Foliar Application of the Exogenous Plant Hormones at ... - Springer Link

143

J. Crop Sci. Biotech. 2011 (June) 14 (2) : 143 ~ 150 DOI No. 10.1007/s12892-010-0070-3 RESEARCH ARTICLE

Foliar Application of the Exogenous Plant Hormones at PreBlooming Stage Improves Flowering and Fruiting in Cashew (Anacardium occidentale L.) Olawale Mashood Aliyu1*, Oluwayemisi Oluwatosin Adeigbe2, Joshua Adedokun Awopetu3 Apomixis Research Group, Department of Cytogenetics and Genome Analysis, Leibniz Institut fur Pflanzengenetik und Kulturpflanzenforschung (IPK), Corrensstrasse 3, D-06466 Gatersleben, Germany 2 Plant Breeding and Genetics Research Group, Cocoa Research Institute of Nigeria,P M B 5244, Ibadan, Nigeria 3 Department of Agronomy, University of Ilorin, P M B 1515, Ilorin, Nigeria 1

Received: July 22, 2010 / Revised: January 31, 2011 / Accepted: May 26, 2011 Ⓒ Korean Society of Crop Science and Springer 2011

Abstract The problem of declining tree yield has led to an investigation into the effectiveness of foliar application of exogenous hormones to improve flowering, fruit set, and fruit retention in cashew. Five exogenous hormones, one Gibberellic Acid (GA3) and four Auxins (IAA, IBA, NAA, and 2,4-D) at seven different rates of application (0 mg L-1, 10 mg L-1, 25 mg L-1, 50 mg L-1, 100 mg L-1, 250 mg L-1, and 500 mg L-1) were tested on six yield-related components of the two Brazilian cashew genotypes. This trial was a factorial split-split-plot design with each treatment replicated five times within a tree and three replications (three trees) per genotype. Responses varied significantly between exogenous hormones, concentrations and genotypes. The cashew plants used showed hormone-specific and optimum concentration response patterns. Of the five exogenous hormones tested, GA3 was most effective as its application at 50 - 100 mg L-1 gave five-fold improvements in flowering (precocity and number of hermaphrodite flowers) and fruiting, and about 69% increase in fruit retention ability and 25% in nut size. Panicles treated with GA3 also produced relatively bigger nuts compared to the untreated. Days to flowering was found to be hormone sensitive, while production of hermaphrodite flowers, fruit set, and nut development tended to be concentration specific. The GA3 exhibited a broad concentration tolerance among the five exogenous hormones investigated. Our data showed that using GA3 at 50 mg L-1 will enhance flowering precocity, shorten flowering duration, increase production of hermaphrodite flowers and fruit set significantly, and resultant nuts develop optimally with high percentage retention. Thus, it suggests cashew yield could be increased by exogenous foliar application of GA3 at 50 - 100 mg L-1 at preblooming stage. Abbreviations: GA3: Gibberellic acid, IAA: Indole-3-acetic acid, IBA: Indole-3-butyric acid, NAA: 1-naphthaleneacetic acid, 2,4D: 2,4-dichlorophenoxyacetic acid Key words: Anacardium occidentale L., breeding, cashew, exogenous hormones (GA3, IAA, IBA, NAA & 2,4-D), flowering, low fruit set, nut yield

Introduction Cashew nut is a high-value export crop for the Third World and main source of cash income for millions of rural households in the major producing countries in Africa and Asia (Topper et Dr. Olawale Mashood Aliyu ( ) E-mail:[email protected] or [email protected] Tel: +049 (0)3948 25673 or +234 (0)803-9548-344 Fax: +049 (0)3948 25137

The Korean Society of Crop Science

al. 2001). It was described as an emerging tree crop in the northern Australia with its imports valued at US$ 30 million per annum about a decade ago (Blaikie et al. 2002). Cashew nut is produced in about 32 countries with the world total production reaching about 3.7 million t in 2008. Based on the nut production statistics for the 2007 season, the leading producing countries were Vietnam (1,190,600 t), India (665,000 t), Nigeria (660,000 t), Brazil (147,629 t), and Indonesia (122,000 t) (FAO

