Interaction effect of age and curing of scion on success rate of softwood grafts in sapota (Achras zapota L.)

Introduction

Sapota (Achras zapota L.) is one of the important tropical fruits belongs to family Sapotaceae. Many fruit growers were attracted towards cultivation of sapota on account of its better adoption to diverse soil and climatic conditions. Hence, there is ample scope for further increasing area under this crop. In the recent past, sapota has shown a phenomenal growth and attained the status of a major fruit crop after mango, banana and citrus. India is considered to be the largest producer of sapota in the world and it is being cultivated in an area of about 1.63 lakh hectare with a production of 14.95 lakh metric tonnes (Anonymous 2013) [1] . Though the fruit crop is having vast scope, the expansion of area under cultivation is limited due to the non availability of sufficient genuine planting material. The major problem in sapota is difficulties in rapid clonal multiplication, since it is a difficult to root plants, comparatively takes longer time for rooting, less success and high percentage of post separation mortality of layers which necessitates resorting to grafting. The most commercial method of propagation in sapota at present is approach grafting. But, there are difficulties in approach grafting, which is being practiced at present. This method is cumbersome, labour intensive and the rate of multiplication is also very low because of limited availability of scion shoots near the ground level of mother trees. So, the branches are to be bent or pandal is to be erected near to the mother tree, which makes watering difficult to the pots and involve additional expenses and labour. To overcome this problem, softwood grafting which is followed in mango and cashew may be tried. Softwood grafting was reported to be very easy, convenient in handling, involve simple skills and can be done within short time. The most important is that, it is a detached scion method. Hence, in order to have more grafting success with low cost and less labour intensive softwood grafting is being mainly practiced.

Success, survivability and growth of grafts depend upon many other factors including variety, grafting method, time of grafting, age of scion and rootstock and environmental conditions (Hartmann et al. 1997) [4] . Pre-curing of the scion and age of the scion shoots plays an important role in the success of grafting. Defoliation of the scion also plays an important role in high success rate of grafting. Successful grafting cannot be possible until the scion sticks of proper age are selected for the grafting. The age of the scion plays a key role in success of grafting. Hence, there is a need for standardizing the age and curing of scion for high success rate in softwood grafting of sapota. Keeping the above factors in view, the present investigation was carried out to know the effect of age and curing of scion on success rate of soft wood grafting in sapota.

Materials and Methods

The investigation was conducted with 18 treatments with three replications adopting factorial randomized block design in the low cost polyhouse, Department of Fruit Science, College of Horticulture, Mudigere, Karnataka State during 2014 to 2015. Soft wood grafting method was followed using four to six months old rootstock. For this, the top growth of rootstock was decapitated with sharp knife and secature. Then longitudinal cut of 5 cm length was given from the terminally pruned rootstock. A scion shoot of about same thickness was selected. The length of scion was about 8-10 cm. The basal end of scion was given two gentle sloping cuts of about 5 cm on opposite sides by removing the bark and a little wood giving a wedge shape. Care was taken to retain some bark on remaining two sides. The wedge shaped scion thus prepared was inserted into the 'V' shaped slit of the stock and secured firmly with 150 gauge thickness transparent polythene strip of 1.5 cm width and 30 to 45 cm length, to keep the stock and scion in firm contact. The scions were covered with small transparent tubular bag to prevent entry of water into the grafted portion and also to avoid desiccation of the scion by creating humidity and microclimate near and above the graft union region. The observations were recorded on number of days for bud initiation, days taken for 50% sprouting, days taken for 100% sprouting, number of bud sprouts per graft, sprout length, sprout girth, number of leaves per graft, length and breadth of the leaves, leaf area index, height of the graft, girth of the graft, graft index, per cent graft success and survivability percentage.

