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Strategies for Flower Induction to


3.3 Strategies for Flower Induction to Improve Orchard Productivity: From Compensation of Alternate Bearing to OffSeason Fruit Production
Pittaya Sruamsiri, Amonnat Chattrakul, Pawin Man

ochai, Martin Hegele, Daruni Naphrom, Winai Wiriya-Alongkorn, Sithidech Roygrong, Fritz Bangerth

3.3.1 Introduction
Due to alternate and irregular bearing of fruit trees, which occurs at various extent amongst different species and cultivars, the yield of many species of fruit tree is erratic. Uncertainties regarding the time of harvest and the quality and quantity of fruit can seriously affect the marketability of the product (MONSELISE and GOLDSCHMIDT, 1982; WESTWOOD, 1995; SUBHADRABANDHU, 1999; SOUZA et al., 2004). Unfavourable climatic conditions during flower induction (FI) or the flowering period are amongst the most important causes of this phenomenon. Often large areas or even whole countries face the same problem simultaneously leading to overproduction and low prices in one year and a low return from fruit production the next. Equalising these fluctuations therefore would help to make fruit production more profitable and sustainable. Another option for raising the return from fruit production would be to extend or totally shift the harvest season by artificially influencing conventional and off-season flowering.

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Figure 3.2: Longan tree with high off-season crop load after KClO3 (Potassium chlorate) application.

In the last four years, research has focused on off-season flower induction of longan (Dimocarpus longan Lour.), lychee (Litchi chinensis Sonn.), and mango (Mangifera indica L.) trees. The work on longans was especially successful and comprehensive. The effects of treatment with potassium chlorate (KClO3) were clearly demonstrated both in the field trials (Figure 3.2) and the laboratory analyses of the resultant hormonal changes. In this sub-chapter, the observed effects of related horticultural procedures are presented, and the factors influencing flower induction are discussed. The overall aim of our research was to optimise fruit production and stabilise the yield of high quality fruit.

3.3.2 Stabilising Yield and Quality of Lychee Fruits
To optimise the water balance of fruit trees, information about local climatic conditions (weather station), water retention capability of the soil and water demands of the trees was gathered at Mae Sa Mai lychee orchard. This plot was shared with other participants in The Uplands Program (for details see Chapter 2). A balanced irrigation system was installed and a proper watering schedule developed which was capable, in combination with

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fertilisation, to improve the yield and quality of lychee fruits. The optimal amount of water per tree in relation to the climatic water balance (100 %) was calculated by means of CROPWAT, a shareware program supplied by the FAO. The first results, summarised in Table 1, show that 66 % irrigation and 2 kg of NPK-fertiliser / tree were sufficient to achieve the highest yields. Stronger vegetative growth might have caused reduced yields. For best fruit size, however, 6 kg of fertiliser / tree and full irrigation were necessary. The fact that the increase in the size and weight of fruit was not simply due to a higher uptake of water is clearly shown by the constant value of total soluble solids (TSS) observed in all variants. Moreover, constant results for the sugar-acid ratio and the peel colour reflected uniform internal quality and ripeness at harvest, irrespective of the treatment. The appearance of fruit cracking, i.e. the cracking of the shell during the last stage of ripening, is another serious problem which often affects the quality of lychees. Although several attempts have been made to solve this problem, the causes are still unknown. Techniques include protecting the peel from external influences by wrapping the fruit clusters in bags made of newspaper or other materials and stabilising the membranes of the shell by spraying the leaves of the trees with solutions containing calcium salts. However none of the treatments was successful in preventing cracking. In the region under study, lychee trees require relatively low temperatures of approximately 17 °C for 10-12 days in order to flower in early January (SETPAKDEE, 1999). Elevated temperatures exceeding 17 °C or lower temperatures with large daily fluctuations during the period of flower induction are among the main factors associated with alternate bearing (i.e. trees only producing fruit every second year).

