Applying Paclobutrazol and Flower Bud Pruning Modify the Fruiting Time and Fruit Quality of ‘Amrapali’ Mango (Mangifera indica L.)

Abstract

Postharvest loss of mango often occurs due to the short harvesting period. An experiment was conducted to determine the impact of paclobutrazol (PBZ) and flower bud pruning (FBP) on regulating flowering, fruiting time and fruit quality of ‘Amrapali’ mango. Different doses of PBZ at 0.5, 1.0, 1.5, 2.0 g per meter canopy diameter and water application along with FBP or without FBP were used, as well as water application without FBP (control). The application of PBZ caused earlier flowering by 22 days and harvesting was also done earlier by 18 days compared to the control. Plants subjected to FBP with PBZ reflowered 36 days later and harvesting was delayed by 16 days compared to the control. Moreover, the combination of PBZ 1.5 g with FBP showed significantly higher flowering percentages, number of panicles, total flowers, total fruits and weight of fruit compared to the control. In addition, the application of PBZ 1.5 g with FBP increased the total soluble solids, reducing sugar, non-reducing sugar, total sugar and β-carotene, while it decreased the vitamin C content. The present findings imply that applying PBZ 1.5 g with FBP to mango can extend the flowering and fruiting time, while the fruit quality was also influenced positively.

Introduction

In the tropics and subtropics, the vegetative growth of mango is year-round, but flowering and fruit setting are generally seasonal. The initiation of flowers occurs during late winter to early spring due to the dependency of flowering on the environment. The mango trees produce fruits only once a year, with the harvest supply between mid-May and mid-August. As a result, there is often an oversupply during harvest season, leading to low prices in the market. Also, due to the highly perishable nature of mango, postharvest mango losses are very significant at around 35% during the main harvest season (Alam et al., 2019). The short availability of fruits in the market has become a major limitation to develop the mango industry. One strategy to solve this problem is the manipulation of flowering time to extend the fruiting season of mango.

The gibberellin content in shoot tips decreased before panicle emergence in mango trees that subsequently flowered (Tongumpai et al., 1997). Gibberellin inhibitors such as paclobutrazol (PBZ) have been successfully applied to reduce vegetative growth and stimulate flowers of several fruit crops including mango (Kumar et al., 2020). Paclobutrazol inhibits gibberellin biosynthesis by inactivating the enzyme ent-kaurene oxidase to ent-kaurenoic acid, which retards plant growth and promotes flowering. Some investigators have reported that PBZ is extremely efficient for flower induction and promotes the early flowering of different mango cultivars (Reddy et al., 2014; Sarker and Rahim, 2018; Upreti et al., 2013). PBZ increases the number of mango fruits set and yields, as well as the quality of fruit (Barman and Mishra, 2018; Sarker et al., 2016). Seasonality problems could also be reduced by delaying flowering. Panicle pruning after blooming can induce secondary inflorescence, which may delay harvest. A recent study showed that de-blossoming of the inflorescence during full bloom could induce reflowering from axillary buds in mango (Chaudhary et al., 2020) and loquat (Peng et al., 2022).

Prolonged flowering may play a vital role to solve the seasonality problem of mango. Usually, paclobutrazol is used to produce early flowering of mango. However, there is no information available on the impact of paclobutrazol and flowering bud pruning on mango production. Therefore, our study aimed to evaluate the impact of different PBZ doses and FBP on the regulation of flowering, harvesting time and the quality of mango fruits.

Materials and Methods

Experimental conditions and plant materials

The experiment was conducted at the horticulture research farm, Hajee Mohammad Danesh Science and Technology University (HSTU), Dinajpur (25.13° N latitude, 88.23° E longitude, and 34.5 m altitude) from July 2020 to August 2021. The average temperature and rainfall level are shown in Figure S1. A uniformly grown 7-year old grafted mango cultivar, ‘Amrapali’ (locally called BARI Mango-3) was used in this study. The cultivar was developed as a hybrid variety of ‘Dashehari’ and ‘Neelum’. It is a medium dwarf, regular and prolific at bearing fruit and with a good fruit quality (Singh, 1996). Plant spacing was 5 m × 5 m. Flowering normally takes place in late January to early February, whereas harvesting of fruit starts in early June.

