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2 FiguresImproving flowering and fruit quality in litchi
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Improving Flowering and Fruit Quality in Litchi: Applying
PGRs and Chemical Regulants
Sanjay Kumar Singh, Amrendra Kumar, S K Purbey and Swati Sharma
ICAR-National Research Centre on Litchi
Flowering is the single most important event in the survival of angiosperms. Fruit
bearing, arboreal species have been selected for cultivation primarily because of their palatable
fruit characteristics and qualities that make them particularly attractive. They can be broadly
categorized into two main groups, deciduous fruit tree species that grow in temperate climates
and evergreen species that thrive in both tropical and subtropical climates. These two groups
display phenologies that incorporate adaptations to each climate, including timing of flowering
to avoid injurious conditions such as freezing winter temperatures in temperate regions and the
desiccating conditions present during dry seasons in the tropics and subtropics.
The availability of fresh litchi fruits in the market may be extended for another few days
by utilizing other genotypes available in the litchi. However, much scope is not there as available
genotypes differ little with regard to their maturity period (Ray and Sharma, 1986). Two pronged
strategy may be employed to solve the problem i.e., either advancing the date of harvest or
delaying the date of harvest. Still, there is no commercial method to be used for either advancing
or delaying the harvesting time of litchi and thus extending the harvesting and marketing season.
An alternative approach to induce early flowering and fruiting by using KNO3 has been
successfully used in mango (Kumar et al., 2003). GA3 has been found to offer suitable means
of controlling ripening process in litchi (Ray and Sharma, 1986) and in other fruit crops (Dilley,
1969). Evidence suggests that cytokinins retards sugar accumulation and pigmentation in litchi
fruits. Yin et al., (2001) demonstrated inhibition of litchi fruit maturation and colouration
following silver thiosulphate (STS) spray, indicated that ethylene is involved in the regulation of
ripening events. Bagging of fruits including litchi can improve ripening and reduce physical
damage.
Flowering is very much related to various morphological (flushing time, shoot maturity),
physiological and biochemical status of shoots especially C: N ratio, balance of endogenous
auxins, cytokinins and gibberellins like substances and floral initiation process especially in
warm subtropical region (Das et al., 2002). The relationship between flowering and vegetative
flushing activity in winter is well established, however floral initiation takes place only after the
shoot has undergone a period of vegetative dormancy. The growers are achieving more reliable
flowering, especially in low-production cultivars by discouraging fall vegetative flushes, by
insuring adequate age of the stems when the cool night temperatures occur in winter (Menzel,
1983).
Flushing pattern and maturity of shoots influence flowering
Litchi is an evergreen, recurrently flushing tree, and vegetative characters are very
susceptible to environment and can change with difference in climate, soil or culture practices.
Tree growth occurs as periodic, ephemeral flushes of shoots emerging from apical or lateral
resting buds before returning to a quiescent state. Periods of stem dormancy are short in young
plants but can last more than 8 months between flushing episodes in mature trees.
The three primary types of shoots (actively growing branch tips or laterals regardless of
type of growth), that typically develop from dormant stems are vegetative (leaves only),
generative (determinate inflorescences or panicles), or mixed (composed of both leaves and
lateral inflorescences inserted at nodes). Vegetative flushes (growth occurring in numerous
shoots, usually in sections of tree canopy or throughout the entire tree) of growth typically occur
one to several times per year on individual stems (branch tips that are in rest). The frequencies
of flushes that occur annually depend upon cultivar, size of the tree, and growing conditions
(especially related to nitrogen and water availability). Reproductive flushes generally occur after
extended periods of stem rest in the low-latitude tropics or immediately following periods of cool
night temperatures in the higher latitude in tropics and subtropics.