144

Exogenous Hormones Improve Fruiting in Cashew

2008). Despite the importance of cashew as a commodity crop with increasing cultivation in major producing countries in South America, Africa, Asia and Australia, the world's average yield of cashew raw nuts is still as low as 780 kg ha-1 (FAO 2008) i.e. the crop is threatened by problem of low yield. In Africa, where about 67% of the world raw cashew nuts are produced, average output per tree is sometimes as low as 2 - 3 kg per tree (Aliyu and Awopetu 2007; Martins and Kasuga 1995; Ohler 1979). Chacko et al. (1990) has suggested that for a sustainable investment and competitiveness in Australian cashew industry, there must be a substantial increase in productivity of trees. Recently, studies on the genetic variability of cashew trees showed considerable variations in nut yield, ranging from 0 to 40 kg nuts per tree (Aliyu and Awopetu 2007; Martins and Kasuga 1995; Wunnachit and Sedgley 1992). From these studies, it was established that less than 20% of the population of trees within a hectare of cashew field produced 80% of the nut yield, while the remaining 20% yield come from the other 80% of the trees (Aliyu 2004; Martins and Kasuga 1995). Moreover, reports of low fruit set have been widely documented with initial fruit-set ranging between 3 and 40% but dropping to about 0.7 and 4.1% at fruit maturity (Ohler 1979). Rao (1956) reported 3% fruit set on the West Coast of India, and 6 - 12% fruit set on the East Coast (Murthy et al. 1975). Several factors have been attributed to the problem of low yield of cashew trees, ranging from poor genetic planting material (Chacko et al. 1990; Foltan and Ludders 1995), poor and irregular flowering to adverse environmental conditions (Parameswaran et al. 1984). There were reports of poor fruit set and excessive premature fruit in the crop (Patnaik et al. 1985). And trees with low yield were also characterized by poor production of hermaphrodite flowers (Aliyu 2006; Parameswaran et al. 1984). Other authors had emphasized nutritional deficiency (Ghosh 1989; Subbaiah 1983), inefficient pollination (Heard et al. 1990), and irregular and prolong flowering (Aliyu 2006; Aliyu and Awopetu 2003; Masawe et al. 1996) as among other factors limiting cashew tree yield. Unfortunately, crop improvement programs to address the problem of declining yield in most major producing countries are at infancy (Topper et al. 2001) and the widely recommended recurrent selection method ideal for improving tropical tree crops, like cashew, also requires a long gestation. In the light of the aforementioned, we considered foliar application of exogenous hormones as a short term strategy to improve on the flowering and fruit set in cashew. Over the years, application of exogenous hormones (i.e. plant growth regulators) has been reported to significantly improve flowering and fruiting in many crop plants. Foliar spraying of gibberellic acid increased fruit set and berry enlargement in seedless grape (Weaver 1958). The use of various plant growth regulators ranging from cytokinins to auxins had improved flowering and fruit set in citrus (El-Otmani 1992), coffee (Schuch et al. 1990), grape (Dokoozlian 2001; Ramming et al. 1995), and mango (Chen 1983; Davenport et al. 2001). Unfortunately, little or no data of such works existed in cashew despite the daunting production challenges confronting the growers. Although foliar

spraying of plant hormones may appear simple in practice, however, studies have shown that successful application to achieve the desire results is based on adequate knowledge of reproductive biology of the target crop, environmental factors, and time of application, which varies between crops and regions. To this end, we evaluated five exogenous hormones applied at seven different concentrations on two genotypes of cashew under mixed rainforest-derived savannah ecology in Nigeria. And the main goal of this study is to determine the effectiveness of the foliar application of exogenous hormones, 1 Gibberelic Acid (GA3) and 4 Auxins {Indole-3-acetic acid (IAA), Indole-3butyric acid (IBA), 1-naphthaleneacetic acid (NAA), 2,4dichlorophenoxyacetic acid (2,4-D)} to improve flowering (i.e. precocity, synchronized flowers sex phases, and increased production of hermaphrodite flowers per panicle), fruit set and fruit retention ability.

Materials and Methods This investigation was carried out at the cashew germplasm/research plot of the Cocoa Research Institute of Nigeria (CRIN), Ibadan (07°10' N 03°52' E) with average annual rainfall ranging between 1,100 mm and 1,500 m. Data on the monthly summary of the weather variables (rainfall, temperature and relative humidity) for the trial site were collected throughout the period of study, but not presented because it was relatively stable and within ranges ideal for growth and fruiting in cashew (Ohler 1979). Progenies of the two Brazilian genotypes (CSO06 and CSO08) planted in 1999/2000 were selected for their characteristic low fruit set based on our previous data (Aliyu and Awopetu 2007). These genotypes produced nuts with average individual nut weight of 9.5 g and three hermaphrodite flowers per panicle (Aliyu and Awopetu 2007). The field planting was a line layout of 5 trees per genotype per plot at plant spacing of 10 × 10 m, but three trees (replicates) per genotype were randomly selected for this investigation. No pesticide was applied to the field during the trial. The experimental layout was a factorial split-split plot design of three treatment factors of genotype (G), exogenous hormone (H) and concentrations of hormone (C) and the trial replicated thrice. Seven rates of application (0, 10, 25, 50, 100, 250, and 500 mg L-1) of five exogenous hormones, 1 Gibberellic acid (GA 3) and 4 auxins {Indole-3-acetic acid (IAA), Indole-3butyric acid (IBA), 1-naphthaleneacetic acid (NAA) and 2,4dichlorophenoxyacetic acid (2,4-D)} were tested on two Brazilian genotypes (CSO06 and CSO08). Each treatment was applied to five twigs at 2° flushing i.e. pre-blooming stage (Aliyu and Awopetu 2003; Ohler 1979) and replicated three times i.e. on three cashew trees per genotype. In total, 1,050 twigs (5 twigs/treatment/tree × 2 genotypes × 3 replications (tree/genotype) × 5 exogenous plant hormones × 7 rates of application i.e. concentrations) were examined. A solution of 10% alcohol and 0.05% Tween 20 was used a surfactant for dissolution of the exogenous hormone (Chalupka 1981). Treatments were applied by controlled foliar spraying of the

JCSB 2011 (June) 14 (2) : 143 ~ 150

hormonal solution to the newly flush twigs that would characteristically produce flowers in the current season. The spraying was carried out thrice with the first application mid-July and repeated at 2-week intervals (Dokoozlian 2001). The treated twigs were tagged and labeled accordingly and were allowed to cross pollinate naturally, e.g. by insect or wind. Data were collected on number of days to 50% flowering (DFF) i.e. when at least three out of five twigs selected per treatment/replication/genotype have commenced flowering, number of hermaphrodite flowers per panicle (HFP) i.e. collected for the whole flowering period, number of fruit set per panicle (FRS), fresh weight of individual (whole) fruit (g) (WTF), weight of individual nut (g) (NWT), and fruit retention ability (%) (FTA). Based on our previous data on cashew flowering in the study area (Aliyu 2004; Aliyu and Awopetu 2003), days to flowering was calculated from September 1 as day 1. Data collected were summarized and statistically analyzed using Genstat Discovery 3.0 version by VSN International Ltd, UK. Split-split plot ANOVA was performed on the data and the means were statistically separated with Duncan Multiple Range Test (DMRT).

