From LycheesOnline.com
by Krystal Folino and Bill Mee




Flowering in Lychee Trees


PART 1: Introduction to Lychee Flowering
PART 2: Flower Bud Formation and Differentiation
PART 3: Flower Panicle Development
PART 4: Lychee Flower Types and Anatomy
PART 5: Flower Pollination
PART 6: Pest of Flowers


PART 1: Introduction to Lychee Flowering

Lychee Fruit Develops from Pollinated Flowers:
Flowers, in particular the female flower, is the predecessor to a lychee fruit. When the ovary of a female flower comes into contact with pollen from male flowers an embryo will develop into a lychee fruit, provided pollination is successful. Most lychee varieties do not flower or fruit consistently every year and others, especially the non-commercial varieties, may go many years without producing much fruit.

'A Brewster' Tree in Full Bloom
Fig. a
'A Brewster' tree in full bloom

Lychee Flowers Develop During Winter Growth Flush:
Flowering in lychee trees occurs once a year in the winter and happens during one growth cycle in the year round continuous growing process. Lychee trees must flower, pollinate and fully complete fertilization during this cycle for there to be fruit set. Understanding this process is a key factor in managing lychees for optimal fruit production.

Mauritius Early Flower Panicle Emerging from Terminal Branch
Fig. b
'Mauritius' early flower panicle emerging from terminal branch

Lychees grow in "recurrent terminal flushes", which means that they grow from the end of branches in periodic spurts separated by short intervals of dormancy. Young lychee trees flush as many as 5-6 or more times a year whereas mature fruit bearing trees flush several times a year with a bloom flush in the winter. The interval of dormancy between successive flushes is 6 weeks minimum and can be much longer depending on the tree size, age, weather conditions, growing conditions and variety, etc.


PART 2: Flower Bud Formation and Differentiation

Your tree(s) must experience bud break of 2mm to 4mm during an extended period of cool weather. Lychees flower during the cooler winter months as it is during this time that the mean temperatures can consistently fall below the 68 degree threshold required for bloom formation (November - February in the northern hemisphere and April - August in the southern hemisphere) During a warm winter (average temperatures above 68 degrees) most lychee trees will grow vegetatively and not produce flowers.

Picture of Emerging Bud
Fig. 1a
Picture of emerging bud

It has been generally established that if the mean daily temperature is above 20 degrees C (68 degrees F) new growth will be vegetative (i.e. leaves and branches) whereas if the mean temperatures are below this temperature flower panicles will form from emergent buds. This process is known as flower bud differentiation and is a function of internal plant hormone levels, environmental conditions and temperature. When the buds first emerge they are a mixture of flower and leaf buds. As the bud continues to grow the leaves fall off and an inflorescence develops if the temperatures remain low. If the temperatures remain high a mixed inflorescence with both flowers and leaves can develop.

'Mauritius' Terminal Bud Break Showing Signs of Differentiation
Fig. 1b
'Mauritius' terminal bud break showing signs of differentiation

If you are thinking that a solution to getting flower panicles during the winter months is to force bud break by pruning prior to the onset of an extended cold period, think again. The problem with this approach is that the remaining leaves left after the pruning are too old to support a good crop because the photosynthetic capacity and nutrient and energy output of the leaves diminishes with age. Lychee tree leaves have a finite lifetime and are shed from the tree periodically.

If you are faced with a situation of winter vegetative growth flush you can selectively pinch prune the growth flush thereby leaving some new leaves to support fruit development if subsequent bloom panicles emerge following the pruning. A better strategy is to time your postharvest pruning so as to insure bud break during the cooler season.


PART 3
Flower Panicle Development

When a lychee tree flowers the flower panicle emerges 4-6 weeks after initiation has taken place as a showy multi flowered cluster, known as a panicle, on the end of branches (terminal inflorescence). In Florida, flower buds emerge in December and sometimes as early as November. The buds develop into a panicle during December and January. The flowers which are borne on these panicles develop from February through March. Fruit resulting from trees which flower very late in the cycle often to not fully mature.

A fully developed panicle consists of a main stem or spine known as a rachis with lateral (axial) branche. These branches are covered with one or more spikelets each having three small greenish yellow non-fragrant flowers approximately 2 to 3 mm in length. Lychee flowers do not have petals, but they do possess the typical anatomical components of a flower: ovary, style, stigma anthers. A panicle can be 1 to 1 ½ feet in length or more long and have as many as 3000 flowers, although only a small percentage of these are ever pollinated (100 - 200).

An example of a flower panicle is shown in Fig. 1c

A Flower Panicle of the 'Brewster' Variety
Fig. 1c
A Flower Panicle of the 'Brewster' Variety

The earlier in the winter a lychee tree flowers the more time the panicle has to elongate and develop and the more fruit the panicle can potentially produce.