The flushing enlarges tree size and produce leaves for utilizing sunlight for carbohydrates
synthesis which supports fruit development in the following seasons. Moreover, leaf flushing is
one of the factors that counteract flowering process (Menzel and Simpson, 1992). The flushing
of shoots in litchi is recurrent with cyclic flush growth. When new shoots emerge, they elongate,
and the new leaves expand, subsequently, the terminal bud becomes dormant, with the leaves
continuing to mature and accumulate chlorophyll. Flush growth after harvest is crucial for tree
recovery and productivity for next season. New growth of shoot responds to low temperatures in
winter and forms reproductive flush (Batten and Mc Conchie, 1995). However, new growth
occurs only from a fully mature shoot. The maturity of flush in winter exerts a large influence on
the flowering. The flush cycle is managed in such a way that the latest flushes are fully matured
before winter, which maximizes flowering success. Hence, prediction of the flush cycle is
needed so that precise and effective measures can be taken to manipulate it and is necessary to
determine the progress of flush maturation, which is reflected by maturity of the new leaves. As
leaves mature, they attain their full size, typical green colour, hard texture and optimal
photosynthetic function.
The poor fruit retention in litchi results from insufficient flushes on the bearing shoot,
whereas poor flowering could occur when the bearing shoots produce immature flush and leaves
in winter. The emerging inflorescence is initially similar to a vegetative flush and it is only when
the lateral meristems develop into secondary inflorescences or start producing small leaves in the
case of mixed shoots, it is possible to identify the shoot. The three flushing cycles in potted ‘Wai
Chee’ litchi that consisted of a mean flushing duration of 20 days and an inter flushing period of
10 days. Each panicle produces ten to hundreds of small flowers (Menzel and Waite, 2005).
Limited data suggested that cultivars with early fruit ripening had a lower alternate-bearing
tendency than late ripening cultivars.
Litchi leaves have a definite lifetime and shed periodically. With a situation of winter
vegetative growth flush, selective pinching/or pruning of the growth flush by leaving some new
leaves to support fruit development can be done for better flowering. A better strategy is to time
your postharvest pruning so as to insure bud break during the cooler season. Floral initiation as
the first step in the productivity of litchi plants in warm subtropical region (Das et al., 2003) is
governed by shoot maturity and sufficient nutrient reserve in the shoot.
When a litchi tree flowers, the flowering panicle emerges 4-6 weeks after initiation has
already taken place as showy multi flowered cluster, known as a panicle on the end of branches
(terminal inflorescence). It is believed that litchi needs a period of vegetative dormancy to
initiate floral buds. Fruit from the trees which flower very late in the cycle often do not fully
mature.
Flushes of vegetative growth occur on groups of stem borne on scaffolding branches in
isolated section of tree canopy. Flushing stems are usually connected at some common branch
point within the tree limbs (Davenport, 2009). Appearance of enlarged leaf primordia and lateral
meristems on the elongating main axis are the first indications of floral differentiation in litchi.
The emerging inflorescence is initially similar to a vegetative flush and when the lateral
meristem develop into secondary inflorescences or start producing small leaves (as in the case of
mixed shoots), each panicle produces ten to hundreds of small flowers (Menzel and Waite,
2005). Out of three flushes in litchi, early (after harvest), mid (August to October) and late (after
November) season; the early and mid-season flushing influenced the yield, whereas the late
season flushing do not contribute towards yield. Thus, the mid-season flush (appearing in
August-October) is of more significance in cultivars Bedana, Bombai and Deshi, whereas the
early season flush (appearing in July) is the desirable vegetative flush in the rest of the cultivars
with respect to yield (Pereira et al., 2005). The floral initiation takes place only after the shoot
has undergone a period of vegetative dormancy (Menzel, 1983).
A minimum cycle would be 6 to 8 weeks for flushing and 4 to 6 weeks between flushes;
consequently, bearing shoot rarely completes more than two flushes between harvest and flower
initiation in Australia (Batten and Lahav, 1994). Flush growth should be restricted to 0.6 – 2.0
cm in October-November and that leaves should be removed from flush growth > 10 cm. These
practices prevent alternate fruit set and stabilise yield. The vegetative flushing in the last week of
November hardly produced any panicle in March apparently due to immaturity of these shoots to
differentiate flower buds in the month of December and January and vegetative growth after
September led to erratic bearing in litchi. The early and mid-season flushing influenced the yield,
whereas the late season flushing did not have any contribution towards yield. The mid-season
flush (appearing in August-October) is of more significance in Litchi cultivars Bedana, Bombai
and Deshi, whereas the early season flush (appearing in July) is the desirable vegetative flush in
the rest of the cultivars with respect to yield (Pareira et al., 2005).