Results and Discussion

All the treatments were significantly differed among all the parameters in sapota after grafting. The days taken for sprout initiation differed significantly (Table 1) among the treatments (age and curing of scion). The significantly less number of days (9.33 days) were taken for initiation of sprout in 3 months with 10 days cured scion and more number of days (15.58 days) was observed for initiation of sprouts in 9 months with 0 days cured scion. The significantly less number of days (19.67 days) were taken for 50 per cent sprouting in 3 months with 10 days cured scion and more number of days (27.00 days) was observed for initiation of sprout in 9 months with 0 days cured scion. The significantly less number of days (30.67 days) were taken for 100 per cent bud sprouting in 3 months with 10 days cured scion and more number of days (37.33 days) were required for 100 per cent bud sprouting in 9 months with 0 days cured scion (Table 1). At 45 days after grafting, higher sprout length (4.07 cm) and higher sprout girth (4.81 cm) were recorded in 3 months with 10 days cured scion (Table 3). The observations confirm that three months age old scion with precured scions defoliated 10 days before grafting took minimum number of days for bud break and increase in length and girth of grafts when compared to without pre-cured scion wood. This might be due to the fact that defoliation causes an immediate rise in sucrose content of phloem sap of the shoots (Zimmerman 1958) [12] , which helped in movement of the solutes towards the apex of the shoot and young scions by resulting in higher meristematic activity at the bud level. This was in conformity with Jha and Shyamal (1995) [5] . [9] and several other workers (Mukherjee and Majumdar 1964, Singh and Srivastava 1979) [7,11] emphasized the importance of pre curing and age of scion. The significantly maximum numbers of leaves (5.13) were found in 3 months with 10 days cured scion, 9 months with 0 days cured scion registered lower (3.43) number of leaves at 60 days after grafting. At 90 days after grafting, significantly more number of leaves (5.50) was noticed in 3 months with 10 days cured scion (Table 4). A lowest mean number of 3.29 leaves were recorded in 6 months with 0 days cured scion. The leaf area index of grafts was found to be significantly differing among treatments. At 60 days after grafting, the 3 months old with 10 days cured scion recorded highest leaf area index, whereas lower leaf area index of 1.06 was noticed in 9 months with 0 days cured scion. At 90 days after grafting, significantly maximum leaf area index (2.70) was recorded in 3 months with 10 days cured scion. Less leaf area index of 1.30 was observed in 9 months with 0 days cured scion. The results confirm that precured scions defoliated 10 days before grafting with 3 months age old scion recorded maximum number of leaves and leaf area index. This variation in different treatments might be due to the effect of defoliation and age of scion as the grafts produced from these scions were more vigorous and juvenile condition of scion sticks had still activated the buds and increase in the leaf area (Zimmerman 1958) [12] . Linear trend of growth was recorded by Dod et al. (1996) [2] . Kumar et al. (2012) [6] in guava also reported that scions cured for nine days gave better results in terms of number of leaves. The treatment with 3 months with 10 days cured scion recorded significantly superior leaf length at 60 days (5.52 cm) and 90 days (6.16 cm) after graft success. Lesser length of leaves (3.00 cm and 3.33 cm) was observed in A3C1 (9 months with 0 days cured scion) at 60 and 90 days after grafting respectively. At 60 days after grafting, maximum breadth of leaves (3.67 cm) was found in 3 months with 10 days cured scion. At 90 days after grafting, more leaf breadth (3.88 cm) was noticed in 3 months with 10 days cured scion. The treatment with 9 months with 0 days cured scion recorded lower leaf breadth of 2.23 cm. It can be attributed to the fact that the curing of scion and younger scions stimulate the meristematic activity in the plant tissues which helps in enhancing more photosynthates and helps in increasing the leaf length and breadth. These findings are in line with those of Jha and Brahmachari (2002) [5] . At 30, 60 and 90 days after grafting, the significantly highest graft heights (36.67 cm) (36.77 cm) and (37.11 cm) were recorded in 3 months with 10 days cured scion. The lowest graft height at 30 (30.50cm), 60 (30.83 cm) and 90 (31.16cm) days after grafting was noticed in 9 months with 0 days cured scion (Table 5). This might be due to swollen terminal buds of cured scion in bulging condition which encouraged earlier sprouting and their continual growth of the grafts. The girth of scion at 30 and 90 days after grafting was found to be non significant. At 60 days after grafting, the treatment 3 months with 10 days cured scion recorded maximum scion girth (4.73mm). Whereas, the minimum scion girth (4.06 mm) was noticed in 9 months with 0 days cured scion. The observations confirm that pre-cured scions defoliated 10 days before grafting with 3 months age old scion recorded maximum diameter of grafts, it might be due to early sprouting of buds giving quick growth as explained by Patil et al. (2012) [8] and the results are in conformity with Jha and Brahmachari (2002) [5] . The percent graft success showed significant differences among different treatments. Per cent graft success was significantly highest in 3 months with 10 days cured scion at 30 (66.67%), 60 (65.33%) and 90 (64.00%) days after grafting. The treatment A3C1 (9 months with 0 days cured scion) recorded lower graft success at 30, 60 and 90 days after grating with 41.66%, 41.33% and 40.67% respectively (Table 6). The graft index showed significant differences among different treatments (age and curing of scion). A perusal of the data revealed that, graft take percentage in soft wood grafting was higher when pre defoliated scions were used, as compared to that of freshly defoliated scions. This might probably because pre-defoliated scions suffer less from desiccation than those of freshly defoliated scions, which retain their leaf stalks for some time after grafting and cut ends of the leaf stalk accelerate the desiccation process. The results are in conformity with Ram and Bist (1982) [10] . Success with age of scions was probably because of activation of the meristematic cells in juvenile condition. The success due to pre-

It is concluded that to achieve maximum success in case of softwood grafting in sapota, the selection of younger scions of three to four months age with ten days curing. The pre-curing of scion is more beneficial in terms of better success rate along with better overall graft growth. In future, more treatments in age of the scion, curing and interactions with two or more different popular varieties may be studied for more robust and accurate results.

References

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