LYCHEE AND LONGAN BECOME MAJOR INDUSTRY IN AUSTRALIA

SCIENTIFIC NAME: Litchi chinensis, Dimocarpus longan
FAMILY: Sapindaceae

For years, Australians knew the lychee (Litchi chinensis) as a tinned fruit commonly served in Chinese restaurants. But now, not only are locally grown lychees readily available as fresh fruit in Australia, the crop is becoming a major export earner.

Lychees, longans (Dimocarpus longan) and rambutans (Nephelium lappaceum) all belong to the Sapindaceae family. Natives of subtropical southern China and Vietnam, lychees and longans grow best from 17-32° latitude, while rambutans are found in the more tropical Malay peninsula.

Many Australians are probably not familiar with the longan, but in Asia, longans comprise a larger share of the market than lychees. The longan has a unique taste and texture - a taste so euphoric that botanists initially named it Euphoria longan. Where the lychee has a red skin, the longan is a yellowy-green to brown colour. The fruits of both crops have an aril (fleshy seed coat) of translucent, firm flesh around a smooth black inedible seed.

In Asia, a crisp, sweet flesh and a small seed are regarded as marks of quality. Varieties with small seeds (called 'chicken-tongue' by the Chinese) have the best flesh-to-seed ratio and so fetch the highest prices. Fruit of lychee and longan ripen on the tree and are best eaten as soon as possible after harvest before they discolour.

World production of lychees exceeds 400,000 tonnes annually, grown mainly in China and India. Of this harvest, much is processed and tinned. The internal market with southern China and northern Indochina is estimated at 300,000 tonnes, but production is seasonal. Australia produces fruit in the northern hemisphere off-season, and export prices are highest in January when many Chinese families traditionally give gifts in the month before the Chinese New Year.

The market for southern hemisphere produce among the growing number of affluent consumers in China and South East Asia appears limitless. However, lychees are a specialty item and fruit has to be of the highest quality to command the best price and to ensure repeat sales.

Total production of lychees in Australia is just a drop in the Asian ocean; we now produce some 3000 tonnes of fruit (worth $15 million); production has increased five-fold over the past five years.

Day temperatures below 20°C are required to cause flowering, and it was thought that the terminal branch buds that give rise to flowers had to be dormant during this time. This belief was based on the observation that trees entering the winter in full growth flush failed to flower, whereas those not flushing tended to flower well. As a result, growers were advised to make every effort to make their trees dormant, for example by reducing fertilisation and withholding irrigation. Unfortunately, successful fruit production did not follow.

The Australian Lychee Growers Association, with funding from the Rural Industries Research and Development Corporation, called in researchers from NSW Agriculture, CSIRO Division of Horticulture and the Queensland Department of Primary Industries to help solve the problem.

Using potted plants in controlled environment chambers and field experiments, the scientists set about investigating the alleged causes of poor fruit set, and soon exposed many as myths. They found that bees are not essential for fruit set, but do help. Generally, Lychee pollen supply and individual pollen grain longevity do not limit fertilisation of the female flowers, which are themselves generally unaffected by the weather conditions that prevail during flowering in the major growing regions. Lychees are responsive to cross pollination, but do not need mixed varieties for successful cropping, and neither dry weather nor water stress induce flowering.

However, poor fruit set can be due to flower infertility, presumably due to unfavourable conditions prior to flowering, while prolonged high temperatures (above 30°C) may increase fruit drop. Provided the trees have adequate nutrition, they do not generally respond to strategically timed fertiliser application.

Irrigation experiments showed that six weeks without rain during flowering and early fruit development, on soils of reasonable water-holding capacity, had no detrimental effect, as the trees can tap soil moisture to 1-2 metres depth. However, severe droughting during early fruit development can reduce fruit size and yield. Dry weather late in fruit development may actually increase production, possibly by redirecting nutrients that would otherwise go to producing new leaves.

