Manipulation of vegetative growth and improvement of yield potential of cashew (Anacardium occidentale L.) by Paclobutrazol

Scientia Horticulturae 257 (2019) 108748

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Manipulation of vegetative growth and improvement of yield potential of cashew (Anacardium occidentale L.) by Paclobutrazol

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Babli Moga, , P. Jananib, M.G. Nayaka, J.D. Adigaa, R. Meenac a

ICAR- Directorate of Cashew Research, Puttur, D.K, Karnataka, 574202, India ICAR- Central Potato Research Station, Shillong, Meghalaya, 793 009, India c ICAR- Central Institute for Arid Horticulture, Beechwal, Bikaner, 334 006, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: Anacardium occidentaleL Paclobutrazol Canopy management High density planting Nut yield

Cashew (Anacardium occidentale L.) is a perennial tree crop that realizes yield after several years of planting due to its long gestation period. Therefore, canopy management by pruning in later stages of growth often affects orchard life and performance of trees. High density planting system (HDP) has been attempted in cashew to obtain early benefits in terms of yield during initial years of planting. Under HDP, maintenance of tree and canopy growth becomes important due to closer spacing and shading of canopy of trees. In cashew, due to non availability of dwarf clones and dwarfing rootstocks, use of growth retardants, paclobutrazol (PBZ), assumes importance. Hence, this study was aimed to evaluate the morpho-physiological responses of cashew to PBZ treatments under field trials. PBZ treatments resulted in reduced vegetative growth and enhanced reproductive growth with most striking responses at PBZ 3 g a.i./tree treatment. PBZ treatments altered cashew tree physiology by modifying tree size, canopy growth, internodal length, branching pattern and overall ground coverage of the tree. Higher total leaf chlorophyll content, better photo assimilation and enhanced leaf photosynthesis contributed in inducing early flowering and development of more flowering panicles with perfect flowers. Enhanced fruit set and increased number of nuts/m2 canopy contributed in yield increment. Regression analysis showed leaf pigments, nut number and number of inflorescence as the most contributing traits for yield enhancement under PBZ. These findings may highlight the exploitation of morpho-physiological traits for better canopy growth and yield maximization by PBZ in cashew under the HDP.

1. Introduction The cashew (Anacardium occidentale L.) was introduced to other parts of the world from Brazil including India during the 16th century mainly for afforestation and soil conservation purpose being a hardy crop. However, cashew has earned a commendable position in the global market as a major foreign exchange earner from its initial beginning as a wasteland crop to control soil erosion and reclamation of degraded land. Cashew belongs to the family Anacardiaceae including 60 genera and 400 species of trees and shrubs. Other important tree and nut crops from the same family are mango (Mangifera indica L.) from North East India and Myanmar and pistachio nut (Pistacia vera L.) from Iran and Central Asia. The area under Indian cashew cultivation is about 9.82 lakh ha and Indian cashew nut production has shown steady growth to the extent of 4.4% over the five decades (Anonymous, 2014). This can be attributed to several factors such as release of varieties with good agronomical

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traits, adoption of proper crop management practices and adoption of improved cultivation technologies, including the high density planting (HDP) system for enhanced cashew nut production. Indian cashew occupies a commendable position in the global market with 20% and 16% share in terms of the area and raw nut production. Being an important horticultural and export oriented crop, Indian cashew earns about $580740, 000 USD per annum through export of cashew kernel globally (Anonymous, 2019). The cashew industry plays a major role in rural economy by employment generation to millions of people including women through processing industries and other allied activities. However, the present domestic production of raw nut is not enough to meet the demand of cashew industry with a processing capacity of two million tons per annum. Hence, India relies heavily on the import of raw nuts from other producing countries including Africa and South East Asia for almost half of their requirements for several decades. Over the decades, the import rate has gone up to 9.71% with only 4.45% as domestic raw nut production (Anonymous, 2014). There is an urgent

Corresponding author. E-mail address: [email protected] (B. Mog).

https://doi.org/10.1016/j.scienta.2019.108748 Received 8 April 2019; Received in revised form 5 August 2019; Accepted 7 August 2019 0304-4238/ © 2019 Elsevier B.V. All rights reserved.

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perennial trees. The pruned mango trees showed uniform canopy growth, more inflorescence development, more uniform flowering and 40–50% higher yield than un pruned trees (Lal and Mishra, 2007, 2010, Reddy and Kurian, 2011; Das and Jana, 2012 and Asrey et al., 2013). The higher yield in pruned trees was the result of increased distribution of light, more canopy photosynthesis and enhanced CO2 assimilation (Pratap et al., 2003; Shinde et al., 2003; Sharma et al., 2006). However, tree orchards planted at high density require frequent maintenance of the canopy by pruning of over shaded branches which may often produce excess vegetative re growth. Hence, its role in controlling tree size and vigor becomes short lived (Kulkarni, 1991).The growth, efficiency and productivity of trees can be regulated by the use of suitable rootstocks. Several studies have been conducted on mango trees to select suitable rootstocks to control tree vigor and reduce variability in yield production. Though many of these studies showed the associations between yield and tree size, there are very limited studies showing dwarfing of trees with medium to large yields with use of root stocks (Reddy and Raj, 2015; Dinesh et al., 2015b). However, these rootstocks may act as dwarfing material to control tree size, yet, these are not properly commercialized. Therefore, the growth of fruit crops and trees can be managed properly by chemical means and dwarf cultivars rather than use of dwarfing rootstock (Iyer and Kurian, 1992). Paclobutrazol (PBZ), a triazole compound, is the most widely used growth retardant for controlling vegetative growth in many fruit crops and trees (Yadav and Singh, 1998; Saran et al., 2008; Hasan et al., 2013 and Shinde et al., 2015). Paclobutrazol affects growth and development of the tree by regulating several physiological processes such as reduction of tree size by regulation of height and internodal length, reduced shoot length and increased shoot flushes per tree, reduction of leaf size and leaf area, production of darker leaves due to the increased synthesis of chlorophylls and enhanced yield by increased production of hermaphrodite flowers, more fruit set, increased accumulation of stored carbohydrates and enhanced source-sink balance for more fruit production (Singh and Ram, 2000; Kurian et al., 2001; Kotur, 2012; Upreti et al., 2014 and Muengkaew and Chaiprasart, 2016). However, the efficacy of paclobutrazol varies with different crop species which mainly depends on the dosage of paclobutrazol, tree physiology and environmental conditions (Blaikie et al., 2004; Hasan et al., 2013; Narvariya et al., 2014 and Shankaraswamy and Neelavathi, 2016). Several workers have reported the management of the canopy by pruning systems in cashew as well. Panda (1990) reported the effects of pruning on 28 year old- cashew trees with more number of panicle/m2 of canopy area and 14.42% higher fruit set compared to 7.75% fruit set in control trees. There was a considerable increase in floral laterals, hermaphrodite flowers, fruits per panicle and ultimately the yield due to pruning of leader shoot at least once every 2–3 years (Chattopadhyay and Ghose, 1994; Mohan and Rao, 1995 and Nayak, 1996). The influence of dwarf rootstocks, namely, NRC 492 and Taliparamba-1 on vigorous cashew cultivars viz., Ullal-3, VRI-3, NRCC Selection- 2 and Vengurla-4 was investigated at ICAR-Directorate of Cashew Research, Puttur, India during 2008-2012. Among the stionic combinations between scions and dwarf rootstocks, NRC 492 recorded 100 percent grafting success against 50% with Taliparamba- 1 and showed reduction in plant height and canopy spread and higher yield when planted under field conditions. Hence, NRC 492 could be exploited well as rootstock in the HDP system to obtain higher yield during the initial years of planting (Adiga et al., 2014). Several experiments have been conducted using pruning techniques and dwarfing rootstocks to evaluate the growth and development of cashew cultivars, however, limited information is available on PBZ induced modifications of tree architectural traits and reproductive growth. In the present study, we have investigated the effects of soil applied paclobutrazol on growth and development of cashew trees. Our aim was to evaluate the changes in tree physiology and subsequent changes in reproductive parameters, including flowering, fruit set and yield as induced by paclobutrazol application. Additionally, we also wanted to quantify the persistence of