Results Analysis of variance of the data presented in Table 1 showed significant difference (P < 0.01) between the exogenous hormones, concentrations and genotypes for flowering (DFF and HFP) and fruiting (FRS and FTA) characteristics. Only concentration was significantly different for fruit size (WTF and NTW) (Table 1). Apart from the significant genotype by concentration interaction (G × C) recorded for hermaphrodite flowers (HFP) and fruit set (FRS), only concentration by hormone interaction (C × H) was significantly different (P < 0.01) for the six yield related components. For clarity and brevity, we summarized the mean values for the five exogenous hormones, seven concentrations and two cashew genotypes in Table 2. The data showed higher variation (CV%) in flowering and fruiting traits compared to the fruit size (WTF and NWT) (Table 2). On the aver-

age, peak of flowering was about 11 weeks, with about 15 hermaphrodite flowers and six fruit set per panicle. About 36% of the fruit set (i.e. about two per panicle) were retained and fruit size of ~63 g and ~10 g for whole fruit and nut weight, respectively, were obtained.

Variation between the exogenous hormones The five exogenous hormones examined showed greater influence on the flowering (DFF and HFP) and fruiting (FRS and FTA) than fruit size (WTF and (NWT) (Table 2). Each hormone performed differently with gibberellic acid (GA3) significantly better in terms of improved flowering (DFF and HFP), fruit set (FRS), fruit retention (FTA), nut weight (NWT), and Indole-3-butyric acid (IBA) for whole fruit (WTF). On the average, GA3 was more effective than the auxins as twigs treated with GA3 flowered as early as 6 weeks and were more prolific than those treated with other exogenous hormones (Table 2). For example, GA3 demonstrated about 120% earliness in flowering (i.e. started flowering about 8 weeks earlier) compared to 2,4-D. This hormone (GA3) not only promotes flower precocity, but also enhanced the production of hermaphrodite flowers and fruit set by about 65 and 88%, respectively, when compared to other exogenous hormones like IAA (Table 2). This hormone (GA3) also showed superior performance in terms of fruit retention ability, for example, about 29% higher than IAA. Although analysis of variance showed that the performance of the five exogenous hormones was not statistically different for the fruit size (Table 1), however we noticed that twigs treated with IBA solution produced the largest fruit (63.96 g), and those treated with GA3 solution gave the biggest nuts (10.14 g) (Table 2).

Variation between the genotypes Although the number of biological replicates in this study was restricted to two for effective management of field data collection, mean values across the yield related traits for the two cashew genotypes deed showed that CSO08 was slightly superior in days to flowering (DFF) i.e. about a week earlier into flowering, number of hermaphrodite flowers production (HFP), fruit

Table 1. Summary of analysis of variance ANOVA (Mean squares) computed for the six yield related components of cashew from experimental study (five exogenous hormones tested at seven different concentrations on two genotypes) Source of variation H Error C CxH Error G GxH GxC GxCxH Error

Yield component characters

Degree of freedom

DFF

HFP

FRS

FTA (%)

WTF (g)

NWT (g)

4 8 6 24 60 1 4 6 24 70

16533.21** 29.89 6095.73** 1162.44** 40.53 2221.38** 48.67ns 64.25ns 31.34ns 41.45

516.08** 8.26 1361.01** 69.84** 5.61 534.40** 22.01ns 33.99* 7.00ns 12.43

109.99** 1.83 441.19** 17.94** 1.28 188.58** 1.04ns 7.32** 2.23ns 1.43

316.75** 62.21 3143.99** 149.03** 32.80 1251.18** 93.87ns 122.95ns 73.09ns 104.01

46.45ns 79.19 480.31** 85.56** 23.04 344.27ns 210.50ns 58.94ns 3.96ns 87.20

4.23ns 1.93 29.28** 3.06** 0.62 2.77ns 0.70ns 0.26ns 0.42ns 1.53

G: genotype, H: hormone, C: concentration. **: significant at 1% level, *: significant at 5% level, ns: not significant. Legend: DFF – days to flowering, HFP - hermaphrodite flowers per panicle, FRS – fruit set per panicle, FTA – fruit retention ability (%), WTF – weight of whole fruit (g), and NWT – nut weight (g).