Treatment and design

A two-factor experiment was conducted in a randomized complete block design with three replications, each consisting of three plants. The first factor was different PBZ doses (0.5, 1.0, 1.5, 2.0 g per meter canopy diameter) and the second one was with FBP. The plants without PBZ or FBP were used as the control.

Cultar^® (Syngenta Chem. Co. Ltd., India), a commercial product containing 25% PBZ, was diluted in one liter of water to make the desired treatment and applied to soil on November 1, 2020. The PBZ solution was applied to the soil around the collar region of the plants. On the other hand, in the control treatment, only water was applied. The newly emerged panicles from PBZ-treated plants were pruned on 4 January, whereas the control plant panicles were pruned on 4^th February. For pruning practice, 20 panicle tips on each plant were selected from the middle portion of the crown. After 80 days of PBZ application, 4 L of 2% calcium nitrate solution was applied to every plant, including controls, to induce flowering according to Oliveira et al. (2017). Calcium nitrate was applied three times on 21 January, 1 February, and 10 February 2021, respectively.

Flower and fruit characteristics measurements

The date of the first panicle emergence was counted at the first panicle appearance at the apex of the branch. Flowering characteristics were measured at the full bloom stage when more than 50% of panicle flowers were open (Delgado et al., 2011). The number of panicles per branch was counted as 10 branches per plant. The flowering percentage in the plant was calculated by the ratio of the number of branches with panicles and the total number of branches on the plant (Olievera et al., 2017). The numbers of males, hermaphrodites and total flowers were counted using a magnifying glass and 10 panicles per plant. The harvest time of fruit was determined by when one or two ripe fruits dropped from the tree and all the fruit were harvested at one time (Sarker et al., 2016). The number of fruits per branch was counted at the time of harvesting. Using an electronic balance, the weight of fruit, pulp and the stone were measured separately. The length and diameter of fruit were measured by digital vernier calipers.

Fruit quality measurement

Ten randomly selected fruits from each treatment after harvest were allowed to ripen at room temperature and fruit quality was determined. Using a refractometer (HI96801; Hanna Instruments, Woonsocket, RI, USA) total soluble solids of mango pulp were measured. Vitamin C was measured by the 2,6-dichlorophenol-indophenol dye method according to Ranganna et al. (1997). Reducing sugar was determined by Lane and Eynon (1923), as described by Ranganna et al. (1997). The total sugar was determined following the procedure of Dubois et al. (1956). Non-reducing sugar was determined by subtracting reducing sugar from total sugars. The β-carotene in mango pulp was measured according to the procedure of Nagata and Yamashita (1992).

Statistical analysis

All the measured data were analyzed appropriately using the ANOVA technique and Tukey’s test adjusted mean differences using a computer-operated program named, Statistix v. 10 (Analytical Software, Tallahassee, FL, USA).

Results

Flowering time and flower characteristics

The application of different doses of Paclobutrazol (PBZ) without Flower bud Pruning (FBP) in plants produced flowers 22 days earlier, whereas in plants treated with FBP and PBZ reflowered 36 days later compared to the control (Table 1).

Table 1

Effects of paclobutrazol and flower bud pruning on flower characteristics of ‘Amrapali’ mango.

All flower-attributing parameters were affected significantly by PBZ and FBP individually or in combination with both (Table 1). The hermaphrodite flowers per panicle showed no significant variations due to FBP. The number of panicles per branch markedly increased with the application of PBZ 1.5 g with FBP compared to the control. The flowering percentage was increased with the higher PBZ dose of 1.5 g giving the best response (Fig. S2A). Moreover, FBP also increased the flowering percentage (Fig. S2B) compared to non-pruned plants. A combination of PBZ and FBP improved flowering percentage of mango plants and the highest was observed with PBZ 1.5 g on FBP-treated plants which was 1.75-fold the control number and 1.96-fold that of the FBP-only treated plants (Table 1). PBZ 1.5 g and FBP also increased the number of males, hermaphrodites and total flowers per panicle by 66%, 97% and 67%, respectively, compared to the control.

Date of harvest and fruit characteristics

The result showed that the harvest occurred 18 days earlier in plants treated with PBZ, but the harvesting was delayed by 16 days in plants treated with PBZ and FBP, compared to the control (Table 2).