Bearing and fruit quality affected by PGRs applications
In litchi, yields are often irregular and suffer from alternate bearing. Productivity in off-
years is unacceptably low and its yield and quality can be substantially improved by application
of PGRs. The plant growth regulators primarily (PGRs) include auxin, gibberellin, cytokinin,
ethylene, and abscisic acid (ABA). Secondarily it also includes florigens, anthesin, vernalin,
morphactins, etc. Each growth regulators have various role and functions which contributes to
economic of the farmers and horticulturists worldwide.
The usage and dosage naphthalene acetic acid used in litchi trees: in the condition of
lychee excessive growth, not germination, using 200-400 mg/L of naphthalene acetic acid
solution sprayed whole tree, can inhibit the growth of new shoots, increasing the flowers
number, improve fruit yield.
Research in Australia, China, Israel and South Africa has shown that synthetic auxin, 3-5-
6 trichloro-2-phridyl-oxyacetic acid (3-5-6 TPA) applied as foliar sprays can reduce fruit drop
and increase fruit size in different lychee cultivars. 3-5-6 TPA (applied at the 2, 4 and 6 g fruit
mass stage at 0, 20, 40 and 60 mg/L) significantly increased fruit size and retention. However,
the applications at the 2 g fruit mass stage at 40 and 60 mg/L resulted in the highest increases in
fruit size and retention, respectively. Thus, a 3-5-6 TPA application in the range of 40 to 60
mg/L at the 2 g fruit mass stage can be recommended to improve fruit size and retention. It (50
ppm) reduced natural fruit drop. If applied too early in 'Tai So', it caused an increase in fruit
drop. The TPA was most effective when natural fruit drop was high; reducing fruit drop from
74.7 to 34.9% in 'Kwai Mai Pink' and least effective when natural fruit drop was low. An
increase in the percentage of fruit with poorly developed (chicken tongue) seed and slightly
larger fruit size was also observed in treated trees.
The litchi plants treated with ethrel (2 ml/L) will have highest C/N ratio (both in leaves
and shoots before flowering), number of flowering panicles (71.58%), number of fruits per
panicle at the initial stage (63.92) and also at harvest (23.09). However, the highest sex ratio
(male: hermaphrodite) (3.26) of flowers found in untreated control plants and maximum
percentage of fertile pollen was observed in plants treated with KNO3 (2%). The ethrel (2 ml/L)
proved to be the most effective for flowering and fruit induction in litchi cv.‘Bombai’. (Mandal
et al., 2014).
GA3 (20 ppm) also was found effective treatment to increase fruit set, fruit retention and
size of fruit being maximum of 42.18 per cent, 21.81 per cent and 3.64 cm x 2.84 cm,
respectively. It also produced maximum number of fruits/tree (5327), weight of individual fruit
(20. 66 g) and fruit yield per tree (104.55 kg). Interaction between borax 0.4 per cent and GA3
(20 ppm) exhibited in maximum retention of fruit (24.64 %) and fruit yield of 123.10 kg/tree
(Kumar et al. 2009). Spray of Gibberellic acid (GA3) (ProGibb®, 20% of GA3) at 5 and 10 mg/l
14 days after full bloom (AFB) over 2 years increased fruit longitudinal and transversal diameter,
and fruit, aril and pericarp weight (40–41 and 37–38 mm, and 27.3–28.4, 21.7–22.7 and 5.0–
5.3 g, respectively) compared with control (35–36 and 33–34 mm, and 22.3–22.4, 17.8–17.9 and
3.9–4.0 g) (Chang and Lin, 2006).
KNO3 (4%) sprayed at 1 cm size of panicle (in the first week of February) and GA3 (20,
40 ppm) and BA (20, 40 ppm) applied two weeks before expected date of harvest (on 15th May).