need for India to double the raw cashew nut production to become selfsufficient. The cashew trees are commonly planted at a wider spacing (8 m × 8 m) accommodating only 62 trees /acre. The nut is realized after the 3rd year of planting and yield gets stabilized after 10–12 years of planting due to its long gestation period. Hence, it takes longer time to recover the cost of planting and orchard establishment. Moreover, it causes several disadvantages such as wastage of spaces, nutrients, water availability, solar radiation and also weeds infestation (Mangalassery et al., 2019). Apart from this, the management of the canopy by pruning in later stages may affect the performance of trees and orchard life. Generally, unlike other tree crops, the canopy expansion rate is slow during the initial years of planting in cashew and there is a possibility of interlocking of canopy branches after 12th year of planting. To harness the maximum benefit, HDP system, has been recommended for enhancement of cashew production. The initial experiments conducted in India showed that cashew varieties planted at 625 plants /ha yielded 4.94 t/ha cumulative nut yield in four to eleven years of planting (Yadukumar et al., 2002). The HDP involves accommodation of more number of plants per unit area to obtain early benefit in terms of higher yield during initial years of planting compared to the normal planting system. These were resultant of deeper root penetration, greater light interception and drying of canopy branches due to mutual shading. The earlier experiments conducted at the ICAR- Directorate of Cashew Research, Puttur studied the performance of cashew varieties under HDP system and revealed that after 8 years of planting, the yield of cashew varieties planted at 500 plants /ha was 10.39 t/ha than the yield of those planted at 200 plants/ha (data unpublished). However, in the HDP system, the dwarf and compact varieties are most preferred. Hence, control of tree size and vigor through manipulation of plant architectural traits play a critical role. The HDP system has been successfully implemented in various other fruit crops such as mango, apple, pear, cherry, olives and stone fruits. The experiments conducted in Uttar Pradesh, India, showed that mango trees planted at 1111 trees per ha yielded 59 t per ha under HDP as compared to 5.9 t per ha with 100 trees per ha (Rajbhar et al., 2016). Similar results were also reported in Amrapali mango orchards where trees planted at 1600 trees per ha yielded 12, 13, 17 and 22 t per ha in the four to seven years after planting under HDP (Majumder and Sharma, 1989). The HDP systems in pear yielded after six years of planting and recovered the costs of orchard establishment after ten years while it took nine years for yield and twenty one years for recovery of costs under normal density (Elkins et al., 2008). Studies on canopy restraining of 15 olive cultivars under HDP showed that low to medium vigour cultivars are more responsive to mechanical and manual pruning and produced more yields whereas pruning affected yields in medium to high vigour cultivars as fruiting shoots are mostly concentrated in the periphery of the canopy (Vivaldi et al., 2015). It is evident from above that success of HDP mainly depends on tree size and vigor, productive scions, the availability of dwarfing root stocks, climatic conditions, including soil types and growing conditions (Tustin, 2014; Trentacoste et al., 2015). Intensive orchard management by HDP showed better canopy management and higher yield with cultivars of low to medium vigor than high vigor due to greater density of plants, better responsiveness to pruning, greater light interception and uniform light penetration throughout the plant canopies (Palmer et al., 2002; Tustin, 2014; Trentacoste et al., 2015; Vivaldiet et al, 2015 and Morales et al., 2016). Pruning techniques, use of dwarfing rootstocks and plant growth retardants, mainly use of paclobutrazol, are few attempted strategies to modify tree architecture in several fruit crops including perennial trees. Pruning affects tree physiology and shows its strong associations with canopy growth, light interception, flowering and yield in many fruit crops (Stassen et al., 1999 and Avilan et al., 2003). A suitable pruning technique can also be successfully used to rejuvenate old orchards in order to modify growth and yield performance of fruit crops and 2

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paclobutrazol in the soil based on observations on growth, flowering, fruiting behavior and yield traits. Here persistence meant the effect of previously applied PBZ on performance of cashew cultivar with respect to vegetative and reproductive growth in the subsequent year. The residual effects of PBZ were assessed in the subsequent year in terms of vegetative and reproductive growth of cashew. The residue content of PBZ was also analyzed in both soil and kernels. The residue content in kernels was found to be below maximum residue level (MRL) (< 0.01 PPM) and hence it is safe for consumption.