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Table 3.3: Effect of irrigation and fertilisation on yield, fruit size and internal quality parameters of lychee fruits harvested at Mae Sa Mai orchard a.
Treatment Yield per tree [ kg ] Control 100% irrigation +2 kg NPK / tree 100% irrigation +6 kg NPK / tree 66% irrigation +2 kg NPK / tree 66% irrigation +6 kg NPK / tree 33% irrigation +2 kg NPK / tree 33% irrigation +6 kg NPK / tree Total mean ± SE e
a b

Size b A-C [%]

Size b D-E [%]

TSS (aril) [%]

TSS/TA Pericarp colour d
c

C*



27± 8.5 4± 2.3 96± 9.0 19.7±0.7 53± 2.4 38.1±1.0 43±3.7 41±17.1 57± 7.1 43± 8.4 19.2±0.1 52±10.9 38.2±0.7 43±3.5 39± 8.5 58± 7.8 42± 7.9 19.2±0.4 51± 1.9 37.9±0.6 42±2.0 41± 6.5 41±12.5 59±15.1 19.4±0.6 45± 4.6 38.2±1.2 44±3.0 29±11.0 15± 7.6 85±17.7 19.3±0.5 48± 3.4 38.3±0.7 43±3.2 33± 4.8 11± 4.9 89±12.6 19.4±0.5 42± 3.1 38.8±0.5 39±1.6 34± 5.1 34± 8.7 66±10.7 19.1±0.4 42± 1.8 38.9±0.9 40±2.1 ------19.3±0.2 48± 1.8 38.3±0.3 42±1.0

CHATTRAKUL et al., to be published. Grading system (g/fruit): A >25, B >22.2 - 25, C >20 - 22.2, D >17 - 20, E 17; fruit size distribution: absolute weight-related frequencies (g fruits per class / 100 g total fruit weight) summarised for size classes A-C and D-E, respectively c Sugar-acid ratio of the edible aril calculated from total soluble solids (TSS in °Brix, i.e. g sucrose / 100 g) and titratable acids (TA, determined as citric acid by titration to pH 8.1) d Colour according to the CIE L* a* b* colour space: Colour intensity and the hue are characterised by the length of the colour vector (chroma C*= (a* 2 + b* 2) 0.5) in the a* x b* colour hue space and by the hue angle H°=180°/ · arctan(b*/a*) of this vector, respectively. H° changing from 90° to 0° marks a transition from pure yellowness to pure redness. e SE = standard error. Total variation among trees: SE for n = 28 trees. Variation within variants: SE for n = 4 trees per treatment.

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As shown in Figure3.3, the harvest period of lychee is quite short, resulting in a seasonal overflow of the markets and low prices for the farmers.
Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov FI Flowering Fruit growth Harvest Pruning Flushing + vegetative growth

Figure 3.3: Schedule for normal-season lychee production / development in northern Thailand (FI = flower induction).

Therefore several experiments were conducted not only to reduce alternate bearing, but also to produce off-season fruits, using already proven measures to induce off-season flowering and hence facilitate off-season harvest of other fruit species. A combination of hard pruning, i.e. a deep cut-back of branches after harvesting, and appropriate fertiliser application, both known to enhance growth, was shown to induce regular flushing of the whole trees. Subsequent girdling of 50 % of the branches, promoted a more regular flowering (Table 3.4). In this technique, which must be carried out not later than end of September, a small stripe of bark around a stem or branch of a tree is removed to repress carbohydrate and hormone transport to the root.
Table 3.4: Inhibition of vegetative flushing of lychee trees (% out of 20 shoots / replication) a.
Treatment 32 Control Girdling Girdling+0.1 % MF Girdling+1.0 % MF LSD 0.05
a

Days after treatment 60 43 a 18 b 14 b 14 b 0.00 5 88 47 a 24 b 20 b 26 b 0.00 8 95 47 27 28 32 0.11 6 102 47 38 35 36 0.45 3 112 47 39 39 41 0.69 2

41 a 11 b 12 b 10 b 0.00 8

CHATTRAKUL et al., to be published; MF: morphactin; LSD: least significant difference. Means in the same column followed by different letters significantly differed at p 0.05

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Table 3.5: Percentage of flushing (after 33 d) and flowering of lychee shoots (after 128 d) a.
Treatment Control Girdled Girdled + KClO3 15 g/m? b Girdled + NaOCl 10 g/m? Girdled + NaOCl 15 g/m? Girdled + NPA 1500 ppm Girdled + NPA 3000 ppm Girdled + PBZ 1000 ppm + Ethrel 500 ppm Girdled + PBZ 2000 ppm + Ethrel 500 ppm Girdled + Morphactin 6000 ppm
a