Table 2

Effects of paclobutrazol and flower bud pruning on fruit characteristics of ‘Amrapali’ mango.

The effects of PBZ and the combination of PBZ and FBP were significant on fruit characteristic-related parameters (Table 2). The length, diameter and weight of the fruit, and pulp weight and stone pulp ratio were found to be non-significant due to the FBP effect. PBZ 1.5 g promoted a substantial increase in fruits per branch by 58% compared to untreated controls (Fig. S3A). Further, the application of FBP enhanced the number of fruits per branch (Fig. S3B), while the combined PBZ 1.5 g and FBP treatment caused a significant 85% rise in fruits per branch compared to the untreated ones (Table 2). The combination of PBZ 1.5 g + FBP treatment produced the longest fruit, maximum weight of fruit and pulp and lowest stone/pulp ratio, although the PBZ 1.5 g without FBP treatment produced fruit with the widest diameter.

Qualitative characteristics

Qualitative fruit characteristics were affected significantly by using only PBZ or combined PBZ and FBP (Table 3). There were no significant differences in all qualitative characteristics of fruit treated with FBP individually. The combined application of PBZ 1.5 g and FBP increased total soluble solids, reducing sugar, non-reducing sugar, total sugar and β-carotene of fruit by 41%, 21%, 18%, 20% and 45% respectively, compared to the control (Table 3). Conversely, vitamin C content was significantly decreased in the PBZ 1.5 g treatment without FBP. Vitamin C of fruit decreased by 5% compared to the control when PBZ 1.5 g with FBP was applied, although there was no significance difference among treated plants.

Table 3

Effects of paclobutrazol and flower bud pruning on chemical characteristics of ‘Amrapali’ mango fruit.

Discussion

Extending the duration of mango production is needed to reduce huge postharvest losses. The role of applying different doses of PBZ with or without FBP to extend the fruiting time was studied in the current study (Tables 1, 2 and 3; Figs. S2 and S3).

Results indicated that PBZ advanced flowering and resulted in higher panicle production compared to the control (Table 1). Early flowering may be induced by PBZ as it changes various physiological actions of mango trees. Some studies have reported that PBZ reduced vegetative growth levels and thereby increased root activity and the C:N ratio, stimulating early flowering (Sarker and Rahim, 2012; Upreti et al., 2013). In addition, the induction of early flowering in PBZ-treated mango shoot apices was found to be due to an increase in ABA and cytokinin, as well as a reduction in gibberellins in buds (Upreti et al., 2013). Also, the use of PBZ may restrict cell expansion and accumulation of phenolic compounds at the shoot tip by inhibiting the final reactions of gibberellin synthesis (Oliveira et al., 2020). Previously, Sarker and Rahim (2012) found that the application of PBZ to ‘Amrapali’ mango grown in Bangladesh showed the earliest panicle emergence, by 19 days, but to our knowledge there have been no studies on the role of PBZ and FBP on the regulation of flowering. Our results revealed that FBP with PBZ delayed the re-flowering time because the pruning of apical flower buds induced axillary buds and these buds usually develop as lateral inflorescences. This effect arises because the removal of the inflorescence bud cancels the suppressive factors controlling the growth of the sub-terminal bud whorl (Yeshitela et al., 2003). The increased percentages of flowered branches for PBZ-treated trees may be due to lower expenditure of tree reserves to the vegetative growth parameters and consequently no assimilate limitations. Moreover, PBZ-treated plants produced the maximum number of panicles (Table 1) and flowering percentages due to minimum expenditure of food reserves for vegetative growth and consequently no assimilate limitations (Yeshitela et al., 2004).

It was also found that the application of PBZ increased flowering percentage and hermaphrodite flowers in mango trees (Fig. S2A; Table 1). Some previous studies reported that PBZ significantly increased the flowering percentage, panicles and hermaphrodite flowers of ‘Uba’ mango in Brazil (Oliveira et al., 2017) and ‘Dashehari’ mango in India (Singh et al., 2005) as compared to the control. An important observation was the increase in the number of panicles in response to FBP with PBZ (Table 1). This happened because the inhibited bud may induce a higher number of axillary buds. Removing the main inflorescence enables a higher number of inflorescences to develop from auxiliary buds in mango (Yeshitla et al., 2003). Surprisingly, the combination of FBP with PBZ-treated trees induced a higher percentage of reflowering, whereas only FBP showed a lower percentage of reflowering. This may have occurred due to PBZ’s influence on flower bud-pruned trees.

