From the Archives of the Rare Fruit Council of Australia, inc.
by V.J. Kulkarni, Fruit Research Station, Sangareddy (A.P. Agricultural University), India


Seasons in Australia are opposite to those in the US.  Summer is Dec. Jan. Feb. Autumn is Mar. Apr. May. Winter is June July Aug. Spring is Sept. Oct. Nov.

Graft-induced Off-season Flowering and Fruiting in the Mango

Scientific Name: Mangifera indica
Family: Anacardiacea


 Summary

Off-season, summer flowering and winter fruiting was induced in mango cvs Dashehari and Totapari, which normally do not flower out of season, when their defoliated shoots were grafted onto the shoots of the off-season cv Royal Special. Graft transmission of the flowering stimulus was possible from March to the first week of May, which is the period when the donor shoots of Royal Special are themselves capable of flowering, after which only vegetative shoots emerged from the receptor scions and donor stocks. When defoliated scion shoots of cv Royal Special were grafted onto non-flowering shoots of cvs Dashehari and Totapari the scion failed to flower. Fruit set and development were markedly influenced by temperature. Most fruits were harvested from the May-grafted shoots, probably due to moderate temperatures during July and August. Very high temperatures from April to June were very detrimental to March- and April-graft-induced plants. The quality of mature graft-induced off-season fruits was excellent.

The florigen hypothesis, originally postulated by Chailakhyan (1936) implicates a flowering hormone. This theory is supported by several successful attempts at graft transmission of floral stimulus, mostly in herbaceous, annual species which initiate flowers in response to specific stimuli such as photoperiod or vernalization (see Zeevaart, 1976). The florigen-hypothesis, however, has not been examined in woody trees, probably because they are generally insensitive to day length in respect of flowering and also because of the difficulty in handling them in glasshouses (Jackson and Sweet, 1976).

Alternation of flowering is a serious problem in several tree fruit species including the mango (Monselise and Goldschmidt. 1982), but the 'off season' mango cultivars, which flower more than once in an annual cycle, provide an interesting contrast to the other cultivars which have a biennial flowering habit. Among the small group of off-season cultivars, Royal Special was found to be the most promising and consistent. In addition to the main appearance of flower buds, which takes place during November-December in south India, the shoots of this cultivar flower during March-May, thus extending their fruiting to September-October. The present investigations were made to test the graft transmission of the off-season flowering habit from cv Royal Special to the commercial cvs Dashehari and Totapari which have never produced any off-season flowering at this station.

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Materials and Method

These investigations started in March 1984 with the onset of off-season flowering in Royal Special. Grafting was continued until the first week of June by which time off-season flowering had finished. Uniform, six-year-old grafts of Royal Special, Dashehari and Totapari, originally grafted on polyembryonic (cv Moovandan) rootstock, were used as rootstocks. For grafting the scion shoots onto the rootstock shoots, the veneer side grafting technique was used. The terminal shoots (c. 12cm long) of the scion cultivars were defoliated on the mother tree a week before grafting in order to activate the apical bud. They were then detached from the mother tree and side-grafted individually onto the terminal shoots of the rootstock tree, one scion being grafted onto a terminal shoot of the rootstock tree. Care was taken that there were at least four to five expanded leaves on the rootstock, both above and below the graft. Flower buds and young vegetative shoots were removed from the rootstock shoots at the time of grafting.

As many as 60 scion shoots were grafted onto an equal number of terminal shoots of matching size on a rootstock tree. The rootstock and other treatments were allocated randomly to the scion shoots so that they could be equally distributed among the rootstock trees in different experiments. Graft union was rapid in all instances, successful 'take' being clearly visible after about 15 days. Over 90% success was achieved in all the graft combinations. After 30 to 40 days from grafting, the rootstock shoots were decapitated above the graft in order to accelerate the growth of the scions. The trees were irrigated at about 10-day intervals during the hot summer months. To evaluate the flowering response, the proportion of flowering scions and reappearance of flowers in grafted rootstock shoots, which were deblossomed at the time of grafting, and the number of days for flowering were used. Fruit retention was also recorded at different stages of fruit development. The mature fruits were ripened in dry rice straw. Total soluble solids (TSS) of pulp were measured with a hand refractometer, and acidity by titration against 0.1 N NaOH and compared against standard citric acid.

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Results

Graft transmission of floral stimulus from Royal Special to Dashehari and Totapari

With the appearance of off-season flower buds in March 1984, defoliated scion shoots of Dashehari and Totapari were grafted onto rootstock shoots of Royal Special. Self-grafts of these cultivars served as controls. Defoliated scion shoots of Royal Special were also grafted onto Dashehari and Totapari shoots. All scions of Dashehari and Totapari (except two in Totapari) flowered when Royal Special was used as the rootstock, whereas none of the self-grafted controls or ungrafted shoots had flowered even after 200 days. Interestingly, all the Royal Special scions grafted onto Dashehari and Totapari rootstocks also remained vegetative. The donor rootstock shoots of Royal Special which were deblossomed at the time of grafting, reflowered after about eight days, whereas all the scion receptors on Royal Special took much longer (about 20 to 30 days). Thus, assuming that about 10 to 15 days were required for the establishment of the graft union, only 10 to 15 more days were required for the appearance of flower buds after the graft had taken.

Duration of the availability of the graft-transmissable stimulus in Royal Special
In some earlier experiments on continuous deblossoming of Royal Special in the off season (March to June), it was found that the continuously deblossomed shoots flowered only until the middle of May, and further deblossoming later in the first week of June failed to re-initiate flowering (Kulkarni and Rameshwar, 1976). Whether graft-induced flowering would follow a similar pattern was tested in this experiment. Dashehari receptor scions were grafted onto Royal Special on different occasions from March to June. Graft induction of off-season flowering was possible only until 1 May, whereas none of the receptor scions grafted in late May or in June could be induced to flower. Thus, the availability of the stimulus must have fallen below a critical value between 1 and 22 May, after which only vegetative shoots emerged from the receptors. This threshold was also found to apply to the deblossomed donor stock shoots of Royal Special which failed to re-flower in the 22 May and 22 June treatments.

Graft-induced off-season fruiting in Dashehari and Totapari
Fruit and set retention were very poor in the March- and April-grafted receptor shoots of Dashehari and Totapari which were induced to flower on Royal Special, whereas excellent fruit set and retention were observed in the 1 May-grafted shoots. This may be attributed to the extremely high temperatures (reaching 45°C) during the time when fruit set and development were in progress in the March and April-grafted shoots. Climatic conditions were much more favourable for the 1 May-grafted Dashehari shoots from which as many as 21 mature fruits could be harvested. Organoleptic evaluation of the quality of the ripe fruits made by a panel of five research workers and two growers indicated that the fruit quality of the off-season fruits was excellent and comparable to main-season fruits. In the case of Totapari, the quality of the off-season fruit was actually better than in the main season. TSS and acidity values confirmed this view.


Temperature Data During March-August 1984

MonthMean daily temperature °C
MaximumMinimum
March40.116.1
April42.424.5
May44.622.0
June39.817.6
July29.917.5
August28.518.4



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Bibliography

Kulkarni, V.J.. "Graft-induced Off-season Flowering and Fruiting in the Mango." rfcarchives.org.au. Archives of the Rare Fruit Council of Australia. Fruit Research Station, Sangareddy. (A.P. Agricultural University), India 502001. May 1995. Web. 30 Mar. 2016.

Published 30 Mar. 2016 LR
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