The word “dormancy” refers to the temporary stop of plant growth. It comprises true dormancy, known as (“rest” or “endodormancy”) triggered by internal factors and climatic dormancy (“quiescence” or “ecodormancy”) controlled by external factors
[72]. As mentioned by reference
[73], dormancy and the breaking of dormancy in buds of bamboos vary with their position on the plant, the season of the year and the species, while seed dormancy is known to occur in many tropical tree species. In seeds, several methods are known to be involved in the induction of dormancy to the germinating state. In this section, the role of plant hormones, various treatments available are discussed for bamboo seed dormancy. Important factors influencing seed germination include the seed quality and their viability. Major causes linked to the loss of seed viability are the endogenous levels of auxins and abscisic acid (ABA) during prolonged storage
[74]. Besides, bamboo seeds are short-lived, germinate within 3–7 days and the germination potential is season-dependent
[75]. To preserve the viability for a longer period of time, seeds are usually stored at 4 °C in desiccators with anhydrous calcium chloride. Furthermore, reference
[76] revealed that prechilling the seeds (4 to 5 °C) for 4 weeks could be the most effective way to extend their life. This process is known as vernalization, and it involves exposing seeds to low temperatures in order to stimulate or to enhance seed development
[72]. For instance, reference
[65] stored D. asper seeds at 4 °C for 3 months before undergoing surface sterilization. However, degradation can occur during storage. Depending on the predominant causes of dormancy, some authors
[77][78][79] have suggested various approaches to break the seed dormancy in order to improve the germination rate and speed up the germination process. Besides, the breaking of seed dormancy varies from species to species. Therefore, it is very important to determine which method and condition are the best for each plant species. Various techniques are available that enhance the vigor of seeds, and these technologies are termed as seed invigoration/seed enhancement techniques
[80]. Seed invigoration is a postharvest treatment that enhances seed production by ameliorating the germinability, storability and yield performance of the seeds
[74]. Hydropriming, seed hardening, on-farm priming, osmo-priming, osmo-hardening, humidification, priming with plant growth regulators, polyamines, ascorbate, salicylate, ethanol, osmolytes, coating technologies and, more recently, pre-sowing dry heat treatments are some of the treatments used to invigorate seeds
[75]. These strategies provide high-value crops with value-added solutions that improve the yield and quality. Generating greater emergence rates, rapid seedling growth and better stand developmental rates are the results of seeds priming
[81]. However, no treatments have been applied to D. asper seeds in order to break their dormancy and improve their viability. In terms of plant growth regulators, reference
[82] indicated that the major gibberellins formed by the germinating embryo are GA1 and GA3. Furthermore, GA3 and GA7 are thought to activate aleurone cells, and GA1 and GA4 are thought to regulate embryo development. GA2 and GA22 are two other active gibberellins, while others like GA12, GA17 and GA26 show no sign of reaction. The importance of endogenous GAs as a seed germination enhancer has also been earlier emphasized by reference
[83]. When the seeds of D. membranaceus Munro were soaked in GA3 solution (50 ppm) overnight, a high percent of seed germination was stimulated, with a corresponding increase in shoot length (2.70 mm) and number of sprouts (7) per explant during culture initiation
[84]. Similarly, reference
[85] discovered that 0.5-mg/L GA3 supplemented in media promotes the germination of D. giganteus Munro seeds under light better than BAP and Kn. In addition, GA3 at 50 ppm was found to be the best pre-sowing treatment on D. hamiltonii seeds, with a statistically significant improvement in seed viability
[75]. Furthermore, seed primed with 1% KNO3 solution increased the germination of D. strictus (Roxb.) by 80.4% at the fastest rate, and no mortality was recorded when transferred to soil
[86]. However, reference
[87] observed that osmopriming with KCl (10%) resulted in a maximum germination percentage of 83.1% when compared to KNO3 and PEG-6000 on D. strictus seeds. Meanwhile, reference
[88] soaked the D. sinicus seeds in 0.5% (v/v) potassium permanganate (KMnO4) for 12 h and resulted in a high germination rate.
The position of explants was found to affect the culture initiation and the quality of the shoots formed under in vitro conditions. During in vitro bamboo propagation, the top and bottom portions of the nodal segment in culm bamboo can hardly regenerate. The initiation of the culture is more efficient when nodal segments from a healthy mature mother plant with disinfected lateral branches are used
[19]. According to reference
[89], the juvenility of lateral shoots, the season of the cultures initiated and the position of axillary bud on the branch highly affect the bud break frequency in D. longispathus. Moreover, reference
[90] reported that nodal segments from mature clumps of B. bambos with pre-existing axillary buds were primarily preferred as explants due to their sufficient availability all-year-round to initiate in vitro cultures, while reference
[91] reported that explants from young lateral buds showed a bud break in B. tulda. Besides, explants from healthy mother stock were found to be good for the regeneration of new plants in D. hamiltonii
[92], P. stocksii Munro
[93], G. angustifolia and D. giganteus
[94]. Explants taken from higher branches were found to respond better to a multiplication medium with an early bud break than explants from lower branches
[53]. In Arundinaria callosa, the position of the nodal buds in the lateral branches affected the efficiency of the bud breaks, resulting in a higher bud break when nodal explants are taken from the basal and middle nodes compared to the distal part of the secondary branches
[95]. Reference
[96] illustrated that the 5th–7th positions of B. nutans explants from the mother stock culm were the best for the maximum regeneration in the vitro culture in bud breaking, while reference
[97] found the best regeneration for D. strictus taken from the 1st and 2nd positions of the base of the secondary branches. Similar findings have also been reported in D. longispathus
[89] and B. vulgaris
[98], which mid-culm nodes of secondary branches are in the best position for axillary shoot initiation explants. Furthermore, reference
[69] stated that the best explants for axillary shoot proliferation in D. asper were taken from the mid-culm nodes of tertiary branches.
