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.
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.
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 45ºC) 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
| Month |
Mean
daily temperature °C |
|
Maximum |
Minimum |
| March |
40.1 |
16.1 |
| April |
42.4 |
24.5 |
| May |
44.6 |
22.0 |
| June |
39.8 |
17.6 |
| July |
29.9 |
17.5 |
| August |
28.5 |
18.4 |
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