water per tree and applied uniformly in the circular trenches (15 cm deep and 10 cm wide) in the soil around the base at a radial distance of 60 cm away from plant trunk. The control trees were treated with only normal water. All treatments were applied once during the 1 st week of September in the year 2014 before flower development. The observations on vegetative and reproductive parameters were recorded in the application year 2014–2015 followed by the subsequent year (i.e. 2015–2016). 2.4. Data collection

2. Materials and method

2.4.1. Measurement of vegetative traits Three healthy trees from each replication were selected and four healthy branches on each tree were chosen to record vegetative growth parameters. Traits like tree height (m), the canopy diameter in m (average of north-south and east-west), stem girth (cm), the average length of new shoots (cm), the average internodal length of new shoots (cm), ground coverage(m2) and the leaf area (cm2) were recorded. Observations for new vegetative growth were recorded in the second fortnight of December. The leaf area of fully matured 30 leaves per tree was recorded according to Nii et al (1995). Ground coverage of the canopy was calculated as GC= π r2 where π = 3.14 and r indicates the canopy diameter (mean of the canopy spread in north-south and east to west). The incremental plant height, stem girth, canopy diameter and ground coverage were recorded 12 months (in September 2015) after paclobutrazol treatments.

2.1. Details of the experimental site The experiment was conducted at the experimental research station of ICAR-Directorate of Cashew Research, Shantigodu, Karnataka, India (12°.25′ N latitude, 75°.4′ N longitude and 90 m above mean sea level) during 2014-2016. The climate was tropical monsoon with distinct dry seasons from January-May. The area receives 3500 mm rainfall per annum. Due to its location along the West Coast region of India, it also experiences the hot and humid climate with the mean annual temperature of 27.6 °C and relative humidity of 60–70%. 2.2. Plant material and adoption of agronomic practices Three year old cashew cultivar Ullal-3 trees were selected for the study. The cultivar is highly vigorous with an average tree height and canopy spread of 6.7 m and 7.5 m respectively. This cultivar is characterized by intense flowering (52.8%/m2 of canopy) with good productivity (cumulative nut yield of 25.95 kg per tree) (Nayak et al., 2014). Initially, the grafts of Ullal-3 were planted at 3 m × 3 m spacing at the Shantigodu research station in March 2010. The physicochemical properties of soil at the study site are given in the Table (1 ) (Mangalassery et al., 2019). All planted trees received standard orchard management practices. Shape pruning was done to keep the main stem free of branches up to a height of 75 cm before the imposition of treatment. Proper fertigation was maintained by the annual application of the recommended dose of fertilizer at the rate of 500 g N, 125 g P2O5 and 125 g K2O on per tree basis. The trees were irrigated through drip irrigation system.

2.4.2. Measurement of the pigment content Pigment contents were estimated in leaf samples of both PBZ treated and non treated trees during both year 1 and year 2. Chlorophyll a (Chl a), chlorophyll b (Chl b), total chlorophyll (Tchl) and total carotenoid (caro) contents were measured following Lichtenthaler and Buschmann (2001) and expressed as mg/g FW. 2.4.3. Measurement of reproductive traits Ten shoots per tree were tagged randomly to record the number of flowering shoots after 10th September 2014 (year 1). Flowering time was defined based on the development of twenty five inflorescence per tree after treatment application. In tagged shoots, the number of inflorescence developed was also recorded. To count the male and hermaphrodite flower, fifteen flowering panicle/inflorescence per shoot were selected to determine hermaphrodite flower to male flower ratio and expressed as sex ratio. During both year 1 and year 2, five healthy branches bearing fruits were tagged to record fruiting behavior. Fruit set was quantified at peanut stage in tagged branches per tree. Number of nuts per m2 of canopy area that reached maturity was also counted on the four directions of the tree. The nuts were harvested, sun dried for 3 days and weighed. The mean nut weight (g/tree) and nut yield (kg/ tree) were calculated for both the year 1 and year 2 to assess the effects of PBZ treatments. The mean nut weight was calculated by dividing the total weight of nuts from the number of nuts. Kernel weight (g) was recorded by manually shelling the kernel from sun dried nuts. The weight of kernels with testa and shells obtained after shelling the nuts were recorded separately. The weight of kernel with testa was divided by the weight of the nut (the weight of kernel with the testa + the weight of shell) and expressed as the percentage which gave the shelling percentage (Bhat, 2005).

2.3. Treatment details Healthy and strong trees with uniform plant height and spread were selected for the study and the experiment was laid out in randomized block design with four treatments. Each treatment had five replications and each replication included five individual trees. Paclobutrazol (CULTAR 23% SC) was applied at 0 (control), 1, 2 and 3 g a.i. (active ingredient) per tree as soil drench treatments. It was obtained from R.V. Agri Corporation, Navsari, Gujarat. The mean plant height and canopy diameter of trees were 4.9 m and 4.5 m before the start of the experiment. Paclobutrazol for each concentration was mixed in 5 litres of Table 1 Physico-chemical properties of soil. Soil physicochemical properties (0-30 cm)

Values

Textural class pH EC (dSm−1) Organic carbon (%) Available N (kg ha−1) Available P (kg ha−1) Available K (kg ha−1) Field capacity Permanent wilting point

Sandy loam 4.8-5.3 0.012-0.03 0.14-1.68 203-247 7-7.3 112-198 20.2% 7.8%

2.5. Statistical analysis The effects of PBZ treatments were assessed by analysis of variance (ANOVA) using the Statistical Analysis System (SAS). Means and standard errors of five replicates presented in both tables and figures were calculated for each level of treatment application. In the ANOVA test, significant differences between means were tested by Least Significant Difference (LSD) test at 0.05 level of significance. The Pearson 3

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Fig. 1. Changes in tree growth after PBZ treatment in Ullal-3 cashew cultivar. (a) Paclobutrazol induced changes in tree growth due to reduction in plant height, canopy diameter and ground coverage and (b) tree growth of normal cashew tree at 110 days after treatment.