The period of explant collection for culture initiation was found to play an important role in reducing the level of contamination, increasing the bud break and increasing the number of shoots per explant
[99]. The environmental conditions during different periods of the year varied the maturity status of the explants, hence influencing the response of explants to the culture initiation
[69]. D. asper responded best to the culture conditions during the pre-monsoon season (May to June) but with a higher contamination rate
[16], while references
[19][99] stated that young branches (nodal segments) of D. asper collected in the spring (February–April) gave a better response in terms of lower contamination, early bud break and a higher number of shoots. On the other hand, reference
[69] stated that the best time in initiating aseptic cultures for D. asper was in January and February, when the maximum bud break was achieved. In the spring, an increased cell division has been observed in trees as young buds produce auxins, which stimulates cell division in the cambium
[100]. Moreover, the months of July–December were discovered to be unsuitable for optimal D. asper bud induction. Reference
[19] found that during the rainy season (July–September), almost 50% of the contamination with moderate bud breaks was due to strong fungal and bacterial contaminants remaining underneath the leaf sheaths, while a poor response during the winter (October–December) was primarily due to the plant’s dormant and slow development. According to reference
[69], the highest rate of contamination was also observed during the time of maximum rainfall (June–August). In a study of B. balcooa by reference
[101], the explants collected during the rainy season in India (June–September) resulted in a high presence of contaminated explants. Furthermore, the establishment of B. oldhamii in vitro was a success when reference
[102] collected the explant material by the end of the rainy season (June and July) in the Central-West Region of Brazil. Therefore, it is important to understand that bud break responsiveness is normally associated with the rainy season of different locations. Similar seasonal effects on bud breaks were also observed in D. giganteus and B. vulgaris
[103], B. nutans
[96][104], B. balcoa
[105], D. hamiltonii Arn. Ex Munro
[99] and B. Bambos
[106].
6. Bamboo Genomics
Genome and transcriptome sequencing of commercially important plant species has led to the discovery of novel genes, the elucidation of biosynthetic pathways and the identification of genomic loci linked to quantitative traits. Bamboo occupies an important phylogenetic node in the grass family and the first attempt to compare the genomes of Oryza sativa and Zea mays was made by estimating the genome size of the tetraploid Moso bamboo (Phyllostachys pubescens) which was determined to be 2034 Mb following which, approximately 1000 genome survey sequence for the analysis of synteny
[107]. Molecular markers from O. sativa were successfully applied and were able to resolve bamboo species into two major groups which concurred with the morphological classification as rhizome type, runner and clumper
[108]. The first high quality of the draft genome of P. heterocycla var. Pubescens provided evidence of genome duplication and led to the identification of 31,967 genes
[109]. The Bamboo genome database (Bamboo GDB) which has been developed as a direct result of multiple genome sequencing projects now provides researchers with a library of functionally annotated genes and pathways as well as tools for analysis and graphical representation of data sets
[110]. Since then, transcriptome analysis of P. edulis has led to the discovery of genes linked to floral transition and flower development in bamboo, both of which are pertinent to the breeding industry
[111]. The cumulative data provides an important resource for the development of molecular markers for the characterization of genome variation in bamboo via genome resequencing
[112]. The wealth of information related to microsatellites has facilitated the reconstruction of high-resolution phylogenetic maps of bamboos
[113]. The discovery of transposable genetic elements within the bamboo genome, which are responsible for somaclonal variation, has also provided insights into the phylogeny of Asian bamboos
[114]. Transcription factors are important for the regulation of genes and their role in growth and development makes them of importance to genetic engineering, the characterization of these transcription factors in P. edulis has provided the foundation for the discovery and application of novel transcription factors for downstream applications in genetic modification
[114]. The recent publication of the draft genome sequence of the diploid, herbaceous bamboo Raddia distichophylla (Schrad. ex Nees) Chase, has provide a clearer understanding of the process of lignification and the genes associated with this biosynthetic pathway
[115]. The increase in the availability of both genome sequencing data from multiple projects when integrated with transcriptomic data from different developmental stages
[116][117] will provide researchers and commercial breeders with data that can be applied for the improvement of bamboo via the application of Marker Assisted Breeding (MAS) program and genetic engineering of important regulatory pathways.