145

146


Table 2. Mean values for the six yield-related components characters showing differences among the five exogenous hormones, seven concentrations and two cashew genotypes studied. Grand mean, range values, standard deviation, and coefficient of variation of the yield-related components were also summarized in the table Treatment factor

Yield and yield related components FRS FTA (%)

DFF

HFP

WTF (g)

NWT (g)

Hormones GA3 IAA IBA NAA 2,4-D

42.42d 82.80b 74.64c 83.02b 94.73a

21.07a 12.74c 13.09c 15.35b 13.14c

8.79a 4.67d 5.55bc 5.76b 5.02cd

41.244a 31.95d 37.65b 35.27bc 33.95c

63.61a 62.65a 63.96a 62.79a 61.24a

10.14a 9.90a 10.05a 9.95a 9.33a

Concentration (mg L-1) 0 10 25 50 100 250 500

90.17b 81.63c 67.37e 60.97f 56.97g 77.63d 93.96a

5.17e 13.00c 19.06b 28.77a 23.60a 11.67d 4.30e

3.03d 4.50c 8.37b 12.33a 8.50b 3.50d 1.50e

35.09c 35.10c 43.95b 51.23a 39.20bc 27.12d 20.41e

62.47c 62.29c 63.57bc 66.90a 65.63a 64.51ab 54.59d

9.58b 9.7b 10.23a 11.42a 10.38a 9.6b 8.17c

Genotype 1 CSO06 2 CSO08

78.78a 72.28b

13.49b 16.68a

5.00b 6.90a

33.57b 38.45a

64.13a 61.57a

9.76a 9.99a

Grand mean Range Standard Deviation Coefficient of variation (%)

75.53 15.00 - 126.00 25.87 34.25

15.08 0.00 - 48.00 8.24 54.64

36.01 0.00 - 71.40 14.27 39.63

62.85 0.00 - 82.50 8.64 13.75

5.95 0.00 - 25.00 4.38 73.61

9.87 0.00 - 14.35 1.46 14.79

NB: The data with the same letters in a column within the same treatment factor are not statistically different at P < 0.05. Legend: DFF – days to flowering, HFP- hermaphrodite flowers per panicle, FRS – fruit set per panicle, FTA – fruit retention ability (%), WTF – weight of whole fruit (g), and NWT – nut weight (g).

set (FRS), and fruit retention ability (FTA) than CSO06 (Table 2). For example, CSO08 was 24% more productive in hermaphrodite flowers than CSO06, and also greater by 38% in fruit set and 15% fruit retention ability (Table 2). However, the variation between the two narrowed with increasing concentration as CSO06 responded better at the higher concentrations (data not shown). This information could be useful for synchronizing flowering in cashew trees with erratic flowering habit. But there was no significant genotypic difference for fruit size (WTF and NWT) traits.

20% for untreated (0 mg L-1) and those treated with 500 mg L-1, respectively (Table 2). Although the concentration of exogenous hormone showed a minimal effect on fruit size (WTF, NWT) (Table 2), nonetheless, nuts harvested from twigs treated with 50 mg L-1 (11.42 g) were 19% larger than those from untreated twigs (9.58 g). On the average, the optimum concentration for foliar application of exogenous hormones was between 50 - 100 mg L-1. In order to identify suitable exogenous hormone(s) at appropriate dosage of application, we summarized the data for each of the five hormones across all the concentrations examined (C × H) for the six yield related components (Table 3).

Variation between the concentrations Similar to exogenous hormone, the effect of concentration of hormone was greater on flowering and fruiting characteristics than fruit size traits (Table 2). Cashew twigs treated with 500 mg L-1 concentration performed poorly than untreated twigs for the 6 yield related components. Twigs treated with 100 mg L-1 recorded peak flowering 4 weeks earlier than untreated (0 mg L1 ). But highest flowering (~29 flowers per panicle) and fruiting (12 fruits per panicle) were recorded from twigs treated with 50 mg L-1 (Table 2). Compared to untreated twigs, those treated with 50 mg L-1 were about 30 days precocious, and about 450 and 300% more prolific in hermaphrodite flowers and fruit production, respectively. Cashew twigs treated with the same concentration (50 mg L-1) retained 51% of the fruits till harvest time, as against 35 and

Performance of the hormones at different concentrations The data showed that flower precocity was somehow hormone-tolerance-specific for GA3 being the most responsive among the five exogenous hormones investigated. For example, significant improvement in days to flowering was recorded at concentration as low as 10 mg L-1 and high as 500 mg L-1 for GA3. This broad concentration tolerance in terms of days to flowering was only recorded for GA3 (Table 3). Highest flowering precocity as early as about 20 days was achieved with application of 100 mg L -1 of GA3 (Table 3). This is about 500% improvement in precocity compared to the untreated. Of all the five exogenous tested, 2,4-D was the least in terms of the response to flowering (precocity), with concentrations beyond

JCSB 2011 (June) 14 (2) : 143 ~ 150

Table 3. Mean values of the interactions between the five exogenous hormones and seven concentrations (C × H) recorded for the six yield-related components of cashew 0

10

25

Concentration (mg L-1) 50

100

250

500

DFF

GA3 IAA IBA NAA 2,4-D

90.05a 90.27a 90.17a 90.17a 90.00a

49.33d 95.67a 84.17bc 87.33b 90.67ab

38.83e 82.83a 78.00b 70.67c 66.67cd

28.17e 69.17b 59.00c 54.17cd 94.17a

19.67e 49.17c 47.17cd 75.67b 93.17a

30.00e 87.50c 65.00d 95.67b 110.93a

41.17d 105.17b 98.50c 107.17b 117.00a

HFP


5.45a 5.10a 5.20a 5.05a 4.95ab

13.50ab 11.17b 10.83c 14.83a 14.67a

23.67a 16.17c 14.00cd 20.33b 21.17ab

31.83a 22.50b 17.33c 23.67b 23.50b

33.17a 19.17c 24.67b 20.00c 16.00d

23.00a 7.33d 10.33bc 12.00b 5.67e

14.17a 4.67d 6.33c 8.50b 2.67e

FRS


3.00a 2.78ab 2.83a 2.82a 2.75ab

5.17a 3.67b 4.33ab 5.50a 3.83b

12.00a 6.33c 6.45c 8.83b 8.33b

19.00a 10.67b 9.67b 10.33b 8.67c

9.83ab 6.83bc 10.33a 7.33b 5.00d

8.17a 1.50c 3.17b 3.17b 1.35c

2.33a 0.83d 1.17c 2.33b 0.83d

FTA (%)