'Mauritius' Mixed Infloresence With Both Flower and Leaves
Fig. 1d
'Mauritius' mixed infloresence with both flower and leaves


PART 4: Lydhee Flower Types and Anatomy

There are three types of flowers of varying distinct sexual forms occurring in lychees. They all form on the same panicle and mature at different times over a two to six week period on most cultivars. Plants that have flowers of both sexes occurring on the same plant are referred to as heterogamous. The ratio of different flower types varies among cultivars and can affect the amount of fruit setting during a season.

The three stages of development are as follows:

Male flowers open and release viable pollen
Hermaphrodite females flowers open which set fruit if pollinated
Male hermaphrodite flowers open and shed viable pollen.

An hermaphrodite flower is one having both male and female parts in the same flower.
The male and male hermaphrodite flowers produce the pollen that fertilizes the females, although the male flowers in the third stage are most responsible for fertilization as the anthers of these flowers tend to shed the most viable pollen. Julia Morton's book Fruits of Warm Climates, suggests that non-viable or damaged pollen may result in abortive seeds. The resulting seed is small and shriveled and often referred to as a "chicken tongue seed."

Lychee Flower Types:

Male flowers:

Male flowers usually have 7 - 9 stamen filaments connecting to the base. The pollen bearing anthers are attached to these long filaments. An example is shown in Figure 2a,b,c. When the pollen is ripe the anthers turn yellow and break open to release the pollen to fertilize the female flowers. Each anther may produce several thousand grains of pollen. When the pollen comes into contact with the stigmatic surface of the female flower the pollen germinates and forms a tube which penetrates into the stigma and then the ovary. Lychee flowers open primarily during the daytime from 8 am to 4 pm.

Male Mauritius Flowers in Full Bloom with Ripe Pollen.
Fig. 2a
Male 'Mauritius' flowers in full bloom with ripe pollen

Male 'No Mai Tze' Flowers with Ripe Pollen and Unopened Females.
Fig. 2b

Male 'No Mai Tze' flowers with ripe pollen and unopened females

Detailed Diagram of a Male Flower
Fig. 2c
Detailed Diagram of a Male Flower
Diagram Courtesy of Agrilink: Department of Primary Industries, Queensland Horticulture Institute


Hermaphrodite Female Flowers:
Fig. 3a,b,c,d shows fully developed hermaphrodite female flowers with three distinctly differentiated well developed female parts: ovary, style and stigma. You will note the two distinct atria on most of the female ovaries. The female flower also has stamen with pollen sacs that never open to release pollen and degenerate. Female flowers usually open in the earlier morning from 7 - 8 am and again in the afternoon between 2 - 5 pm. The stigmatic surfaces are stickiest and most receptive immediately after they open.

Fully Developed Hermaphrodite Female Flowers
Fig. 3a

Fully developed hermaphrodite female flowers - Note the fully developed anthers, stigma and style

Fully Developed Hermaphrodite Female Just Opened Flower on the 'Sweetheart' Variety
Fig. 3b
Fully developed hermaphrodite female just opened flower on the 'Sweetheart' variety - Note the ovaries and receptive stigmatic surface. Flower has probably not been pollinated

Female Flower Diagram Showing the Distinctive Parts
Female flower diagram showing the distinctive parts: stigma, style, ovary and pollen bearing anthers
Diagram Courtesy of Agrilink: Department of Primary Industries, Queensland Horticulture Institute


Hermaphrodite Male Flowers:
Hermaphrodite male flowers have degenerated pistils (the receptive female part that has a stigmatic surface that receives pollen) with undeveloped ovaries and no stigma on their styles. These cannot form fruits. An example of this type of male flower is shown in Figure 4. You will note that the male flower to the left and right of the already fertilized female flower at the top of the photo clearly display the degenerated pistils and undeveloped ovaries.

Hermaphrodite Mauritius Male Flowers with Ripe Pollen
Fig. 4a
Hermaphrodite 'Mauritius' male flowers with ripe pollen - Note the vestigial ovary in the center of the stamen

Hermaphrodite Brewster Male Flowers with Ripe Pollen
Fig. 4b

Hermaphrodite 'Brewster' male flowers with ripe pollen - Note the degenerated ovary of the male flower in the upper right hand side - Note also the pollinated female on top whose stigma is degenerating

Hermaphrodite Flower Diagram Showing the Distinctive Parts
Fig. 4c
Hermaphrodite flower diagram showing the distinctive parts: stigma, style, ovary and pollen bearing anthers
Diagram Courtesy of Agrilink: Department of Primary Industries, Queensland Horticulture Institute


PART 5: Flower Pollination

There must be an overlap of the stages in order to assure good pollination. On a single tree panicles emerge and develop at different times. This insures that there will be pollen available to fertilize female flowers and obtain good fruit set assuming that there is a pollinator like bees to carry the ripe pollen from the male to the female flowers. Having multiple trees, such as in a grove situation, or having a tree of a different cultivar with different stages of panicle development assures that there will be improved overlap of pollen bearing male flowers with receptive female flowers.


