Effect of genotype, embryo maturity and culture medium on in vitro embryo germination of Sri Lankan coconut ( Cocos nucifera L. ) varieties

: Poor in vitro germination of embryos of exotic varieties was reported as one of the major constraints faced during coconut germplasm exchange programmes. In this study, the effect of genotype, embryo maturity and culture medium on in vitro germination of coconut embryos was investigated. (cid:36)(cid:3) (cid:86)(cid:76)(cid:74)(cid:81)(cid:76)(cid:191)(cid:70)(cid:68)(cid:81)(cid:87)(cid:3) (cid:72)(cid:73)(cid:73)(cid:72)(cid:70)(cid:87)(cid:3) (cid:11)(cid:83)(cid:3) (cid:31)(cid:3) (cid:19)(cid:17)(cid:19)(cid:24)(cid:12)(cid:3) (cid:82)(cid:73)(cid:3) germination the cultivars, %). Addition of growth hormones favoured root growth of in vitro raised SLRD (cid:83)(cid:79)(cid:68)(cid:81)(cid:87)(cid:86)(cid:3) (cid:90)(cid:76)(cid:87)(cid:75)(cid:3) (cid:68)(cid:3) (cid:86)(cid:76)(cid:74)(cid:81)(cid:76)(cid:191)(cid:70)(cid:68)(cid:81)(cid:87)(cid:3) (cid:76)(cid:81)(cid:70)(cid:85)(cid:72)(cid:68)(cid:86)(cid:72)(cid:3) (cid:76)(cid:81)(cid:3) (cid:87)(cid:75)(cid:72)(cid:3) (cid:81)(cid:88)(cid:80)(cid:69)(cid:72)(cid:85)(cid:3) (cid:82)(cid:73)(cid:3) (cid:83)(cid:85)(cid:76)(cid:80)(cid:68)(cid:85)(cid:92)(cid:3) roots and the number of leaves (p < 0.05) when 10 µM BAP, 10 µM Kinetin and 200 µM NAA were added to the embryo culture medium.


INTRODUCTION
Conservation of coconut genetic resources is of vital importance for present and future breeding programmes for maintaining the genetic diversity . Based on loss of palms, the rate of reduction of the existing coconut genotypes in Sri Lanka was estimated to be around 2 % annually, leading to genetic erosion. The main causes of genetic erosion were land fragmentation for urbanisation/industrialisation and replacement of locally adapted old coconut plantations with improved coconut cultivars, which has a narrow genetic base. Biotic and abiotic factors such as drought and cyclones have also extent. Therefore, systematic collection and conservation of coconut germplasm at the Coconut Research Institute, Sri Lanka (CRISL) was initiated in 1984 in order to conserve the existing germplasm and to obtain locally adapted materials for future breeding programmes. However, DNA based germplasm characterisation studies carried out at the CRISL revealed that the genetic base of the available coconut gene pool in Sri Lanka is narrow, and therefore further improvement of coconut by utilising only locally available materials is limited (Perera et al., 1999). Moreover, it was also revealed that genetic diversity when compared with the South Asian and African regions (Perera et al., 2003). Therefore, it was decided that incorporation of exotic germplasm, for further improvement of coconut in Sri Lanka.
Coconut is one of the largest seed producing plant species within the Plant Kingdom. Quick germination can be observed after maturation, which negatively affect the collection and exchange of coconut germplasm among countries. The introduction of exotic germplasm

