Kalpana Sengar¹,  R.S. Sengar¹ and Sanjay  Kumar Garg²

¹Tissue culture Lab, College of Biotechnology, Sardar Vallabh Bhai Patel University of Agriculture & Technology, Meerut-250110

² Department of  Plant Sciences, M.J.P.  Roheilkhand University,  Bareilly.

Commercial Sugarcane belonging to the genus Saccharum (Poaceae) is an important industrial crop accounting for nearly 70% of sugar produced world wide. Compared to other major crops efforts to improve Sugarcane are limited and relatively recent, with the first induction of interspecific hybrids about 80 years ago. Production of sufficient quantity of seed material of a new variety of Sugarcane for planting in a vast area generally takes over 10 years if multiplied through conventional methods of seed multiplication. There are also chances of perpetuation of sett-borne diseases. In vitro micropropogation technique is emerging as a powerful tool for rapid and large scale production of disease free planting material in a number of crops. Several agro industries and research institutes are now engaged in the micropropogation activities for faster multiplication of newly released and commercially important varieties of Sugarcane (Yadav et al. 2004).

Callus Induction and subsequent shoot regeneration in sugarcane was first demonstrated by Heinz and Mee ( 1986) and Barba and Nickell (1969). Later, Heinz and Mee (1971) and Liu and Chen (1976) demonstrated that sugarcane plants regenerated from callus showed wide variation in chromosome  number. Callus cultures of sugarcane have been successfully established by several investigators also (Nadar et al . 1978, Bhansali and Singht 1984, Visessuwan et al . 1999, Lal  2003) using shoot apices , leaf sheath and young inflorescence as explants on Murashige and skoog (MS) medium supplemented wth 2,4-D  and coconut milk. Among these explants, young leaf sheath has  been widely used as the explants for callus induction and subsequent shoot regeneration. Maximum frequency of callus formation from leaf sheath explants was demonstrated on MS medium containing various concentration of 2,4-dichlorophenoxyacetic acid (2,4-D) (Mannan and Amin1999, Lal 2003, Ramamand et al . 2006) Effect of tissue culture explant sources on sugarcane yield component (Hoy et al 2003) was also studied. Development of sugarcane saccharum spp. Commercial cross, based on maximum likelihood approach for estimation of linkage and linkage phase (Garcial  et al 2006).

Plant tissue culture offers a methodology for crop improvement through direct and indirect regeneration of plants in sugarcane. The potential benefit of somaclonal variation for improved cane yield and sugar quality along with increased resistence against the disease like Fiji disease, Downy Mildew and those caused by sugarcane mosaic virus have been documented (Krishnamurthi and Tlaskal 1974, Naik and babu 1989, Wagih and Adkins 1998). An assessment of somaclonal variation in micropropagation plant of sugarcane by RAPD marker has been done by P.N. Tawar (2008). Genetic diversity associated with in vitro and bud propagation of saccharum varieties using RAPD analysis  (Da Silva et al 2008  and Patel et al 2008) also been done. New varieties through somaclonal variation (Jalaja NC et el 2006) also have been developed. Rapid micro propagation of sugarcane (Saccharum officinarium L.) varieties by shoot tip culture is done by Shabaz et al (2008). Several reports on induction of callus and shoot regeneration in sugarcane are available. However phenotypic instability has often been reported among in vitro raised plants of sugarcane (Irvine et al 1991, Naggi et al.1991, Buner and Grisham 1995, Taylor et al 1995).This tissue culture generated variation termed as somaclonal variation ( Larkin and Scowcraft 1981) largely depends on various modes of plant regeneration. Among various factors, the role of growth regulators on in vitro morphogenesis has been extensively studied, however, other factors like light intensity, photoperiod and growth room temperature have not been thoroughly worked out in sugarcane.

The present investigation was therefore proposed to study the effect of light intensity, photoperiod and growth room temperature on morphogenteic responses of leaf sheath explants of sugarcane varieties CoS 96258 and CoS 99259.