Harvesting time was advanced due to the application of PBZ in ‘Amrapali’ mango as reported in this study (Table 2). Early flowering may result in advancement of fruit maturity. Spraying of PBZ on mango trees promoted early fruit ripening by two weeks (Sarker and Rahim, 2012). On the other hand, FBP with PBZ application induced delayed flowering, which may be the reason for late flower initiation. This delaying method was formerly studied by Oosthuyse and Jacobs (1997). The maximum number of panicles was observed in plants following application of PBZ and FBP may have contributed to the development of more fruits per branch (Fig. S2; Table 2). PBZ considerably increased the fruit-set percentage of mango trees (Sarker et al., 2016; Singh et al., 2005). The present study showed that the PBZ application with FBP enhanced fruit weight in mango (Table 2). Kamran et al. (2020) reported that PBZ-treated maize plants had more chlorophyll in their leaves resulting in a higher photosynthetic rate. Thus, an increase in individual fruit weight with PBZ application (Table 2) may be due to a higher photosynthetic rate. Water use efficiency in photosynthesis by leaves was increased due to the application of PBZ (Quinlan, 1981). PBZ-treated plants produced larger mango fruits, which agrees with the previous reports of Benjawan et al. (2006) and Sarker and Rahim (2012). Interestingly, PBZ 1.5 g with FBP showed a better result in terms of the fruit weight of mango compared to the control (Table 2). Yeshitela et al. (2003) observed that panicle pruning improved the fruit weight of ‘Tommy Atkins’ mango, which is consistent with present results.

PBZ-treated fruit exhibited better quality regarding total soluble solids (TSS), reducing sugar, total sugar and β-carotene (Table 3) that could be related to photosynthetic assimilate partitioning of the plant. PBZ-treated plants had suppressed vegetative growth because of less competition between shoot growth and fruit development (Yeshitela et al., 2004) and the assimilation was unidirectional to the developing fruit, resulting in increased sucrose and cellulose levels in fruits that further resulted in an increase in TSS (Abdel Rahim et al., 2011). The application of PBZ increased sugar contents in fruits due to the favorable translocation of assimilates to the developing sink created in maturing fruits (Reddy et al., 2013). Moreover, abscisic acid is possibly induced in plants by PBZ application, and this may contribute to an increase in sugars (Upreti et al., 2013). Further, mango fruit was significantly influenced by abscisic acid treatment on total sugar according to Zaharah et al. (2013). Paclobutrazol possibly helps to increase antioxidant contents such as carotenoids in plants to fight against oxidative stress, as reported in an earlier study (Tuna, 2014). Our results showed that the application of PBZ reduced the amount of vitamin C in mango. Consistent with our result, PBZ treatments reduced vitamin C content in strawberry (Shahrokhi et al., 2008). The improvement in fruit quality such as higher TSS, increased carotenoid and high reducing and total sugars with PBZ treatment was reported by Sarker and Rahim (2018) and Abdel Rahim et al. (2011). Due to pruning treatment, plants got enough time to produce higher reserve levels and these fruits could receive an adequate supply of reserve food in the plant body (Yeshitela et al., 2003). Paclobutrazol treatment with FBP showed distinct improvements in fruit quality in terms of TSS, sugars and carotenoids contents.

In summary, we conclude that PBZ 1.5 g helps in early panicle emergence and advanced the harvesting of mango by 22 and 18 days, respectively, whereas in plants treated with FBP with PBZ 1.5 g reflowered after a delay of 36 days and harvesting of fruits was delayed by 16 days compared to the control. Also, PBZ 1.5 g with FBP exhibited superior results in terms of fruiting percentages and mango quality. This study was based on single cultivars and single-year data, but further studies should be carried out to evaluate the effects of PPZ and FBP on different cultivars and in other agroecological zones.

Acknowledgements

We appreciate the help of Professor Mirza Hasanuzzaman, Department of Agronomy, Sher-e-Bangla Agricultural University, Dhaka, Bangladesh, for his assistance in writing this article. The authors thank Mr. Habibullah, UAO, Sitakunda, Chittagong, Bangladesh, for his encouragement during the field experiments.

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