KNO3 (4%) advanced the harvesting date only for 2 days in comparison to control. GA3 (20 and
40 ppm) delayed the harvest date for 2 and 5 days, respectively while BA (20 ppm and 40 ppm)
delayed the harvest date for 5-6 days. In all the treated trees, fruit weight was found to be more
than 21 g. Higher fruit quality attributes were recorded with GA3 (40 ppm) followed by GA3 (20
ppm) and reduced fruit cracking was also observed in trees which were sprayed with GA3 and
BA. Higher dose of cultar (5 ml/m2 plant spread) proved better than the lower dose (3
ml/m2 plant spread) in controlling vegetative flush and increasing flowering and yield. Similarly,
cultar (paclobutrazol) application 90 days before bud break was found to be more effective than
its application 60 days before bud break. Paclobutrazol, thus holds promise in increasing
flowering, fruit set, yield and quality of fruits.
Application of paclobutrazol (3 ml a.i./m2 canopy surface area) advanced the flower
emergence by six to seven days. The paclobutrazol induced flowering in China. There is increase
in C: N ratio and leaf water potential, by the paclobutrazol with drastic increase at the bud break.
C: N ratio in shoot is positively related to ABA content in buds. The doses of 1.0 and 1.5 g of
PBZ resulted in the reduction in the concentration of nitrogen and carbon in leaves and, therefore
in C: N Ratio. The proportion of pure panicles can be increased by 4-fold with 0.1 g
paclobutrazol spray in litchi cv. Xiangli. Paclobutrazol besides affecting gibberellins also
increases ABA and cytokinin contents concomitant with C: N ratio and leaf water in mango buds
to elicit flowering responses.
TIBA (tri-iodobenzoic acid) is considered a polar auxin transport inhibitor and increases
the endogenous cytokinin level in the lateral buds. There may be a positive relationship between
cytokinin level and flower bud formation which may be due to the positive impact of TIBA
(Negi et al. 2010). TIBA @ 1 g/L resulted in early panicle emergence, more flowering shoots
percentage, more panicle length, and more fruit retention per panicle of the fruit. TIBA gave
higher percentage of fertile pollen in both years (Mitra and Sanyal, 2001).
Treatment of ethrel when early litchi had winter flushes at 400, 600, 800 and 1,000 ppm
could remove winter flushes, but 800 ppm of ethrel could remove 95.6% of winter flushes,
increasing C/N proportion to enable flowering, increasing fruit setting (28.1%), fruit yield
(47.8%) compared with control, not affecting fruit quality and plant growth. Using ethrel of
1,000 ppm, which could remove winter flushes, dropped mature leaves, thereby affecting plant
growth and development. The litchi plants sprayed with 1000-2000 dilution of Ethrel in late fall
inhibited the shooting of new flushes and increased 29.7-36.7% of panicle formation.
Need of Cincturing/Girdling
The girdling treatment delayed the initiation of flowering and reduced about 15.9% of
male flowers and increased 17.7% of hermaphrodite functioning as male flowers. However,
girdling of the trunks also increased 31.2% of panicle formation. Ethrel treatment increased the
yield of litchi fruits by 57.1%. Closed girdling, spiral girdling led to increase in flowering in
litchi with increases in soluble sugars and starch content in the shoot. Girdling of trunks or
primary branches inhibits the downward transport of photosynthates, and promotes accumulation
in the upper canopy. Girdling of branches having 3 to 4 cm diameter or foliar application of 0.5 g
paclobutrazol + 0.4 g of ethephon per litre with hardened flush in September (North) promote
flowering in unproductive litchi trees. Cincturing at the 3 mm deep resulted less flowering shoots
percentage, low ascorbic content of the fruit and early harvest of the fruit. Cincturing the north
and west side shoots showed induced flowering.
Mineral nutrient requirement for flowering
KNO3 increased flowering, number of fruits per panicle and yield. Potassium nitrate
could replace the need for vegetative dormancy period, and induced higher flowering rates than
plant growth regulators (Figure 1). The higher flowering resulted in higher yields, mainly in
“off” years and thus produced highest yields also on 4-years basis, 52% higher than the control
(Figure 2).