treated trees were significantly reduced (P < 0.05) after 10 months of PBZ application (Fig. 2). On an average, the reduction corresponded to 29.1% in plant height, 43.3% in stem girth, 36.8% in canopy diameter and 76.4% in ground coverage after PBZ treatments over control. However, among treatments, most drastic reduction in tree growth was observed at PBZ 3 g a.i./tree treatment. Although the most striking reduction of tree growth was observed after PBZ application, it lasted only for a year. In the subsequent year, there were no significant differences in tree growth between treated and control trees (data not shown). The most striking reduction in shoot length (11.92 cm) was found at PBZ 3 g a.i./tree treatment among other treatments compared to control (16.30 cm) (Table 2). However, in the year 2, the reduction in shoot length was less evident among PBZ treatments. Even at higher PBZ concentration (3 g a.i./tree), shoot length increased to 13.14 cm from 11.92 cm. The relative shoot growth was 9.01% over two years after PBZ treatments. As with shoot length, the effects of PBZ induced reduction in internodal length was more prominent during the year 1.

correlation coefficients were calculated for both vegetative and reproductive traits under PBZ treatments for year 1. To establish the most important vegetative traits in modifying tree growth and also important reproductive traits in enhancing yield under PBZ treatments, step wise multiple regression analysis was also performed by using SAS analysis. In this analysis for vegetative and reproductive traits, ground coverage (GC) and yield were considered as dependent variables and remaining traits were independent. 3. Results and discussion 3.1. Vegetative parameters Tree growth and vigor of Ullal-3 cashew cultivar was significantly influenced by soil applied paclobutrazol (PBZ) treatments. The most striking response was the reduction in plant height, canopy diameter and ground coverage in PBZ treated trees (Fig. 1). The incremental plant height, stem girth, canopy diameter and ground coverage of

Fig. 2. Changes in growth attributes of cashew (Anacardium occidentale L.) as affected by paclobutrazol (PBZ) treatments. (a) Incremental plant height (m), (b) incremental stem girth (cm), (c) incremental canopy diameter (m) and (d) incremental ground coverage (m2) after 10 months of treatment. The values shown are means with SE (n = 5). Vertical bars indicate standard error (n = 5).

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reduction of leaf surface area with the lower individual leaf area of treated trees. Generally, maturity of leaf depends on the division of plate meristem. PBZ usually affects the multiplication of plate meristem thereby reduces leaf size and ultimately decreases total leaf area as observed in our study (Esau, 1977). This PBZ induced reduction in leaf area may also be attributed to reduced levels of endogenous gibberellins, which is responsible for cell elongation in plants. However, our data do not support the fact where PBZ treatments tend to increase leaf area (Vijayalakshmi and Srinivasan, 1999). Our results also revealed that the effects of soil drench PBZ remained in the soil were short lived and the effect was disappeared after 2nd year of application. This might be due to precipitation and leaching away of most of soil applied PBZ as indicated by Salazar-garcia and Perez-rosales (1991) in mango. However, the persistence of PBZ in soil depends on varieties, type of soil and environmental conditions. It remained up to longer duration in soil as reported in apple (Ma et al., 1990), pears (Erez, 1986), apricot (Jacyna et al., 1989) and eucalyptus (Hasan and Reid, 1995). Modifications in ground coverage due to changes in tree size and canopy growth caused by PBZ application offer several benefits including efficient interception of sunlight for enhanced photosynthesis and minimize the necessity of pruning which in the long run can affect production efficiency of trees (Ghosh et al., 2010 and Kumar et al., 2012). Pruning may also reduce the leaf area available for the synthesis of the photo assimilates to support leaf photosynthesis and also plant growth (Shinde et al., 2015).

Table 2 Treatment effects of paclobutrazol (PBZ) on vegetative parameters of Ullal-3 cashew cultivar during year 1(2014–2015) and year 2 (.2015–2016). Rates of PBZ (g a.i./tree)

0 (control) 1 2 3 SE(d) LSD (0.05)

Shoot length (cm)

Internodal length (cm)

Leaf area (cm2)

Year 1 16.30 15.60 14.40 11.92 0.566 1.233

Year 1 15.06 10.38 8.69 8.04 1.510 3.290

Year 1 70.70 68.20 67.00 66.20 1.103 3.369

Year 2 20.20 16.68 15.88 13.14 0.533 1.162

Year 2 23.90 12.08 9.80 9.00 0.425 0.926

Year 2 88.70 70.00 68.90 67.90 1.587 3.457

Values are the mean of five replicates ± S.E. Means within a column are significantly different at P < 0.05 according to LSD test.

There was 40.03% reduction in internodal length in treated trees with PBZ 3 g a.i./tree treatment showing higher reduction (8.04 cm) over control (15.06 cm) (Table 2). The relative internodal length was 13.95% over two years. The plant morphology, growth and productivity can be regulated by the use of plant growth regulators including synthetic growth retardants at required concentrations. The potential role of growth retardant, paclobutrazol (PBZ) has been exploited well in many agricultural and horticultural crops for controlling vegetative growth and also subsequent improvement in flowering parameters, fruit quality and yield (Berova and Zlatev, 2000; Blaikie et al., 2004; Asin et al., 2007 and Ghosh et al., 2010). In the present study, we have investigated the morphological and physiological responses, growth and yield of cashew after PBZ treatments under field trials. We have also explored the use of PBZ in canopy management and further yield improvement in cashew. In the present study, significant changes in vegetative parameters were observed after PBZ treatments at higher concentrations. When vegetative parameters were compared between treatments and control after 3 and 6 months of application, no significant differences were observed (data not shown). However, the effects were more prominent after 10 months of PBZ application. This might be due to the period which coincided with the final stage of fruit development (at harvest stage) and higher partitioning of the photo assimilates into developing fruits leading to restriction of new vegetative growth in PBZ treated trees (Yeshitela et al., 2004). In our study, reduction in vegetative growth in PBZ treatments were associated with reduction in tree height, branch growth, canopy diameter and ground coverage at higher concentrations (2 and 3 g a.i./tree). These results are in agreement with earlier studies by Banon et al. (2002); Yeshitela et al. (2004); Kumar et al., (2012) and Ghosh et al., (2010). PBZ treatments also reduced the shoot growth and internodal length as reported earlier by Kumar et al., (2012). Inhibition of gibberellins biosynthesis and reduction of endogenous auxin (mainly IAA) content due to PBZ might have contributed in growth inhibition (Dalziel and Lawrence, 1984; Graebe, 1987 and Browning et al., 1992). The reduction in vegetative growth observed in this study may also be due to the reduction in xylem thickness, thereby limiting the uptake of water and nutrients of trees due to PBZ (Wang et al., 1989). There was a significant reduction in the functional leaf area after PBZ treatments over control. The highest reduction was observed at PBZ 3 g a.i./tree treatment (66.20 cm2, P < 0.05) as compared to control (70.70 cm2). Over all, the reduction in the total leaf area was 5.19% in treated trees over control (Table 2). On the other hand, the second year of PBZ application showed a different trend with increased leaf area due to PBZ treatments with 2.68% relative leaf growth over the two years. The PBZ treatments affected leaf area. In our study, significant reduction in leaf area was observed with PBZ treatments. Our studies are in agreement with studies by Yeshitela et al. (2004) who reported the similar results citing the positive relationship between internodal length and leaf area. Reduction in internodal length resulted in the

