Agriculture Reference
In-Depth Information
initiation and yields by SNA treatment during years with dry autumn months
and on trees that yielded heavily during the previous season.
A combination of ethephon and kinetin is more ef ective in reducing
litchi shoot length and inducing high proportions of fl oral buds than either
chemical alone (Chen and Ku, 1988), while unsprayed shoots remain
vegetative. Killing winter fl ushes by the use of chemicals to promote litchi
fl owering is also practised. A 25% urea solution, ethephon (1500 mg/l),
maleic hydrazide (0.125%) and DNOC (dinitro-orthocresol) at 0.1-0.5% forces
the young winter leaf fl ush to abscise, with subsequent emergence of axillary
fl owering. A 0.5% urea spray applied to the late-summer growth fl ush causes
the soft leaves to turn green, mature quickly and produce fl owers in the spring.
The higher the concentration of ethephon applied at the time when trees
approached fl owering, the lower the percentage of fl owers produced.
Girdling (cincturing) branches or the trunk in late summer increases
fl owering and fruiting (40-800%) in litchi trees that would have fl owered
poorly in the spring. If no pronounced growth fl ush has occurred in the 6
months before the fl oral initiation period, or the trees are in poor condition
or have had a late-autumn vegetative fl ush, they have increased fl owering
(Menzel and Simpson, 1987). Trees to be girdled should be fertilized
immediately after harvest and irrigated to induce fl ushing and then girdled in
late summer when the leaves have become fully matured (Fig. 9.1). Girdling
of limbs and trunks of trees that start fl ushing in late summer has no signifi -
cant ef ect on trees that would have fl owered profusely on their own. Early
recovery from girdling is associated with vegetative fl ushing during winter
(Menzel and Paxton, 1986b).
Retention of litchi fruit after setting is a universal problem. Fruit
development is punctuated by four phases of abscission (Fig. 9.6), the fi rst
occurring within 15 days of anthesis, in which more than 92% of pollinated
fl owers drop due to endosperm degeneration and subsequent embryo abortion.
The second period of abscission is possibly due to degeneration of the embryo
sacs. Phase IV does not occur in fruits that have aborted seeds. There are many
causes of fruit drop, including unfavourable climatic conditions and nutrient
defi ciencies. Spraying newly set fruit twice at 15-day intervals with 35-100 ppm
of 2,4,5-T and NAA progressively reduces fruit drop, improves fruit size and
even hastens ripening (Prasad and Jauhari, 1963). Lower concentrations also
reduce litchi fruit splitting, another serious problem in many areas. Growth
regulators (GA 100 ppm, NAA 20 ppm, 2,4,5-T 10 ppm, chlormequate 250
ppm) sprayed on litchi cv. 'Rose Scented' at the pea stage had reduced fruit
drop (Khan
et al.
, 1976). Spraying 'Early Seedless' and 'Calcuttia' with 20
ppm IAA enhances fruit set; 50 ppm GA-3 increases fruit retention and 100
ppm GA-3 improves fruit weight, with a combination of IAA and GA-3 being
suggested (Singh and Lal, 1980).
Reduction of seed size or the production of seedless fruit has also been an
objective in the use of growth regulators. Kadman and Gazit (1970), using