Fig.1: Effect of flower induction treatments on flowering shoots (%) in litchi trees in ‘off’-years.
Fig. 2: Effect of flower induction treatment on the yield of litchi trees in ‘off’-years.
Zinc plays a vital role in the metabolic activities of plants. The principal functions of zinc
in plant are as a metal activator of enzymes like dehydrogenase (pyridine nucleotide, glucose-6
phosphodiesterase, carbonic anhydrase etc). It is involved in the synthesis of tryptophane, a
precursor of IAA. It is associated with water uptake and water retention in plant bodies.
Boron on the other hand, is considered to be necessary for hormone metabolism,
photosynthetic activities, cellular differentiation and water absorption in plant parts. It is also
involved in reproduction, germination of pollen tube and fertilization. In case of boron
deficiency, flowers are produced in less number and are mostly sterile; fruits are deformed and
render themselves commercially useless. The application of borax 0.4 per cent resulted in
maximum fruit set (42.50 per cent), fruit retention (22.60 per cent), size of fruit (3.72 cm x 2.90
cm), number of fruit per tree (5422), weight of individual fruit (20.91 gm) and fruit yield per tree
(111.05 Kg). Aril percentage was high in borax 0.2 per cent and 2, 4-D 10 ppm. Minimum fruit
crack of 10.91 per cent was observed in borax 0.4 per cent (Kumar et al. 2009).The application
of micro-nutrients was beneficial in improving the fruit yield which was observed maximum
under the trees treated with 1.0 per cent borax (Lal et al., 2010).
Foliar application of Zinc (0.6%), Copper (0.3%) and Boron (0.3%) was found to
accelerate the growth and vigour of the plant (Babu and Singh, 2002). The foliar sprays of
ZnSO4 @ 0.5, 1.0 and 1.5% on litchi considerably increased the fruit yield and reduced fruit
drop. The maximum length and diameter of fruit is found with ZnSO4 at 0.4% spray whereas the
weight of fruit was obtained highest in ZnSO4 at 0.2% and 0.4%. The fruit size and weight of
fruit were increased greatly with borax applied at 0.4% and ZnSO4 at 1.0% through foliar spray
(Rani and Brahmachari, 2001).
Maximum edible percentage and minimum non-edible percentage in fruits was observed
in treatment with 400 ppm SADH. Trees sprayed with 1.5 per cent potassium nitrate and 2.0 per
cent calcium nitrate gave the highest fruit weight of 20.41 and 20.37 g, respectively. TSS,
Ascorbic acid content, Total sugar, Juice percentage was found to be significantly higher by the
sprays of borax at 1.0 per cent. Acidity was also lowest with the application of borax 1.0 per
cent. Although the application of two sprays of the aqueous solution of 1.0 per cent borax
technically proved to be most effective in improving yield and quality of fruits over control
significantly, yet the most economic treatment proved to be the spraying of 0.5 per cent borax on
litchi trees at 15 day interval during the period of growth and development of fruits.
PGRs for extending post harvest life
The synthetic cytokinin N-(2-chloro-4-pyridyl);N’-phenylurea (CPPU) at 5 – 10 mg l–1,
applied to green or slightly red fruitlets (25 or 30 mm in diameter), delayed harvesting by 2–3
weeks compared with control trees. At harvest, CPPU-treated fruit that attained a red colour
comparable to that of earlier harvested control fruit, were 20–25% larger with total soluable solid
contents: titratable acidity (SSC:TA) ratios more than 50% higher than the controls. Despite their
high SSC:TA ratios, CPPU-fruit stored well for 6 weeks at 1°C due to reduced browning, lower
decay development and less aril discolouration, and maintained an acceptable flavour. These
results suggest that CPPU can be used to extend the harvest season for litchi fruit. Silver
thiosulphate gave a harvest delay of 8 days, however, a few brown spots on fruit skin were
observed after the spray.
Thus, applying various plant growth regulators and chemicals can help litchi orchardist to
manage their orchard in such a way that it will have better quality production. It is mentioned
here that above PGRs is effective in particular climate and its exact time of application must be
known from researcher before orchard application.
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