MATERIAL  AND METHOD

Fresh tops of sugarcane varieties CoS 96258 and CoS 99259 grown at the research farm of  Sardar Vallabh Bhai Patel University of Agriculture and Technology, Meerut were collected. After peeling out the outer leaf sheath the material was wiped with 70 % ethanol. Six to eight cm long spindle segment having growing tips and furled young leaf sheath were then excised out from the tops. For preparation of leaf sheath explants the segments were washed thoroughly under running tap water for 25-30 min to remove the dust particles. After washing with tap water, the segments were soaked in 1% (v/v) Cetavlon solution for 10-15 min followed  by thorough washing with clean  water. The material was then rinsed with 70% alcohol for 30-40 seconds followed by washing with sterile distilled water. The material was then surface sterilized with 0.1% (w/v) aqueous solution of mercuric chloride (Hgcl₂) containing a few drop of Triton X-100 for 8-10 min with continuous shaking. Finally the segments were washed 4-5 times with sterile double distilled water to remove the traces of chemicals.

The surface sterilized spindle segments were aseptically dissected out into 1 cm long pieces and spit longitudinally into equal halves with the help of sterile surgical blades and forceps. The leaf sheath explants thus prepared were immediately inoculated in Agar (8.0/l) solidified MS medium (Murashige and Skoog 1962) containing 2,4-D (3.0 mg/l) and incubated under varying conditions of light intensities, photoperiods and temperatures.

RESULTS AND DISSCUSSION

Effect of light intensity on callus initiation

After inoculation the explants were incubated under different light intensities. One set of cultures was also maintained under complete darkness for 3 weeks and then transferred to the light condition (3000 lux).The responses regarding callus formation were recorded 4 weeks after inoculation.

The data presented in Table 1 showed that the highest frequency of callus initiation from leaf sheath explants took place in cultures receiving dark treatment as compared to those maintained under different light intensities in both varieties. Maximum 59.4 ± 5.3 % explants induced callus formation in variety CoS 99259  and 55.3 ± 4.7 % in CoS 96258. The frequency of callus formation gradually decreased with the increasing intensity of light in both the varieties. The explants initially incubated under complete dark retained their original cream colour for a longer period while those incubated under light turned green with the time. The callus initiation occurred at the cut margins of explants incubated in dark within 10-15 days whereas in explants incubated directly under light condition (1000-2000 lux), the callus initiation took more than 20 days. Under 4000-5000 lux light intensity the explants turned green within a week and become non-response.

Better growth of callus was observed in the explants initially incubated under dark than in those incubated under low light intensities (1000-2000 lux). However, high light conditions (3000-5000 lux) did not favour the growth of initiated calli. The calli initiated from explants receiving dark treatment grew rapidly and gave rise to compact, pale yellow and morphogenic callus. At 5000 lux, callus initiation could be observed in >10% explants and also the callus growth was very poor. The results revealed that the frequency of callus formation reduced with the increasing intensity of light. This might be due to increased morphogenetic potential caused by low chlorophyll contents in the cells. A loss of callusing in presence of light occurred possibly due to synthesis of chlorophyll which would reduce the callusing potential of leaf sheath explants.

Effect of light intensity on shoot regeneration from leaf sheath callus

In order to study the effect of light intensity on shoot regeneration from leaf sheath callus, the actively growing calli of both varieties were inoculated on agar gelled (8.0 g/l) MS medium supplemented with BAP, Kn and NAA (0.5 mg/l each).The callus culture were incubated under different light intensities under 16 h photoperiod at 25±2ºC and also under complete darkness. Data enumerated in Table 1 showed that the regeneration frequency increased in both varieties with the increasing light intensity. Maximum 58.7±6.1 % shoot regeneration was recorded in CoS 99259  whereas it was 51.1±6.3 % in variety CoS 96258 at a light intensity of 4000 lux. No shoot regeneration took place from the callus in both varieties in cultures incubated in dark.

The number of shoots per culture also increased with the increasing intensity of light. At 4000 lux, maximum 13.1±1.7 and 12.3±1.5 shoots per culture were recorded in CoS 99259  and CoS 96258 respectively, however the number of shoots per culture was numerically lower beyond 3000 lux in both the varieties. High frequency shoot regeneration from leaf callus at 4000 lux of light intensity have also been reported in several other varieties of sugarcane by earlier workers (Pawar et al. 2002, Ramanand and Lal 2004, Singh 2005).