September 2016
Journal of the National Science Foundation of Sri Lanka 44 (3) has also been restricted by quarantine regulations considering possible inadvertent introduction of many pests and lethal diseases prevalent in other coconut growing countries.
Therefore, coconut germplasm exchange is possible only as embryos or embryo containing endosperm plugs and raising of seedlings through in vitro embryo culture technique, which prevent the introduction of pests and diseases inadvertently (Assy-Bah et al., 1987). Mature embryos of 10 -12 month old nuts are collected, sterilised and pre-cultured in storage medium in cryovials individually. These cryovials can be hand carried or be sent by air mail through courier services to recipient countries. Once they arrive at the recipient laboratories the embryos are transferred to normal embryo germination medium One of the major drawbacks in germplasm exchange programmes is poor germination of embryos of some are the effect of genotypes, maturity of nuts and long storage time during collection and transport (Cueto et al., 2012). These should be minimised to achieve an acceptable level of embryo germination in order to develop a viable germplasm exchange programme. Therefore, this study was focused on evaluating the genotype effect on in vitro embryo germination and possible improvements to the germination medium. The effect of nut maturity on in vitro germination of embryos was also evaluated. In addition to the above drawbacks at the germination stage, poor root growth of in vitro raised seedlings is one of the reasons for low survival rates at the acclimatisation stage. Therefore, to improve root growth of in vitro seedlings different auxin treatments were tested with plants four (04) months after germination.

Materials
Four locally grown coconut cultivars, namely, Sri Lanka Red Dwarf (SLRD), Sri Lanka Green Dwarf (PGD), San Ramon Tall (SNRT) and King Coconut (RTB) were used in this experiment. Mature nuts were harvested de-husked and split opened. Endosperm plugs were extracted using a cork borer. The extracted endosperm plugs were properly rinsed out 4 -5 times in tap water after adding a few drops of liquid soap. Zygotic embryos were dissected from endosperm plugs with a scalpel blade and collected ( Figure 1).

Embryo culture
Surface sterilisation of the embryos was carried out using newly prepared 5 % bleach solution with a few with sterilised distilled water. The sterilised embryos 3 medium (Karunaratne et al., 1985) and incubated under dark conditions at 28 °C for 2 months. Germinated embryos were transferred to light with 16 hrs photoperiod (Temperature 28 ± 1 °C , PAR: about 75 µmol m -2 s -1 ). Germinated embryos and week intervals.

Effect of genotype on in vitro embryo germination
Embryo germination of Sri Lanka Red Dwarf (SLRD), Sri Lanka Green Dwarf (PGD), San Ramon Tall (SNRT) and King Coconut (RTB) were evaluated. Four replicates consisting of 20 embryos were used from each cultivar 3 medium (Karunaratne et al., 1985) was used as the culture medium.

Effect of germination medium on in vitro embryo germination of SLRD
3 medium (Karunaratne et al., 1985) was used as the basal culture medium with two levels of sucrose (60 g/L and 75 g/L), and solid (by adding 0.7 % agar) and liquid states of medium were combined to get four different treatments. Each treatment was replicated 6 times with 15 embryos per each replicate. Embryos of SLRD were used as the explants and embryo germination and plant development were recorded.

Effect of embryo maturity on in vitro germination of PGD
Zygotic embryos were obtained from PGD to determine the most suitable maturity stage of nuts for embryo culture. Nuts were obtained from 10, 11 and 12 month old 3 medium (Karunaratne et al., 1985). Two replicates with 20 embryos per replicate were used for each treatment and embryo germination was recorded.

Effect of hormones on root development of SLRD
In vitro raised plants of SLRD were used to evaluate the effect of indoleacetic acid (IAA), benzyl amino purine (BAP), kinetin and naphthaleneacetic acid (NAA) on root development. Cutting of initial primary root of 4 months old in vitro grown plants was done (leaving about 0.5 -1 cm root) as practiced in coconut embryo culture . Root development in 3 treatments viz., dipping shoots in IAA solution (10 -4 M) for 3 days followed by transfer to normal embryo culture media (Fernando & Gamage, 1995); culturing in 10 µM BAP and 10 µM kinetin added media for 1 month followed by transfer to 200 µM NAA added media; culturing in 10 µM BAP, 10 µM kinetin and 200 µM NAA added media for 1 month followed by transfer to normal embryo culture media, were compared with the control (culturing in normal embryo culture media). Three replicates with 15 plants in one replicate were used for each treatment. Development of new primary roots and plant growth was recorded.

Data analysis
Continuous data were analysed using GLM procedure and count data were analysed using CATMOD procedure in SAS statistical package (SAS Institute Inc., 1999).