The number of lychee growers is estimated at about 300, with roughly a third in north Queensland, a third in central Queensland, and the remainder in southern Queensland and northern New South Wales. Many north Queensland growers are based around Mareeba on the Atherton Tablelands behind Cairns, a district once famous for its tobacco growing, but rapidly branching into fruit orchards since deregulation of that industry. As with many other fruit crops, the first fruit to reach the market come from the north, beginning in November and fetching the highest prices, and supply then extends southwards with the final harvests from Lismore and Coffs Harbour in March.

Over the years, Australia has introduced about 50 accessions of lychee, with some varieties chosen more for their ease of propagation than for fruit quality. Four varieties now dominate: Tai So, with 40 per cent of production, has large fruit but tends to have a less attractive brown skin colour; Kwai Mai Pink (25%) attracts a 20-30 per cent price premium due to its higher flesh-to-seed ratio; Bengal (15%) is a large-seeded variety; Wai Chee has 10 per cent of production while the remaining 10 per cent comprises a mixture of varieties including the promising small-seeded Salathiel - a local selection from one old lychee tree on a property near Cairns.

The major problem for Lychees growers has been inconsistent yield - some years few trees flower, and sometimes flowering occurs but fruit fail to set. Researchers internationally have blamed poor fruit set on many causes, among them: too few bees; lack of cross pollination between varieties; and too much dry weather.

Research explodes myths
Lychees, like other subtropical fruit, need a period of cool weather to induce flowering. If the leaves of the plant are cooled (15-20°C) when new buds start to grow, the shoot is all floral (above); if buds are between 2 and 3 mm long, the new shoot is part floral, part vegetative; if buds are over 3 mm, the new shoot is all vegetative.

Wet weather can cause problems; male and female flowers rapidly lose fertility if wetted, but only if they are open. Rain for one or two days is all right because new flowers open sequentially, over several days. Extended rainfall or repeated overhead irrigation during flowering could reduce fruit set. (Persistent rain is unlikely during flowering in the current production area.)

Growth and temperature
Lychee specialists, Dr Don Batten of NSW Agriculture, and Dr Cameron McConchie of the Division of Horticulture, found growth and temperature to be the most important factors for flowering with these subtropical species. However, contrary to earlier beliefs, they found that cool conditions trigger the growing, not the dormant buds.

In experiments with potted plants in controlled environment chambers, the researchers forced plants to begin growth at high temperatures and then transferred them to a low temperature environment when buds were at various sizes. With low temperatures, small buds reliably produced flowers with no leaves, whereas larger buds produced progressively fewer flowers but more leaves at the base.

Low temperatures induce flowering for only a short time, from when the buds for a new flush start to swell, to when they are just a few millimetres in length. After this, new growth will be entirely vegetative. It only takes a couple of days of low temperature to trigger flowering.

The optimum temperature to induce flowering is between 15 and 20°C, whereas the optimum for vegetative growth is around 29°C. Fruit set is reduced if temperatures are consistently above 26°C or below 15°C, as the number or viability of the female flowers is reduced. Thus, the key to economic yields in commercial orchards is to induce flowering by getting the trees to begin growth just before or during cold weather. If growers could promote growth at the latest time that low temperatures are expected, they could reduce the likelihood of sterile flowers, as flower development would be delayed until the weather is warmer. These new insights are radically transforming the prospects for commercial lychee production.

Lychees grow by producing flushes of new reddish coloured leaves. The leaves of each flush have to harden before there can be another flush. Dr Batten showed that the pattern of flushing can be predicted because the period between flushes while the leaves harden depends largely on temperature. With this knowledge, bud growth can be synchronised to occur with cool conditions.

Growth normally occurs after harvest, but trees can be synchronised to begin growth again during winter - thus ensuring flowering. At the calculated date, trees are tip pruned with a hedging machine, stimulating buds in the axils of leaves immediately below the cut to produce a new flush. This new growth then hardens and the buds of the next flush form - by which time the weather is cold. The resulting new shoots develop into inflorescences full of male and female flowers. However if the weather is too warm during early bud formation, leaves, rather than flowers form.