35.25a 34.09a 35.10a 33.60a 32.05b

37.25b 32.92c 40.06a 37.75b 27.50d

51.78a 39.89c 44.87b 43.32b 45.85b

59.49a 47.70b 56.05a 44.11c 39.86d

31.25c 35.23b 42.79ab 45.76a 37.12b

36.15a 23.47c 31.25b 26.67c 19.72d

27.39a 17.85b 17.94b 28.98a 12.67c

WTF (g)


61.67ab 63.00a 63.17a 62.33a 62.00a

62.29a 61.54a 60.67ab 62.88a 63.46a

63.49a 63.01a 62.83a 63.67a 64.50a

66.50b 65.45b 66.33b 65.94b 68.55a

65.51b 64.09b 74.21a 63.17b 66.68b

65.74a 63.65a 65.83a 62.64ab 64.26a

58.86a 57.87ab 60.42a 57.45ab 55.27b

NWT (g)


9.50a 9.61a 9.58a 9.35a 9.85a

9.60a 9.58a 9.81a 9.62a 9.62a

9.70a 10.28a 10.48a 10.28a 10.19a

11.91a 11.43a 10.87ab 11.05a 10.41ab

9.99ab 10.57a 11.32a 10.25a 9.80ab

9.76a 9.31ab 10.08a 9.53a 9.30ab

9.45a 8.58a 8.27a 9.01a 8.38a

Character+

Hormone++

NB: The data with the same letters in a column within the same treatment factor are not statistically different at P < 0.05. Figures in bold indicate optimum performance for each trait. + Character: DFF – days to flowering, HFP – hermaphrodite flowers per panicle, FRS – fruit set per panicle, FTA – fruit retention ability (%), WTF – weight of whole fruit (g), and NWT – nut weight (g). ++ Hormone: GA3: Gibberellic acid, IAA: Indole-3-acetic acid, IBA: Indole-3- butyric acid, NAA: 1-naphthaleneacetic acid, 2,4-D: 2,4-dichlorophenoxyacetic acid.

250 mg L-1 delayed flowering up to about 16th week (Table 3). Highest flowers production (33 per panicle) was obtained from twigs treated with 100 mg L -1 of GA 3 and other hormones attained peak of flowers production between 50 mg L-1 and 100 mg L-1 concentrations (see Table 3). There was no significant difference between hermaphrodite flowers production between twigs treated with 50 mg L-1 and 100 mg L-1 of GA3 (see Table 3). This suggests that GA3 solution around 50 - 100 mg L-1 of concentrations will give optimal flower production. The improvement in flowers production (about 500%) obtained with 100 mg L -1 GA 3 was quite significant when compared with untreated (0 mg L-1). Effect of higher dosage was recorded for IAA and 2,4-D, with significant reduction in flowering at concentrations higher than 100 mg L-1. Peak of fruit set was generally recorded at 50 mg L-1 except for IBA that gave highest fruit set at 100 mg L-1. Highest fruit set (19 per panicle) recorded from twigs treated with 50 mg L-1 GA3

was almost twice the yield obtained from other hormones at the same 50 mg L-1 concentration and about 500% higher than the untreated (0 mg L-1). Considerable improvement in fruit retention ability was recorded from twigs treated with 50 mg L-1 of GA3 (59%) and 50 mg L-1 IBA (56%) (Table 3). Although harvested fruits seem to be relatively uniform in size across all the treatments, however, largest nuts (11.91 g) was recorded from twigs treated with 50 mg L-1 GA3, which was 25% higher than those harvested from untreated twigs (9.50 g) under GA3 treatment plot. The same concentration (50 mg L-1) gave the best nut size for IAA, 2,4-D and NAA, except for IBA that produced largest nut and fruit at 100 mg L-1 concentration (Table 3). Concentrations in the range of 25 to 100 mg L-1 appear to promote optimal fruit development, as nuts harvested from twigs treated with these concentrations were comparably bigger than those harvested from untreated twigs (0 mg L-1). Overall, improvement due to GA3 applied at 50 mg L-1 increases flowers production

147

148


(500%), fruit set (500%), fruit retention (68%), and nut size (25%) when compared to untreated twigs under GA3 (Table 3).