Effect of photoperiod on shoot regeneration from leaf sheath callus

The actively growing callus cultures were incubated under different photoperiods (Table II).The results showed that the frequency of shoot regeneration increased with the increasing photoperiod in both the varieties. About 36.3±3.9 % callus culture showed shoot regeneration in CoS 99259  and 32.7 ±4.3 % in CoS 96258 under 8 h photoperiod. Maximum 71.4±6.9% callus cultures showed shoot regeneration in CoS 95255 and 67.9 ±7.2 % in CoS 96258 under 16 h photoperiod .When the callus cultures of both the varieties were incubated above 16 h photoperiod (up to 20 h or continuous light conditions), there was a significant decrease in shoot regeneration frequency.

The number of shoots per culture was also increased significantly with the increasing photoperiod up to 16 h, however, it was drastically reduced under photoperiods above 16 h. Maximum 13.6±1.6 and 15.6±1.7 shoots per culture could be produced in variety CoS 96258 and CoS 99259  respectively under 16 h photoperiod while it was minimum 3.3±0.8 and 4.7±0.7 shoots per culture respectively under 8 h photoperiod. Normal healthy plants with dark green leaves were observed under 16 h photoperiod. It is apparent from the data that a 16 h photoperiod was optimum both for shoot regeneration and production of maximum number of vigorous shoots per culture.

The results indicated that an appropriate photoperiod was essential for optimum proliferation of shoots. Under continuous light the morphogenetic responses were greatly reduced suggesting that a dark period was also essential and equally important for regeneration of shoots. These results are in conformity with the reports of earlier workers (Geetha et al. 2000, Lal 2003, Wagih et al. 2004, Lal et al 2008). 

Effects of temperature on shoot regeneration from leaf sheath derived callus

Data presented in Table III demonstrated that the temperature of growth room had a noticeable effect on shoot regeneration frequency and number of shoots per culture in the both experimental varieties. At a temperature of 20±2⁰C, shoot inducted was observed in 45.4±4.9% callus cultures in variety CoS 99259  and 38.6±3.6% in CoS 96258. The regeneration frequency was significantly increased upto a maximum of 54.7±6.1% and 63.3±6.8% in CoS 96268 and CoS 99259  respectively by raising the temperature upto 25±2⁰C. At 30±2⁰C and 35±2⁰C, the frequency of shoot regeneration was decreased in both varieties.

As regards the number of shoots per culture, maximum 12.6±1.6 and 14.8±1.5 shoots per culture were recorded in variety CoS 96258 and CoS 99259  respectively at 25±2⁰C. At 20±2⁰C, the multiplication rate was however lowered down in both varieties At 35±2⁰C, a marked reduction in number of shoots per culture was noticed (Table III). At 25±2⁰C, healthy and highly green shoots were formed, however at the temperatures beyond 25±2⁰C, thin, light green shoots could be regenerated. These results indicated that metabolic activities of growing tissues are directly/indirectly controlled by the growth room temperature.

Although some species were grown successfully at much lower temperatures i.e. Galanthus and Potato tubes at 15⁰C and some cultivars of Narcissus and Allium at 18⁰C, some tropical  species usually required higher temperature, e.g. palm tree grew at 27⁰C (Tisserat  1981) and Manastera diliciosa at 30⁰C (Fonnesbech and Fonnesbech  1980). An optimum temperature of 25±2⁰C is commonly reported for several other crops under in vitro condition and also observed in case of sugarcane in the present investigation.

Based on above observations it may be concluded that in vitro morphogenetic responses of leaf sheath explants of sugarcane are highly influenced by the environmental conditions of growth room. Maximum shoot regeneration with more number of shoots per culture could be obtained under 16 h photoperiod of 4000 lux light intensity at a growth room temperature of 25±2⁰C in both the varieties of sugarcane.

ACKNOWLEDGEMENTS

Authors are thankful to Department of Biotechnology, New Delhi, for financial assistance.

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