Bee Taking Nectar from the Calyx Gland of a Lychee Female Flower.
Fig. 5

Bee taking nectar from the calyx gland of a lychee female flower.

To a small degree you can get some minimal wind pollination, but insects such as bees, wasps, beetles and flies do most of the work. In particular, honeybees, are the primary carriers of pollen and for good fruit set care should be taken to maintain bee hives around a grove. The bees are attracted to the nectar which is secreted in the morning by a gland within the base (calyx) of the flower to which the stamen and pistils are attached.

Typical Beehives in a Lychee Grove
Fig. 6
Typical beehives in a lychee grove
Bee hives in a typical grove - Note the removal of most vegetation away from the area around the hives.

You should consult the services of a professional bee keeper (apiarist) if you plan to maintain bees in your grove. Keeping bees is a science in and of itself. There are many insect predators that affect bees, such as hive beetle and wax worm and for effective grove management you need an apiary professional.

The honey that is derived from the nectar of lychee flowers is a very superior variety. Unlike citrus based honeys lychee honey does not crystalize. The color is darker than orange blossom honey and the flavor is very pleasant.

Lychees can be artificially pollinated. The ripe pollen must be collected by lightly tapping male flowers onto a towel and washed into a solution which can then be immediately prayed on the receptive female flower clusters.


PART 6: Flower Pests

Depending on where in the world lychees are grown there are different pests that affect the flowers. In China lychee brown blight is one of the most serious diseases. This is a fungal pathogen whose spores infect panicles when it is rainy causing them to dry up and turn brown. Anthracnose can damage lychee flowers if we experience heavy rains during the winter which is usually not the case in Florida. Anthracnose can be controlled through the use of approved fungicides; however fungicides can destroy essential soil mycorrhizal fungi that are an important symbiont for lychee tree roots and should probably be avoided.

The most significant pathogen affecting both Australian and Florida lychee growers are flower caterpillars. In Australia there are caterpillars of the moth species Isotenes miserana and Platypeplus aprobola and they feed on the flowers and young fruit.

In Florida the adult webworm moth (Crocidosema sp.). oviposits (lays eggs) on the apical buds of flower panicles or new growth. It is believed that this moth was introduced into Florida from the Caribbean. The eggs hatch and the larvae may bore into the succulent new growth or feed on newly emerging flower parts. Symptoms of damage include webbing of newly emerged panicles or leaves. Very small, recently set fruit (green, pea size) may also be bored into and under close inspection may show an entry hole. The moth populations begins to build up during November and peaks during January and February, which unfortunately peaks during the critical phase of the lychee flowering season.. There are no alternative host plants that this moth feeds on and the population of this pest appears to be coincident with the flowering and flushing cycle of lychees and longans.

Lychee Webworm Damage to a Flower Panicle
Fig. 7
Lychee webworm damage to a flower panicle

Parasitic wasps are being tested for use as a bio-control for this moth species, but most growers are using an insecticide known as Intrepid 2F, which is effective in controlling lepidopteran pest (moths).

Intrepid 2F is manufactured by Dow Agrosciences and has been approved by the EPA for use on lychees and longans. The active ingredient in Intrepid - methoxyfenozide - mimics the natural insect molting hormone, inducing a premature lethal molt of the larvae within hours of ingestion. Intrepid offers extended residual activity of at least 14 days on treated foliage.

Supposedly Intrepid 2F has not demonstrated any toxicological effects to most beneficial insects, mites, and pollinators such as lady beetles, lacewings, parasitic wasps, and bees.

Intrepid 2F has low toxicity to mammals, birds, and fish, but is toxic to aquatic invertebrates. Please remember that a wide variety of invertebrates comprise the inter-related eco-systems of the soil food web and use of topical pesticides may have a very adverse affect on soil health. A certain percentage of what gets sprayed topically ends up in the soil around a tree so you should remember this before using any insecticides.

While you may solve the immediate problem of caterpillar larvae you may be causing serious damage to the environment essential to good root system health.

Another insecticide Admire 2F (imidacloprid) has a similar mode of operation however this compound is highly toxic to honeybees and should not be used on lychees for obvious reasons.



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Bibliography

Folino, Krystal and Bill Mee. "Flowering in Lychee Trees, Part One, Two, Three, Four, Five and Six." lycheesonline.com. N.d. Web. 24 Jan. 2014.

Photographs

Fig. a,1c,2b,3a,3b,5,6,7 Folino, Krystal and Bill Mee. Flowering in Lychee Trees, Part One, Two, Three, Four, Five and Six. N.d. lycheesonline.com. Web 3 June 2014.
Fig. b,1a,1b,1d,2a,4a,4b Maguire, Ian. Mauritius Lychee. N.d. trec.ifas.ufl.edu. Web. 3 June 2014.

Published 24 Jan. 2014. LR. Last update 3 June 2014 LR
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