Discussion Generally, the level of responses recorded between treated and untreated twigs in this study demonstrated spectrum of improvement in flowering, fruiting and total yield that can be achieved through the use of exogenous hormones in cashew nut production. Our data consistently showed that application of exogenous hormones can improve fruiting/yield significantly through the flowering components (DFF and HFP). Kurian and Iyer (1993) reported similar yield increase through intense flowering in mango trees sprayed with cytokinin-based plant growth regulators. The significant reduction in the period of flowering will considerably enhance flowering synchronization in cashew. These two events couples with the increased number of hermaphrodite flowers were likely responsible for higher fruit set through improved pollination. Indeed, there was correlation (Pearson correlation r = 0.86, table not shown) between number of hermaphrodite flowers (HFP) and number of fruit set per panicle (FRS). Moreover, exogenous hormones such as IAA, GA, and NAA had been extensively reported to enhance flowering and pollination in various crops, including perennial trees. Chen (1985) reported precocious flowering with application of 6-benzylaminopurine in mango, with treated plants flowering at 4 weeks after application and untreated flowering 12 weeks later. In this study, we recorded flowering precocity as early as 2 - 3 weeks (Tables 2 and 3), which is quite unprecedented. Reduction and synchronization of the period of flowering and fruiting will afford treated cashew trees the opportunity to produce regularly with a short harvest period. Such trees will also have the advantage of the long period of vegetative recuperation before the next production season. The considerable improvement in flowering and fruit set recorded in this study further support the notion that factors controlling yield in cashew are complex and very little knowledge are available on this aspect. Improvement as high as about 500% in both flowering and fruiting improvement, due to application of exogenous hormone, precisely GA3, suggests that physiological factors such as floral bud dormancy, imbalance endogenous hormones and differential level of abscisic acid (Davenport 2003, 2009) may be critical to flowering and pollination of the cashew tree. Thus, requisite understanding of these multifaceted factors relative to specific ecology and soil nutrient dynamics will be useful in tackling the problem of poor and declining yield in cashew. As shown from our data, there was variation in the performance of each of the hormones both for flowering and fruiting characters. Out of two categories of growth regulators investigated, cashew showed a preference for gibberellins over auxins. Similar improvement in flowering and fruit set with the use of GA3 was reported in tangelos and tangerine citrus cultivars in Florida (Wright 2004 unpublished report). There have been contrasting reports on the role of GA3 to flowering and fruiting in

crop plant. Dokoozlian (2001) opined that GA3-7 would only promote flowering and reduced fruit set. While GA3 was widely reported to delay flowering especially in mango, Kachru et al. (1972); Nunez-Elisea and Davenport (1998) have shown that such effect is dependent upon concentration and prevailing environmental conditions at the time of application. They concluded that lower concentrations applied at long cool temperatures would lead to flower ignition and development. It thus implies that, in as much as the effect of GA3 could be beneficial in terms of promoting flowering as obtained in this study, wrong application, i.e. higher concentration at the wrong time, especially during high temperature > 33°C day / 27°C night (Davenport 2003) can cause inhibition of floral bud initiation and delay and/or irregular flowering and finally, poor fruit set. In this investigation, we observed broad dosage tolerance for GA3 compared to other exogenous hormones. These two factors (dosage and time of application) should be taken into consideration in the adoption of this technology by cashew farmers. Here, we do not only report improvement in flowering, but also an increased fruit set and slight nut size increase. In essence, it is probable that poor yield of cashew, especially in the tropical sub-Sahara region could have been partly due to delay and non-synchronization (erratic) of flowering that often led to late and sparsely fruit set. With significant reduction in days to flowering i.e. with possible full bloom attained as early as late October to early November (under West African conditions) through GA3 treatment, there will be sufficient moisture in the soil to support higher sinks and efficient assimilate transportation during fruit development (Aliyu and Hammed 2000; Gawankar et al. 2010). Unfortunately, most of the cashew farms in this sub-region, attained peak blooming at the peak of dry season (February to March), when average monthly temperature usually ranged between 36 and 40°C and water stress is at the peak. Such high temperature and excessive drought during flowering and fruiting will not only increase fruit drop (abortion), but result in fruits with wide variation in size with shrunken nuts due to incomplete kernel development. In addition to increased flowering and fruit set due to application of GA 3, significant improvement in fruit retention was recorded. Imbalance in the endogenous hormone is known to aid embryo/seed abortion and low fruit set (Nunez-Elisea and Davenport 1983; Ram et al. 1976). Application of exogenous hormone might in a way enhance balancing of endogenous hormones. Despite the improvement in fruit retention ability of the treated twigs, it was surprising that none of the twigs (treated or untreated) successfully retained 100% of the fruit set. The highest was about 60% from twigs treated with 50 mg L-1 of GA3 (Table 3). This observation corroborated previous studies on the complexity of factors controlling yield and the likely role of nutrition to fruit development and retention in cashew (Aliyu 2008). It is becoming clearer that the upper limit regulatory mechanism and compensatory effect of the number of fruits vs. weight of fruit per tree will continue to play a significant role in cashew tree productivity (Aliyu et al. 2011 submitted). Furthermore, GA 3 is known to enhance photosynthate production through an efficient use of nitrogen "N" (Khan et al. 2004).

