri Lanka in 1856 (MacMillan, 1907; Petch, 1925). Our studies as well as reports of past curators of the RBG, showed that some of the plants in the RBG flowered, set seed and did not die at the end of flowering (Macmillan, 1907; Petch, 1925; Ramanayake and Yakandawala, 1998). We were also able to induce flowering in vitro in axillary shoots of a 75-year-old clump that did not flower in nature (Ramanayake et al., 2001a). Thus if bamboo flowering is caused exclusively by the internal mechanism of a biological clock running in harmony in the field clump and its shoots growing in tissue culture, flowering is expected to occur together in both. Since this did not happen, it indicated that there maybe more than one pathway that lead to flowering in bamboo. However, a clump of M. baccifera in the RBG Peradeniya flowered in synchrony with others of the species that commenced gregarious flowering in their native habitat in Northeast India (Ramanayake and Weerawardene, 2003).
A flowering bamboo was considered to be bad luck even in Sri Lanka where bamboo flowering is rare. Flowering of giant bamboo in home gardens was a problem to inhabitants due to the fall of vast quantities of flowers that even polluted water in wells and streams. The dying culms that tended to collapse over houses and electric wires were a hazard.
A few yellow bamboo (Bambusa vulgaris var vitata) clumps also flowered sporadically and all individuals that flowered died without regeneration (Ramanayake and Yakandawala, 1995). Flowering of many individuals of the same species recently in India leading to death with no seed set caused a speculation that the species was heading for extinction (Koshy, 1997).
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Figure 1. Flowering in Dendrocalamus giganteus and D. hookeri in nature and in vitro. A. spikelets of D. giganteus in a flowering culm in the field, bearing florets at anthesis with anthers exerted. B. Caryopsis (top) and seeds (bottom) of D. giganteus after dehusking. C. Spikelets (pointed by arrow) that developed in vitro in D. hookeri.
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Figure 2. A clump of Dendrocalamus giganteus in Kandy with dying culms after flowering
Bambusa atra, is a species introduced to Sri Lanka from Indonesia. This is a constantly flowering species and does not die when flowering culminates in the shoots. It also flowered in vitro (Ramanayake et al., 2001b).
Although D. asper has not been observed to flower in Sri Lanka, we were able to induce in vitro flowering in axillary shoot cultures raised from seeds obtained from Thailand when the species mast flowered.
Gene expression during flowering
It is now known that the transition to flowering in plants is governed by genes. With the recent development of advanced techniques of tracing of biological events at the molecular level, the molecular mechanism of flowering is being revealed (Weigel and Nilsson, 1995; Ruize-Garcia et al., 1997; Pineiro and Coupland, 1998; Somers, 1999). Flowering is accompanied by the transition of the apical meristem from a vegetative to a reproductive mode of growth to develop into a flower or an inflorescence instead of vegetative shoots. A large number of genes associated with flowering are expressed during this transition (Lin, 2000). Perception of environmental cues and the proceeding events that lead to floral development have been extensively studied. A number of pathways that lead to flowering have been discovered (Simpson and Dean, 2002). However, the genetic mechanism of flowering related to age, as in bamboo, is yet to be discovered. The intriguing phenomena of gregarious flowering indicate that it is not brought about by external factors but is defined by an internal mechanism of gene activity. Flowering, when bamboo reaches a certain age, and the regular fixed flowering cycles in certain species, indicate an age-related gene activity. At the same time, it does not imply that environmental factors do not have an influence. Incidences of flowering in vitro or damage due to fire or cutting down may also sometimes trigger this internal mechanism by alternate pathways of flowering discovered in many other plants.
During flowering, a bamboo clump remains almost leafless (Fig. 2). It does not produce new culms during flowering or in the year before flowering (Rao, 1998). Thus it would be necessary to study these well in order to understand the causal factors and relationship between vegetative growth and flowering.
Large-scale plantation establishment and prevention of flowering
It is worth understanding the flowering behavior of bamboo especially in large-scale propagation and plantation establishment. At present, Sri Lanka has a very limited bamboo
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resource both qualitatively and quantitatively. Mast flowering in bamboo does not occur in Sri Lanka possibly due to occurrence of only small populations. Bamboo has a vast economic potential for Sri Lanka. Seifriz (1950) speculated that gregarious flowering could be due to gregarious seeding in nature or planting by man. Therefore, in our quest for expansion of bamboo plantings, it is necessary to be cautious in selecting planting material of known origin and establishing a wide genetic base to alleviate problems of gregarious flowering. Selection of
non-flowering cohorts and cohorts that flower out of phase during gregarious flowering and using them in breeding programs are also important.
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