Tip-pruning is not the only method of regulating growth. Chinese researchers have developed methods using sprays of ethephon to kill or defoliate the shoot tips and restart growth. In Australia, this method could be used as a back-up if growers have been unable to prune at the right time. However, the concentration of ethephon is critical- too little is ineffective, too much will cause all the leaves to drop. The response depends on the weather and differs between varieties.

Pruned trees smaller
Strategic pruning has other benefits. Growth and yield depend on the reserves stored in the fruit-bearing branches and on the amount of photosynthesis in the leaves supporting the shoot. As excessive foliage causes shading of lower leaves, a pruned canopy allows all leaves to be exposed to sunlight and can increase yields.

Small trees are easier to net against birds, flying foxes, and fruit piercing moths, and to protect from hail. Pruned trees also need fewer pesticides and are easier and cheaper to harvest.

However, smaller trees mean wasted solar radiation and more weed growth unless they are planted closer together. Conventional lychee orchard spacing has been 6 x 6 or 6 x 8 metres, giving a density of 230-250 trees per hectare; densities for small trees may need to exceed 1000 per hectare, with a spacing of 4 x 2.5 metres at establishment, but with pruning and thinning as the trees grow to prevent overshading.

Chinese researchers are investigating even closer spacings of 2 x 1 metres, similar to the mini-apple tree orchards. Another advantage of small trees and closer spacing is the shorter time to first productive harvest after planting - an important factor in the economics of orchard establishment.

The researchers have planted experimental orchards with close spacing at Mareeba and Bundaberg in Queensland, and at Brooklet in northern New South Wales, to verify the mod- els they have developed to calculate the correct timing for pruning according to latitude and local weather conditions.

The introduction of smaller trees brings a new problem. Traditionally, lychees have been propagated by air layering or marcotting. Marcotting is done by wrapping a small branch, which has had a ring of bark removed, in damp moss inside a plastic bag. Over the next three months roots develop in the ball of peatmoss, the branch is cut off and grown in a pot, and then planted in an orchard. Marcots require a lot of plant material and are too expensive for high density plantings.

Dr McConchie has shown that using rooting hormones (auxins) on cut leafy shoots in propagation beds, plantlets can be produced just as successfully and much faster. Each plant needs less plant material, although the rooted cuttings are smaller than marcotted plants, and may take an extra year to reach first harvest.

The longan
The patterns of inflorescence initiation and tip growth in the longan are similar to those of the lychee. While lychees have been the main crop in northern Australia so far, the longan may have an even more profitable future. During the northern hemisphere season, consumers eat more longans than lychees, but there are no major southern hemisphere producers.

Longans tend to be biennial, but can produce yields of 60-100 kg/tree. The longan has an interesting flavour and different texture to lychee. It can fetch an equivalent price on export markets.

The Australian longan industry at present is small, at only three per cent of the weight of lychees in the Sydney market, but is expected to reach 25 per cent over the next three years.

With only a couple of commercial growers, production was negligible in 1990, but reached 200 tonnes (worth $1.4 million) by 1994. More than 10,000 trees have been planted around the Dimbula-Mareeba area, and new orchards are being established. A major limit to more rapid expansion is the availability of planting material of the best cultivars. The QDPI predicts that Australian production will reach 1200 tonnes by 1997, rising to 2000 tonnes by the year 2000.

Further reading:
D. Batten and C. McConchie: Floral induction in growing buds of lychee (Litchi chinensis) and mango (Mangifera indica). Australian Journal of Plant Physiology, 1995,22,783-91.

D. Batten, C. McConchie and J. Lloyd: Effects of soil water deficit on gas exchange characteristics and water relations of orchard lychee (Litchi chinensis Sonn.) trees. Tree Physiology, 1994,14,1177-89.

'From marcot to market'. Abstracts from 4th National Lychee Seminar, September 1996.

IAN PARTRIDGE,
Rural Research PO Box 1139, Collingwood, Vic 3066, Australia
BASED ON AN ARTICLE IN THE AUTUMN 1997 ISSUE OF THE CSIRO MAGAZINE "RURAL RESEARCH"
Extracted from WANATCA Yearbook 21.

DATE: November 1998

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