JCSB 2011 (June) 14 (2) : 143 ~ 150

The remarkable response recorded in cashew twigs treated with GA3 over other exogenous hormones suggests hormonespecific nature of cashew tree (Davenport 2003). Meanwhile, genotypic response for example in fruit and nut development was concentration sensitive. Similar, genotype/cultivar specific response had been widely reported in citrus (Roitsch and Ehneb 2000), coffee (Schuch et al. 1990), and mango (Davenport 2009). Such genotypic variation has been attributed to differential rate of photoassimilation, breeding history of the cultivars (Chacko et al. 1995; Searle et al. 1995) and level of abscisic acid (Roitsch and Ehneb 2000). Reduction in the variation between the two cashew genotypes with increased concentration of exogenous hormone could imply a differential inhibitory level caused by abscisic acid, with each genotype requiring different dosage to overcome dormancy effect. In his tri-factor hypothesis of flowering in mango, Kulkarni (2004) opined that level of the floral stimulus determines the level of response and ratio of cytokinin to auxin is critical to bud break and floral initiation (Nordstrom et al. 2004). Twigs treated with 500 mg L-1 consistently performed lower than untreated (0 mg L-1) in flowering and fruiting traits, suggesting a negative effect of higher concentration of the exogenous hormone on the floral initiation and development in cashew. Here, we observed that 100 mg L-1 was the ideal dosage for promoting flowering and 50 mg L-1 would increase fruit set. It seems that efficient pollination in cashew could only be achieved with treatment at lower concentrations < 50 mg L-1. It is probable that gibberellins may be playing important role in the germination of pollens during pollination and fertilization, while auxins are essential for seed development. Auxins are well known for promoting growth through elongation of initials and activation of cells. In our opinion, these are stages (pollination and fertilization) where the effect of higher concentration of exogenous hormones can significantly impede the reproductive process. While we found only gibberellin, GA3 as the most beneficial exogenous hormone for improving flowering and fruiting, the performances of auxins were comparatively low and less suitable for use in cashew. Perhaps, it would be better to combine GA3 with any of the auxins, as the synergetic effect between auxins and other plant growth regulators has been reported (Fujioka et al. 1983). Climatic variables are also known to play a significant role in the growth and production of cashew trees. There is little data on the effect of day length on cashew, but equatorial behavior, i.e. equal day and night lengths have been found most favorable to flowering. A sub-tropical climate with about 4 to 6 dry months and well-spread rainfall, ranging between 1,000 and 2,000 mm with monthly average temperature of 27°C will be suitable for optimum flowering and commercial cashew cultivation. These climatic variables must be factored into programs targeting yield improvement in cashew. Here, we have demonstrated how application of Gibberellic acid (GA3) applied at the rate of 50 mg L-1 improved flower precocity by 500%, increased the numbers of hermaphrodite flowers by 500%, fruit set by 500%, fruit retention by 69% and nut size by 25%, which would significantly improve output/tree in

cashew. Further research with more genotypes, years and locations are needed to validate this preliminary result.

Acknowledgments The authors thank Messr. Victor Enagu of the Plant Breeding Group, Cocoa Research Institute of Nigeria, Ibadan, Nigeria, for his support on field data collection. We also acknowledged the anonymous reviewers of this paper for their useful comments.

References Aliyu OM. 2004. Characterization and compatibility studies in Cashew (Anacardium occidentale L). Ph.D Thesis, University of Ilorin, Nigeria. pp 266 Aliyu OM. 2006. Phenotypic correlation and path coefficient analysis of nut yield and yield related components in cashew. Silvae Genet.55: 19-24 Aliyu OM. 2008. Compatibility and fruit-set in cashew (Anacardium occidentale L.). Euphytica 160: 25-33 Aliyu OM, Hammed LA. 2000. A study of nut and apple devel opment in cashew (Anacardium occidentale L.). Nigerian J. Tree Crop Res. 4: 1-10 Aliyu OM, Awopetu JA. 2003. Studies on flowering pattern in cashew (Anacardium occidentale L.). Nigerian J. Genet.18: 29-35 Aliyu OM, Awopetu JA. 2007. Multivariate analyses of cashew (Anacardium occidentale L.) germplasm in Nigeria. Silvae Genet. 56: 170-179 Blaikie S, O'Farrell P, Muller W, Wei X, Scott N, Skyes S, Chacko EK. 2002. Assessment and selection of new cashew hybrids. A Report for the Rural Industries Research and Development Corporation (RIRDC), Australia, RIRDC Publication No. 01/177, pp 21 Chacko EK, Baker I, Downton JWS. 1990. Towards a sustainable cashew industry for Australia. J. Aust. Inst. Agric. Sci. 3: 39-43 Chacko EK, Lu P, Kohli RR. 1995. Photoassimilate production and distribution in relation to productivity in mango (Mangifera indica L.). In: Mango 2000 - Marketing Seminar and Production Workshop Proceedings. Department of Primary Industries, Brisbane, pp. 109-124 Chalupka W. 1981. Influence of growth regulators and poly thene covers on flowering of Scots pine and Norway spruce grafts. Silvae Genet. 30: 142-146 Chen WS. 1983. Cytokinins of the developing mango fruit. Plant Physiol. 71: 356-362 Chen WS. 1985. Flowering induction in mango (Mangifera indica L.) with plant growth substances. Proceedings National Science Council Part B, Life Sciences (Republic of China) 5: 49-55 Davenport TL. 2003. Management of flowering in three tropical and subtropical fruit tree species. HortScience 38: 1331-1335