Flushing [%] 53 13 20 35 48 22 35 15 18 5

Flowering [%] 0 50 48 52 40 45 58 20 28 18

CHATTRAKUL et al., to be published; NPA: naphthylphthalamic acid; PBZ: paclobutrazol, MF: morphactin b Dosage of agrochemicals in g active ingredient per square metre of ground area under canopy

Other potentially flower inducing horticultural procedures that were tested included bending and the use of various chemicals either sprayed onto the leaves (Ethrel1) or applied directly into the girdling wound (auxin transport-inhibitors etc.). In most cases, girdling in combination with flower inducing chemicals had no additional beneficial effect on flowering in lychee trees (Table 3.5). This was probably because the desired inhibiting effect of girdling on vegetative flushing had been partly replaced by that of the other treatments. An exception was the case of morphactin, where enhanced growth inhibition could be observed at least in the beginning, but flower induction was reduced (Tables 3.4 and 3.5). Although girdling was able to improve flowering by 50 %, it could not completely replace stimulation by cool temperatures. Hence in none of the experiments, induction of off-season flowering was achieved by these procedures, but the undesirable
1 Contains ethephon (IUPAC name: 2-chlorethyl phosphonic acid) as active ingredient and decomposes into ethylene, a volatile plant hormone

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vegetative flushing during the period of flower induction was reduced and flowering was considerably improved by some of the treatments. Young leaves inhibited the process of flower induction. In their presence, manipulations that usually provoked flowering, became either less effective or totally ineffective as is clearly demonstrated in Table 4 for the example of girdling in lychee production.
Table 3.6: Percentage of flowering lychee shoots at different age of the leaves a. Treatment Non-girdled + young leaves Non-girdled + old leaves Girdled + young leaves Girdled + old leaves
a

Flowering (%) 3.3 17.3 3.3 41.3

CHATTRAKUL et al., to be published

3.3.3 Possible Factors Influencing the Response of Longan Trees to Potassium Chlorate
In contrast to its lack of effectiveness in lychee cultivation, the use of potassium chlorate (KClO3) in longan production, whether applied as a soil drench, foliar spray or stem injection, successfully prevented alternate bearing. It even proved to be the ideal method for inducing flowering. Thus off-season fruits could be produced almost all the year round, whereas the normal harvesting period is limited to July-August in the area under study. Since recovery periods are no longer required after harvest, three harvests could theoretically be achieved from the same tree within 2 years. The harvest time could be chosen for each individual tree according to the schedule of KClO3 application, but there would be some limitations due to leaching during the rainy season (paragraph 3.3). Although KClO3 (Potassium chlorate) is well known amongst Thai farmers as a means of inducing flowering in longan trees, severe problems are frequently encountered when they try to use it for off-

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season fruit production. In many cases, application of inappropriate doses results in a poor response by the longan trees to KClO3 and poor fruit quality. Farmers will sometimes double or even triple the recommended dose, which eventually damages or kills the trees, since chlorates are also potent herbicides (HANSEN, 1929). Consequently, studies of the factors affecting the response of longan trees to KClO3 were necessary, focusing particularly on the development of proper technologies for its safe use. As summarised in Table 3.7, many relevant factors emerged. They will be discussed in detail in the following paragraphs.
Table 3.7: Factors affecting the response of longan trees to KClO3
Factor Cultivar Leaf age Application frequency Environmental conditions Influence Variability in sensitivity to KClO3 Influences susceptibility to KClO3 Health and yield of the trees Low soil humidity and full sunshine enhance the quality of the flowering response

3.3.3.1 Impact of the Longan Cultivar The sensitivity of different longan cultivars to various concentrations of KClO3 was examined in order to find out the minimum dosage necessary to achieve maximum flowering response.

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Percentage of flowering
10 9 8 7 6 5 4 3 2 1 0 8 1

KClO3 (g/m2canopy)

n.d.
Si-Chompoo E-Daw

n.d. n.d . 0 1

4 2

.

Figure 3.4: Effect of various concentrations of KClO3 applied as soil drench on flowering of longan cv. E-Daw and Si-Chompoo at 3 weeks after treatment (MANOCHAI et al., 2005).