3.2. Leaf pigments Significant changes in leaf pigment contents, including chlorophyll a (Chl a), chlorophyll b (Chl b), total chlorophyll (Tchl) and total carotenoids (Caro) were observed after PBZ treatments. The data revealed that pigment contents increased with PBZ treatments to an extent of 27.35% increase in Chl a, 54.54% in Chl b, 30.98% in Tchl and 13.55% in Caro content compared to control during the 1st year of application (Fig. 3). Among PBZ treatments, PBZ at 3 g a.i./tree treatment recorded highest leaf pigment contents (1.43, 0.59, 2.02 and 1.43 mg g−1 FW) than control. During 2nd year, different trend in leaf pigment contents were found between treated and control trees. Pigment contents tend to decrease exhibiting 8.1% reduction in Chl a, 11.7% in Chl b, 6.7% in Tchl and 5.2% in Caro contents after PBZ treatments. Higher leaf photosynthesis in plants mainly depends on the increased rate of leaf pigment contents. In the present study, PBZ treated trees exhibited intense dark green leaves which ultimately resulted in higher pigment contents including chlorophylls and carotenoids. Similar studies have also been reported in different crop plants (Belakbir, 1998; Berova and Zlatev, 2000; Sebastian et al., 2002 and Kumar et al., 2012). The PBZ treatments increased the chloroplast size to the extent of 30–34% than control with a subsequent increase in leaf chlorophyll content per unit leaf area as reported in sugar beet (Dalziel and Lawrence, 1984 and Gao et al., 1987). Studies have also indicated that phytol, a part of chlorophyll molecule, play role in regulation of chlorophyll synthesis. Under PBZ treatments, due to the degradation of active gibberellins, several intermediates in gibberellins biosynthetic pathway get activated which in turn increase phytol synthesis (Chaney, 2003). This may have contributed in higher pigment contents as observed in our study. However, in the second year, reduction in leaf pigment contents was observed in PBZ treated tress. This may be attributed to less intense concentration of pigments in leaves due to increased leaf area and increased synthesis of gibberellins that might have affected chlorophyll biosynthesis (Monge et al., 1993; Kwack and Kwack, 1990). 3.3. Flowering parameters The average length and width of inflorescence of control trees were 5

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Fig. 3. Changes in concentrations of chlorophylls and carotenoids in Ullal-3 cashew cultivar at different PBZ treatments. Chlorophyll a(a), Chlorophyll b (b), total chlorophyll (c) and total carotenoid contents (d) expressed in mg/g FW were determined in leaves of cashew trees under both PBZ treatments and control during year 1 and year 2.The values shown are means with SE (n = 5). Vertical bars indicate standard error (n = 5).

Table 3 Effect of soil applied paclobutrazol on flowering parameters of Ullal-3 cashew cultivar during year 1(2014–2015) and year 2 (.2015–2016). Rates of PBZ (g a.i./tree)

Number of inflorescence

Days for flowering

Male flower

0 1 2 3 SE(d) LSD (0.05)

Year 1 134 171 196 140 22.799 49.674

Year 1 80 64 58 55 5.696 12.41

Year 1 236 280 317 340 31.388 68.388

Year 2 138 159 172 198 4.444 9.682

Year 2 83 86 68 65 5.022 10.941

Year 2 238 186 210 212 15.693 34.193

Female flower

Sex ratio

Year 1 46 65 85 102 4.909 10.695

Year 1 0.22 0.30 0.43 0.55 0.012 0.0256

Year 2 51 60 70 92 4.035 8.792

Year 2 0.23 0.28 0.36 0.45 0.011 0.0256

Values are the mean of five replicates ± S.E. Means within a column are significantly different at P < 0.05 according to LSD test.

number of floral buds on trees that resulted in more intense flowering. Burondkar and Gunjate (1993) also reported about the bimodal flowering nature of trees after PBZ treatment. The PBZ reduced the bimodal flowering and resulted in early flowering and increased flowering intensity as observed in our study. In our data, at PBZ 3 g a.i./tree, time for flowering was 55 days compared to 80 days in control after year 1. Flower initiation was 25 days earlier at PBZ treatments than control. Similar results have been reported by Kulkarni (1988); Von (1991) and Burondkar and Gunjate (1993). In addition, the earliness of flowering in PBZ treated trees may be due to increase in total non structural carbohydrates (TNC) in shoots of treated trees which may have contributed in increased flowering intensity (Voon et al., 1991 and Phavaphutanon et al., 2000). The average number of hermaphrodite flowers increased due to PBZ treatments (102 at PBZ 3 g a.i./tree and 85 at PBZ 2 g a.i./tree) than control (46) and showed significant differences (P < 0.05) in the application year (Table 3). However, in the subsequent year, the average number of hermaphrodite flowers was reduced with a range of 60–92 in PBZ treatments. The sex ratio expressed as the hermaphrodite to male flower ratio was also increased after PBZ treatments with 90% increase over control (Table 3). Generally, trees have less number (less than 0.1%) of the