149

150


Davenport TL. 2009. Reproductive physiology, In RE Litz, eds., The Mango: Botany, Production and uses, CABI, Oxford, pp 97-169 Davenport TL, Pearce DW, Rood SB. 2001. Correlation of endogenous gibberellic acid with initiation of mango shoot growth. J. Plant Growth Regul. 20: 308-315 Dokoozlian NK. 2001. Gibberellic acid applied at bloom reduces fruit set and improves size of 'Crimson Seedless' table grapes. Hortscience 364: 706-709 El-Otmani M. 1992. Principal growth regulator uses in citrus production. Proc.2nd Seminar on Citrus Physiology, Bebedouro Ed. Cargill. pp. 55-69 FAO. 2008. Food and Agriculture Organization of the United Nations, Crop Production Statistics Division. http:// faostat.fao.org/site/567/DesktopDefault.aspx?PageID=567. website last visited 19th June 2010 Foltan H, Ludders P. 1995. Flowering, fruit set and genotype compatibility in cashew. Ange. Bot. 69: 215-220 Fujioka S, Yamaguchi I, Murofushi N,Takahashi N, Kaihara S, Takimoto A. 1983. The role of plant hormones and benzoic acid in flowering of Lemna aucicostata 151 and 381. Plant Cell Physiol. 24: 241-246 Gawankar MS, Sawale RD, Pawar SN, Chavan SA. 2010. Effect of Ethrel Ⓡ on flowering, sex-expression and yield in cashew. J. Hort. Sci. 51: 68-70 Ghosh SN. 1989. Effect of nitrogen, phosphorus and potassium on flowering duration, yield and shelling percentage of cashew (Anacardium occidentale L.). Indian Cashew J. 19: 19-23 Heard TA, Vithanage V. and Chacko EK. (1990). Pollination biology of cashew in the Northern Territory of Australia. Aust. J. Agric. Res.41: 1101-1114 Kachru RB, Singh RN, Chacko EK. 1972. Inhibition of flowering in Mangifera indica L. by gibberellic acid. Acta Hort. 24: 206-209 Khan NA, Mobin M, Samiullah M. 2004. The influence of gibberellic acid and sulfur fertilization rate on growth and S-use efficiency of mustard (Brassica juncea). Plant Soil 270: 269-274 Kulkarni VJ. 2004. The tri-factor hypothesis of flowering in mango. Acta Hort. 645: 61-70 Kurian RM, Iyer CPA. 1993. Chemical regulation of tree size in mango (Mangifera indica L.) cv. Alphonso. III. Effects of growth retardants on yield and quality of fruits. J. Hort. Sci. 68: 361-364 Martins PJ, Kasuga LJ. 1995. Variation in cashew tree yield in Southeast Tanzania and the implication for management of cashew smallholder farmers in Tanzania. Exp. Agric. 72: 261-268 Masawe PAL, Cundall EP, Caligari PDS. 1996. Distribution of cashew flower sex-types between clones and sides of tree canopies in Tanzania. Ann. Bot. 78: 553-55. Murthy KN, Yadava RBR, Bigger J. 1975. Increasing fruit set in cashew by hormone spraying. J. Plantation Crops (India), 3: 81-82 Nordstrom A, Tarkowski P, Tarkowska D, Norbaek R, Astot C,

Dolezal K, Sandberg G. 2004. Auxin regulation of cytokinin biosynthesis in Arabidopsis thaliana: a factor of potential importance for auxin-cytokinin-regulated development. Proc. Natl. Acad. Sci. USA 101: 8039-8044. Nunez-Elisea R, Davenport TL. 1983. Abscission and ethylene production of mango (Mangifera indica L.) fruit cv. Tommy Atkin. Proc. Florida State Hort. Soc. 96: 185-188 Nunez-Elisea R, Davenport TL. 1998. Gibberellin and temperature effects on dormancy release and shoot morphogenesis of mango (Mangifera indica L.). Sci. Hort. 77: 11-21 Ohler JG. 1979. Cashew. Koninklijk instituut voor de Tropen. Teskin, Zutphen Co. Amsterdam, The Netherlands Parameswaran NK, Daramodaran VK, Prabhakaran PV. 1984. Factors influencing yield in cashew (Anacardium occidentale L). Indian Cashew J. 16: 15-19 Patnaik HP, Das MS, Panda JM. 1985. Studies on fruit set and fruit drop in cashew (Anacardium occidentale L.) under Orissa conditions. Cashew Causerie 7: 7-8 Ram SL, Bist D, Lakhanpal SC, Jamwal IS. 1976. Search of suitable pollinizers for mango cultivars. Acta Hort. 57: 253263 Ramming DW, Tarailo R, Badr SA. 1995. 'Crimson Seedless': A new late maturing, red seedless table grape. Hortscience 30: 1473-1474 Rao VNM. 1956. Multiply the better yielding cashew nut. Indian Farming (India). 6: 19-23 Roitsch T, Ehneb R. 2000. Regulation of source/sink relations by cytokinins. Plant Growth Regu. 32: 359-367 Schuch UK, Fuchigami HL, Nagao MA. 1990. Gibberellic acid causes earlier flowering and synchronizes fruit ripening of coffee. Plant Growth Regul. 9: 59-64 Searle C, Whiley AW, Simpson DR, Saranah JB. 1995. A pre liminary phenol-physiological model for 'Kensington' mango in subtropical Australia. In Mango 2000-Marketing Seminar and Production Workshop Proceedings. Department of Primary Industries, Brisbane, pp. 109-124 Subbaiah CC. 1983. Fruiting and abscission patterns in cashew. J. Agric. Sci. (Cambridge) 100: 423-427 Topper CP, Caligari PDS, Camara M, Diaora S, Djaha A, Coulibay F, Asante AK, Boamah A, Ayodele EA, Adebola PO. 2001. Report of the West African regional cashew survey covering Guinea, Guinea Bissau, Cote d'Ivore, Ghana and Nigeria. Sustainable Tree Crop Programme (STCP) Report No. BHA-01109. Biohybrids Agrisystem Ltd. P. O. Box 2411, Earley, Reading RG6 5FY, UK Weaver RJ. 1958. Effect of gibberellic acid on fruit set and berry enlargement in seedless grapes of Vitis vinifera. Nature 181: 851-852 Wright G. 2004. Plant growth regulators used in Citrus production. http://ag.arizona.edu/crops/presentations/2004/wrightpgrho051204.pdf An unpublished presentation to the University of Arizona, USA. Website last visited 19th June 2010 Wunnachit W, Sedgley M. 1992. Floral structure and Phenology of cashew in relation to yield. J. Hort. Sci. 67: 769-777