In healthy trees, a soil drench of KClO3 at a dose of only 4 g/m2 of ground area under the canopy could successfully induce off-season flowering in the ‘Si-Chompoo’ cultivar. For the cultivars ‘E-Daw’ and ‘Haew’, the concentration had to be increased to 8 g/m2 to achieve the best flowering response (Figure 3.4). These results show that, the cultivar is the first factor that has to be considered when choosing the proper KClO3 dose for successful off-season longan production. 3.3.3.2 Influence of Leaf Age Under natural conditions in northern Thailand, longan trees start to flower in January to February after being subjected to cool temperatures in November and December. If vegetative shoot flushing occurs during the cold-temperature period, e.g. in November, trees will not produce flowers in the following season unless treated with KClO3. Similarly, flowering was considerably reduced when KClO3 was applied to trees at the young leaf stage (Table 3.8). The best application time was at the “full mature” leaf stage, resulting in flowering within 4 weeks after KClO3 application. When trees were treated at the “leaf expansion” stage,

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they required up to 7 weeks after the use of KClO3 before they flowered.
Table 3.8: Flowering response of longan shoots depending on leaf stage and treatment with KClO3 (8 g/m? of ground area under the canopy) as a soil drench (HEGELE et al., 2004).
Leaf stage/treatment 4 Young leaf Mature leaf KClO3 at young leaf KClO3 at mature leaf 0.0 b 1.0 b 9.0 b 88.0 a Percentage of flowering after application (wks) 5 0.0 b 11.0 b 12.0 b 100.0 a 6 0.0 d 32.0 b 16.0 c 100.0 a 7 0.0 c 39.0 b 78.0 a 100.0 a

Means in the same column followed by different letters significantly differed at p 0.05 Young leaf = 15 days after flushing Mature leaf = full green mature (45 days after flushing) Table 3.9: Effect of leaf stage and treatment of longan trees with KClO3 (8 g/m?) as a soil drench on the size of inflorescences (HEGELE et al., 2004).
Leaf stage/treatment Young leaf Mature leaf KClO3 at young leaf KClO3 at mature leaf 0Length [cm] 0.0 13.8 b 17.4 b 23.0 a Width [cm] 0.0 20.1 b 26.3 b 33.5 a

Means in the same column followed by different letters significantly differed at p 0.05 Young leaf = 15 days after flushing Mature leaf = full green mature (45 days after flushing)

The experimental findings in Tables 3.8 and 3.9 confirmed the necessity of leaf maturation for a good response by the plant to KClO3. In many cases, farmers want to treat the plant at the time of shoot emergence, in order to obtain a good market price. Optimal crop management techniques should therefore allow the farmer to produce fruits all the year round and without interference from

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young shoots. Eradication of young leaves was found to be one way of enhancing the ability of longan trees to respond to KClO3. Young shoots could be eradicated by labor-intensive cutting. Alternatively, foliar application of Ethephon at a concentration of 300 mg/L could dry out the youngest shoots and retain a good response to KClO3 (Table 3.10).
Table 3.10: Percentage of flowering of longan trees treated with KClO3 at the young shoot stage, after shoot cutting or young shoot eradication with Ethephon.
Treatment 3 KClO3 at young shoot stage Shoot cutting + KClO3 Shoot eradication with Ethephon + KClO3 Significance 10.0 b 51.3 a 66.3 a + Percentage of flowering 4 13.8 b 57.3 a 67.6 a + 5 13.8 b 61.3 a 70.0 a + 6 13.8 b 66.3 a 80.0 a +

Means in the same column followed by different letters significantly differed at p 0.05

3.3.3.3 Environmental Conditions at the Time of Potassium Chlorate Application Farmers observed a poor response of longan trees to KClO3 in the rainy season, especially when the treatment was performed on rainy days. Researchers also found that shoots on the southern/south-western sides of trees flowered better. Concerning the season, plants responded better to KClO3 and produced more flowers in the hot and cool dry seasons than during the rainy season (Figure 3.5 in paragraph 5). Experiments with potted trees showed that this effect might be related to a leaching of KClO3 during heavy rains (MANOCHAI unpubl.). This general information confirmed the influence of environmental conditions on the reaction of trees to KClO3. Results of this research program demonstrated the enhancing effects of light intensity and dryness of the soil on the efficiency

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with which KClO3 promoted flowering. Temperature fluctuations in the range of 15 to 40 C had no significant effect on flowering.