22.14 cm and 18.62 cm, respectively. The inflorescence length and width got reduced more significantly after PBZ treatments with PBZ 3 g a.i./tree treatment showing higher reduction (12.28 cm and 14.90 cm, P < 0.05) during first year of application. However, no significant differences were observed during year 2 (data not shown). The soil applied PBZ treatments induced early flowering than control during the application year. The flowering duration was 55 days (PBZ 3 g a.i./tree) and 58 days (PBZ 2 g a.i./tree) compared to 80 days in control (Table 3). During year 2, the effect of PBZ on flowering data began to decline. The flowering duration was increased to 65 days (PBZ 3 g a.i./ tree) and 68 days (PBZ 2 g a.i./tree) in PBZ treatments. In the present study, a significant increase in flowering parameters was observed after PBZ treatments than control. This may be due to some physiological modifications such as the low expenditure of the photo assimilates for vegetative growth (Yeshitela et al., 2004), enhancement of total phenolic content in terminal buds, alteration of phloem to xylem ratio of stem (Kurian and Iyer, 1992) and florigenic properties (Asin et al., 2007). Such modifications can in turn limit vegetative growth and induce flowering by balanced regulation of the pattern of assimilate partitioning and rate of nutrient supply. The availability of increased photo assimilates to reproductive growth after PBZ treatments might have contributed to the development of more 6

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Fig. 4. Fruiting pattern of Ullal-3 cashew cultivar after PBZ treatment. (a) Bunch bearing trait with more fruit set in PBZ treated trees and (b) with fewer fruits in non treated trees at 150 days after treatment.

bearing trait with more fruit set per inflorescence than control trees (Fig. 4). The most striking differences were found after one year of PBZ application. On an average, the mean fruit set ranged from 6.5 to 10.2 after PBZ treatments as compared to 5.4 in control (Table 4). The PBZ treatments ranging from 2 g and 3 g a.i./tree produced more number of mature nuts (8.4 and 10.5) than control (6.3). However, PBZ treatment at 1 g a.i./tree produced the same number of nuts as that of control during the first year of application (Table 4). In the second year of application, PBZ treated trees tended to produce fewer nuts but PBZ 3 g a.i./tree recorded relatively higher manure nuts (8.9). No significant differences were observed between PBZ 2 g a.i./tree and control in terms of mature nuts. The least number of mature nuts was recorded at PBZ 1 g a.i./tree treatment. Flowering, fruiting and yield of plants are the interplay of the amount of photo assimilates partitioning into the reproductive organ and their efficient utilization. The pattern of photo assimilate allocation within plant organ can be regulated by PBZ in order to distribute them more towards reproductive development specially to increase sink strength capacity and ultimately to increase more yield (Addo-Quaye et al., 1985). In the present study, we observed more fruit set, increased number of nuts and enhanced nut yield in PBZ treated cashew trees which were also reported by Hoda et al. (2001) and Kumar (2012).

Table 4 Effect of soil applied paclobutrazol on fruiting parameters of Ullal-3 cashew cultivar during year 1(2014–2015) and year 2 (.2015–2016). Rates of PBZ (g a.i./ tree)

No. of fruit set per inflorescence

No. of nuts/m2 canopy

0 1 2 3 SE(d) LSD (0.05)

Year 1 5.4 6.5 7.7 10.2 0.628 1.367

Year 1 6.3 6.4 8.4 10.5 0.858 1.868

Year 2 5.6 5.0 6.1 8.5 0.586 1.276

Year 2 6.5 5.5 6.8 8.9 0.871 1.898

Values are the mean of five replicates ± S.E. Means within a column are significantly different at P < 0.05 according to LSD test.

hermaphrodite (perfect flower) flowers that develop into mature fruits and also require more nutrient reserves for its development than male flowers (Singh, 1987). In our study, the higher number of hermaphrodite flowers was observed at PBZ 3 g a.i./tree during the1st year of application. This might be due to low vegetative growth and low sinksource competition for nutrient reserves in treated trees. Gibberellins are mainly responsible for mobilization of carbohydrates by degrading them to glucose. Thus higher gibberellins content in plants may cause less accumulation of starch and affects reproductive growth (Jacobson and Chandler, 1987). A higher percentage of hermaphrodite flowers to male flowers after PBZ treatments were also reported by Vijayalakshmi and Srinivasan, (2002).

3.5. Nut and kernel parameters The soil applied PBZ treatments significantly reduced weight and size of nuts. It was evident from the reduction in nut weight (20.25% reduction over control), nut length (12.27% over control), nut width (9.96% over control) and nut thickness (13.02% over control) during year 1 (Table 5). However, the weight and size of nuts tended to increase during 2nd year of application. The relative growth in mean nut

3.4. Fruit set In case of the number of fruit set, PBZ treated trees exhibited bunch

Table 5 Effect of soil drench paclobutrazol on nut parameters of Ullal-3 cashew cultivar during year 1(2014–2015) and year 2 (.2015–2016). Rates of PBZ (g a.i./tree

Nut weight (g)

0 1 2 3 SE(d) LSD (0.05)

Year 1 7.9 7.0 6.2 5.9 0.492 1.072

Nut length (mm) Year 2 8.3 7.7 7.1 6.7 0.235 0.5119

Nut width (mm)

Year 1 33.4 30.8 29.2 28.0 0.518 1.128

Year 2 35.9 31.8 30.2 29.6 1.137 2.477

Year 1 26.1 24.5 23.8 22.3 0.473 1.030

Nut thickness (mm) Year 2 27.6 26.3 25.6 24.8 0.557 1.214

Values are the mean of five replicates ± S.E. Means within a column are significantly different at P < 0.05 according to LSD test. 7

Year 1 19.2 17.5 16.9 15.8 0.327 0.712

Year 2 21.1 20.1 19.3 18.9 0.340 0.741

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Table 6 Effect of soil drench paclobutrazol on kernel parameters of Ullal-3 cashew cultivar during year 1(2014–2015) and year 2 (.2015–2016). Rates of PBZ (g a.i./tree

Kernel weight (g)

0 1 2 3 SE(d) LSD (0.05)