Effect of genotype on in vitro embryo germination
in vitro embryo germination was observed among cultivars used in this experiment ( Table 1). The highest percentage of in vitro embryo germination was observed in the cultivar San Ramon Tall (77.48 %) and the lowest was observed in the cultivar King Coconut (52.5 %). Therefore, the effect of genotype can be considered as a factor contributing to variation of in vitro embryo germination of exotic varieties. Karun et al effect of the genotype and interaction between media composition and the genotype when four coconut cultivars (West Coast Tall, Laccadive Ordinary, Chowghat Orange Dwarf, and Malayan Yellow Dwarf) were cultured in embryo culture media used in the Philippines, India and France. Parallel studies conducted in vitro embryo germination between the cultivar Sri Lanka Tall 3 medium . In the present study cultivars which were not tested previously were selected. SNRT, which is a tall cultivar and SLT performed better than SLRD, RTB and PGD, which are dwarf and intermediate cultivars. 3 medium used in the present study was very similar to the media (Philippines, UPLB) used in the study by Karun et al. (2002). Cueto et al disinfection and storage procedures, screenhouse and tissue culture facilities on the establishment of embryos in culture and survival of embryo cultured seedlings during germplasm exchange programmes. Therefore, optimisation of culture media composition and culture conditions is required for different varieties to improve in vitro embryo germination. In addition, during germplasm exchange programmes more embryos can be collected and cultured from low germinating cultivars to get the desired number of plants from those cultivars.

Effect of germination medium on in vitro embryo germination of SLRD
in vitro embryo germination in liquid and solid medium was observed. A high in vitro embryo germination percentage was observed in solid medium compared to that in liquid medium (Table 2). effect on in vitro embryo germination.
Shoot height in monthly intervals up to three months different culture media (Table 3). A higher average shoot height was observed in solid medium compared to that in liquid medium. Moreover, 60 g/L sucrose content in the medium favoured shoot growth by achieving the highest shoot height. Further, a better root growth was also observed in solid medium (data not statistically one passage in solid medium and reduction of sugar during the growth and development phase improve in vitro growth in terms of earlier leaf and root formation, and higher percentage of cultures with simultaneous shoot and root formation. Karun et al. (2002) also reported hyperhydricity of embryos when germinated in liquid medium.

Effect of embryo maturity on in vitro germination of PGD
The highest percentage of in vitro germination was observed in embryos of the 12 month old bunch (97.67 %), while the lowest percentage of germination was observed in the 10 month old bunch (52.17 %). These results clearly showed that in vitro embryo germination was affected by maturity of the nuts (Table 4).
A similar trend was observed in a previous experiment using a 9 month old bunch and 10 month old bunch from the cultivar Sri Lanka Tall (Weerakoon, 1999). An embryo germination percentage of 91.3 % was observed in the 10 month old bunch compared to 27.3 % in the 9 month old bunch (Weerakoon, 1999). Further, the results of an experiment conducted using embryos from 9 th to 11 th bunches of the cultivar Mapanget Tall   Figure 2). Adding any growth hormone combination had a positive effect on rooting compared to the control (Table 5).
Although the shoot height was lower (272.0 mm) in the above medium the number of leaves was the highest (3.6), which is a more favourable character for successful weaning. Therefore, considering the 3 parameters In preliminary experiments when 200 µM NAA alone was tested, retardation of shoot growth (shoot height and number of leaves) was observed and this was also evident in the study by Ashburner et al. (1993). Shoot retardation due to root enhancement was also observed by IAA treatment (dipping shoots in IAA solution for 3 days followed by transfer to normal embryo culture media). This effect was compensated by adding 10 µM BAP, 10 µM kinetin at the same time of 200 µM NAA addition.

CONCLUSION
This study indicated that factors such as genotype, embryo maturity and culture medium affected in vitro germination of coconut embryos. Therefore, these factors have to be considered during embryo collection and culture. During embryo collection for varieties, which show low in vitro germination, more embryos have to be collected. Higher in vitro embryo germination can be achieved by culturing embryos from the 12 month old bunch. Embryo germination percentages could be further improved by culturing initially in solid media than in liquid media. By adding growth hormones plant growth in the number of primary roots and the number of leaves.