3.3.4 Options for Optimised Mango Production
Paclobutrazol (PBZ) is a substance that inhibits gibberellin biosynthesis. Similarly to KClO3 in longan cultivation, it has proved to be suitable for inducing flowering in mango trees when applied as a soil drench (TONGUMPAI et al., 1989). In this project, the appropriate dose for local varieties in northern Thailand was investigated. For the mango cultivars `Nam Dokmai?, `Falan? and `Chok Anan?, 1.0 g a.i. / m of canopy diameter proved to be sufficient for successful flower induction, whereas in the case of cv. `Khiew Sawoei?, `Rad?, `Kaew? and `Nga? up to 1.5 g a.i. / m seemed to be necessary.2 When relating the PBZ dose to the ground area under the canopy as shown in Figure 3.4 for KClO3 applications to longan trees, corresponding PBZ dosages of 0.25 and 0.38 g a.i. / m2 resulted for trees with 5 m of canopy diameter in the first and the second cultivar group, respectively. Additionally strong pruning and spraying with thiourea and KNO3 were required to force and synchronise a new leaf flush, since the hardening stage of a flush was found to be the optimal time for PBZ application. The removal of normal season flower clusters by hand pinching, spraying of Ethephon or severe branch cut-back proved to be essential to delay normal or off-season flowering. In mango trees cv. `Kaew?, the flower promoting activity of PBZ applied to the soil was also compared with that of prohexadione-calcium applied as a spray. The latter is a less persistent and therefore a less critical inhibitor of gibberellin biosynthesis than PBZ. Both substances showed positive effects after 21 days of treatment3 and will be further tested on a larger scale, especially prohexadione-calcium whose effectiveness still needs to be improved.

2 SRUAMSIRI, P., MANOCHAI, P., to be published 3 NAPHROM, D. et al. to be published.

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3.3.5 Conclusion: Challenges in Sustainable and Economical Fruit Production in Mountainous Areas of Northern Thailand
During our investigation of flower induction in three fruit species in the region under study, it became evident that the techniques used must be specific for each species. Nevertheless, some common factors promoting flowering emerged. In mango, lychee and longan trees, a strong correlation between the inhibition of young leaves and the promotion of flowering could be observed. When young leaves were present, manipulations or chemical treatments, which normally promote flowering became either less or non-responsive as clearly demonstrated in Tables 3.6, 3.8, 3.9 and 3.10 for the examples of girdling in lychee and KClO3 application in longan production, respectively. In this research program, appropriate techniques for year-round longan production by use of KClO3 have been developed and disseminated (Figure 3.5). Similar developmental steps will also be monitored in lychee and mango cultivation. If these investigations are successful, regular, year-round bearing and high yields can be expected from these three fruit crops. This will facilitate sustainable and economical fruit production in the mountainous areas. To achieve this, more research and development is still required. New strategies for marketing, storage, drying etc. of off-season fruit have to be worked out. In principle, two major fields of knowledge and technology must be developed for successful marketing and export of fruit products.

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Flowering (%) 100 Flowering (%) 90 80 70 60 50 40 30 20 10 0 Aug. Sep. Oct.
rainy season

Days to flower

Nov.

Dec.

Mar.

Apr.

May

Days to flower 40 36 32 28 24 20 16 12 8 4 0 Jun. Jul.

cool season

hot season

rainy season

Month of the year

Figure 3.5: Effect of KClO3 on flowering of longan trees and days until flowering after KClO3 application by soil drench at different months all the year round (MANOCHAI et al., 2005).

Firstly, the products must meet international standards of food safety and be produced with low investment costs, using environmentally friendly horticultural practices and processing techniques. Secondly, management systems within the supply chain from handling raw materials, through processing, logistics, and marketing to delivery to the consumer must all be developed in a way that ensures a better understanding of the interactions between the single links in the chain so that the system as a whole will be better integrated. Both aspects require a bigger labor force, more time and more finance, but we are confident that these challenges can be met.


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