Year 1 2.4 2.3 2.1 2.0 0.040 0.086

Kernel length(mm) Year 2 2.7 2.5 2.4 2.4 0.026 0.057

Year 1 17.7 16.4 15.2 14.9 0.046 0.0998

Kernel width (mm) Year 2 18.3 17.2 15.9 15.3 0.405 0.883

Year 1 14.7 13.6 13.4 13.1 0.032 0.069

Shelling percentage Year 2 15.2 14.8 14.1 13.9 0.403 0.877

Year 1 30.1 29.5 29.5 29.0 0.400 1.222

Year 2 34.3 31.1 30.7 29.4 0.954 2.079

Values are the mean of five replicates ± S.E. Means within a column are significantly different at P < 0.05 according to LSD test. Fig. 5. Changes in reproductive parameters in Ullal-3 cashew cultivars at different PBZ treatments. Mean values of nut yield (kg) as affected by paclobutrazol application in (a) application year (2014–2015) and (b) subsequent year (2015–2016). The values shown are means with SE (n = 5). Vertical bars indicate standard error (n = 5).

size of nuts after PBZ applications during the application year. Our results were in consistent with the findings of Budhianto et al. (1994); Bayat and Sepehri (2012) and Xu (2013) who reported the similar results in maize, white clover and jatropha. However, there are contrasting results depicting positive effects of PBZ on quality and size of fruits (Blanco, 1988 and Martin, 1989). The reduction in weight and size can be attributed to the negative impact of PBZ on nutrient element contents of seeds, restriction of water and nutrient uptake into plants due to the reduction in growth of xylem cells and roots (Wang and Gregg, 1989; Pequerul et al., 1997 and Pardos et al., 2005).

Table 7 Pearson correlation coefficients of studied vegetative traits under paclobutrazol (PBZ) application. Trait

PHT

SG

CD

IL

SL

GC

PHT SG CD IL SL GC

1.00

0.634* 1.00

0.334 0.450 1.00

0.465 0.600* 0.081 1.00

0.697** 0.810** 0.199 0.702** 1.00

0.605* 0.598* 0.827** 0.227 0.491 1.00

*, ** significant at P < 0.05 and P < 0.01 respectively. PHT plant height, SG stem girth, CD canopy diameter, IL intermodal length, SL shoot length. GC ground coverage.

3.6. Analysis of trait association Pearson correlation coefficient analysis was performed among vegetative traits under PBZ treatments for year 1 (Table 7). The data indicated that plant height was positively and significantly correlated to stem girth (P < 0.05), shoot length (P < 0.01) and ground coverage (P < 0.05). The ground coverage showed significant positive relationships between stem girth (P < 0.05) and canopy diameter (P < 0.01). A strong positive associations were also found between internodal length and shoot length (P < 0.01). The backward regression analysis was also performed to study the main vegetative traits responsible for maintaining tree vigor under PBZ treatments (Table 8). The results showed that plant height, stem girth and canopy diameter are the major vegetative traits controlling tree size and vigor in terms of ground coverage under PBZ treatments. The results also indicated that the canopy diameter is the major contributing vegetative traits (Table 8). The relationships between physiological and reproductive traits were also studied for year 1 after PBZ treatments (Table 9). The data revealed that hermaphrodite flower, fruit set, nut number, sex ratio and nut yield showed significant positive correlations with chlorophyll a, chlorophyll b and total chlorophyll. Among the reproductive traits, hermaphrodite flower was related to the fruit set, nut number, sex ration and nut yield. Fruit set was mainly depended on hermaphrodite flower, number of inflorescence, nut number, sex ratio and yield. The nut number and sex ratio showed significant negative relationships with nut weight. However, nut weight showed no significant positive correlation with physiological and other reproductive traits.

Table 8 Results of multiple regression analysis with ground coverage (GC) as dependentvariable in PBZ treatments. Independent variable

R2

Adjusted R2

F value

P value

PHT SG CD

0.36 0.35 0.68

0.31 0.30 0.66

7.52 7.25 28.19

0.016 0.018 0.0014

PHT plant height, SG stem girth and CD canopy diameter.

weight over two years was 12.6% after PBZ treatments. As observed in case of nut weight, soil applied PBZ treatments also reduced the kernel weight and kernel size (Table 6). The relative difference in kernel weight between year 1 and year 2 was 14.2%. Though nut weight got reduced with PBZ treatments, a significant increase in nut yield was observed in PBZ treated trees after one year of application (Fig. 5). On an average, mean nut yield was 1.86 kg/tree in PBZ treatments as compared to 1.62 kg/tree in control. PBZ treatment at higher concentration (3 g a.i./tree) recorded highest nut yield (1.96 kg/tree) than control among other treatments. However, reduction in nut yield was observed in the 2nd year of application. Nut yield was reduced to 4.3% in PBZ treatments during the year 2. Although significant improvement in nut yield was observed after PBZ treatments in our study, we could find a reduction in weight and 8

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Table 9 Pearson correlation coefficients of studied physiological and reproductive traits under paclobutrazol (PBZ) application. Trait

Chla

Chlb

Tchl

dif

ffl

fst

mfl

m/f

nif

nnt

sr

nwt

Nyld

Chla Chlb Tchl dif ffl fst mfl nif nnt sr nwt nyld

1.00

0.52* 1.00

0.85** 0.88** 1.00

−0.19 −0.21 −0.23 1.00

0.75** 0.56* 0.74** −0.36 1.00

0.71** 0.55* 0.72** −0.22 0.77** 1.00

0.49 0.09 0.32 0.11 0.39 0.57* 1.00

−0/39 −0.46 −0.49 0.52* −0.71** −0.36 0.32

0.38 0.35 0.41 0.11 −0.06 0.61* 0.03 1.00

0.74** 0.62* 0.78** −0.04 0.81** 0.67** 0.42 0.06 1.00

0.65* 0.75** 0.79** −0.54* 0.72** 0.68** 0.39 0.59* 0.60* 1.00

0.33 0.09 0.14 0.40 0.29 0.31 0.10 0.36 −0.56* −0.71** 1.00

0.52* 0.89** 0.73** 0.02 0.54* 0.87** 0.35 0.68** 0.80** 0.78** 0.01 1.00

*,** Significant at P < 0.05 and P < 0.01 respectively. Chla chlorophyll a, Chlb chlorophyll b, Tchl total chlorophyll, dif days for inflorescence development, ffl female flower, fst number of fruit set, mfl male flower, nif number of inflorescence, nnt number of nuts, sr sex ratio, nwt nut weight and nyld nut yield.

influence the work reported in this paper.

Table 10 Results of multiple regression analysis with nut yield as dependable variable in PBZ treatments. Independent variable

R2

Adjusted R2

F value

P value

Chla Chlb Tchl ffl fst nif nnt sr

0.38 0.31 0.56 0.31 0.45 0.51 0.60 0.46

0.33 0.26 0.52 0.26 0.41 0.47 0.57 0.42

8.15 5.93 16.54 6.11 10.79 13.87 20.24 11.29

0.013 0.030 0.0013 0.027 0.0059 0.0025 0.0005 0.0051

Acknowledgement The funding was done by ICAR- Directorate of Cashew Research, Puttur, Karnataka, India. References Addo-Quaye, A.A., Daniels, R.W., Scarisbrick, D.H., 1985. The influence of paclobutrazol on the distribution and utilization of 14C-labelled assimilate fixed at anthesis in oilseed rape (Brassica napu L.). J. Agric. Sci. (Cambridge) 105, 365–373. Adiga, J.D., Kalaivanan, D., Meena, R.K., Mohana, G.S., Lakshmipathi, 2014. Performance of vigorous cashew cultivars as influenced by dwarf rootstocks. Vegetos 27 (2), 242–248. Anonymous, 2019. Export of Cashew Kernel, CNSL and Import of Raw Cashew Nut in India. Directorate of Cashew nut and Cocoa Development (DCCD), Kochi, Kerala. https://dccd.gov.in. Anonymous, 2014. Trends in world raw cashew nut (RCN) Area and production. Cashew Handbook 2014- Global Perspective, Section 1. pp. 23–26. www.cashewinfo.com. Asin, L., Alegre, S., Montserrat, R., 2007. Effect of paclobutrazol, prohexadione-Ca, deficit irrigation, summer pruning and root pruning on shoot growth, yield, and return bloom, in a ‘Blanquilla’ pear orchard. Sci. Hort. 113, 142–148. Asrey, R., Patel, V.B., Barman, K., Pal, R.K., 2013. Pruning affects fruit yield and postharvest quality in mango (Mangifera indica L.) cv. Amrapali. Fruits 68, 367–381. Avilan, L., Martinez, G., Marin, R.C., Rodriquez, M., Ruiz, J., Escalante, H., 2003. Square and pyramidial pruning effects on mango production. Agron. Trop. 53, 239–257. Banon, S., Gonzalez, A., Cano, E.A., Franco, J.A., Fernandez, J.A., 2002. Growth, development and colour response of potted Dianthus caryophyllu cv. Mondriaan to paclobutrazol treatment. Sci. Hort. 94, 371–377. Bayat, S., Sepehri, A., 2012. Paclobutrazol and salicylic acid application ameliorates the negative effect of water stress on growth and yield of maize plants. J. Res. Agric. Sci. 8 (2), 127–139. Belakbir, A., 1998. Yield and fruit quality of pepper (Capsicum annu L.) in response to bio regulators. HortScience 33, 85–87. Bhat, M.G., 2005. Evaluation of Genotypes and Hybrids and Technique of Hybridization. Experimental Manual on Cashew, pp. 19–21. Berova, M., Zlatev, Z., 2000. Physiological response and yield of paclobutrazol treated tomato plants (Lycopersicon esculentum Mill.). Plant Growth Regul. 30, 117–123. Blaikie, S.J., Kulkarni, V.J., Muller, W.J., 2004. Effects of morphactin and paclobutrazol flowering treatments on shoot and root phenology in mango cv.KEnsington Pride. Sci. Hort. 101, 51–68. Blanco, A., 1988. Control of shoot growth of peach and nectarine trees with paclobutrazol. J. Hortic. Sci. 63 (2), 201–207. Browning, G., Kuden, A., Blanke, P., 1992. Site of (2rs, 3rs) paclobutrazol promotion of axillary flower initiation in pear cv. Doyenne du Comice. J. Hortic. Sci. 67 (1), 121–128. Budhianto, B., Hampton, J.G., Hill, M.J., 1994. Effect of plant growth regulators on a white clover (Trifolium repens L.) seed crop II. Seed yield components and seed yield. J. Appl. Seed Prod. 12, 53–58. Burondkar, M.M., Gunjate, R.T., 1993. Control of vegetative growth and induction of regular and early cropping in’ Alphonso’ mango with paclobutrazol. Acta Hortic. 341, 206–215. Chaney, W.R., 2003. Tree Growth Retardants: arborists discovering new uses for an old tool. Tree Care Industry Magazine. 54, 2–6. Chattopadhyay, N., Ghose, S.N., 1994. Studies on the effect of time and extent of pruning in increasing the yield of cashew. J. Plant. Crops 22, 111–114. Dalziel, J., Lawrence, D.K., 1984. Biochemical and biological effects of kaurene oxidase inhibitors, such as paclobutrazol. In: British Plant Growth Regulator Group.

Chla chlorophyll a, Chlb chlorophyll b, Tchl total chlorophyll, ffl female flower, fst number of fruit set, nif number of inflorescence, nnt number of nuts and sr sex ratio.

The results of backward multiple regression analysis revealed that chlorophyll a, chlorophyll b and total chlorophyll are the important physiological traits which play a major role in predicting yield (Table 10). Among the reproductive traits, hermaphrodite flowers, fruit set, number of inflorescence, nut number and sex ratio are the contributing traits. However, among physiological and reproductive traits, nut number and total chlorophyll had major contribution in predicting nut yield. 4. Conclusions The present study revealed that PBZ treatments are effective in arresting vegetative growth and promoting reproductive growth of cashew. The PBZ treatments altered cashew tree physiology through reduction in vegetative growth, enhancement of flowering, production of more fruits and more fruit set due to efficient distribution of photosynthates, enhanced total leaf chlorophyll contents and increased leaf photosynthesis. These ultimately resulted in enhanced nut yield. Therefore, the findings of our data may provide useful insights on finding solutions to tackle low productivity of cashew by proper regulation of endogenous growth hormones that can relate to enhanced nut yield. In addition, these findings may also throw light on induction of the desired physiological effects in cashew trees that can help in modifications of canopy growth and tree vigor. These in turn can be exploited well under the HDP system to harness early benefits with enhanced yield in cashew. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to 9

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