Extent of pollination and fruit set in cashew

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Short scientific report Journal of Plantation Crops, 2011, 39 (1): 157-160

The cashew is an andromonoecious plant withmale (staminate) and hermaphrodite flowers on the sameterminal inflorescence and thus amenable forxenogamous or geitonogamous pollination. As cashewpollen grains are sticky in nature and most of them willbe held as clumped, pollination by insects is very muchcommon. For maximum fruitset, hermaphrodite flowersneed to be pollinated within 3-6 h of anthesis (Wunnachit,et al., 1992). Since maximum flowers open duringforenoon hours, visit of pollinators during late forenoonand early afternoon hours may result in high rate ofpollination and as such high level of pollination upto 98percent was reported in Australia (Heard et. al., 1990).Under these circumstances, studies were undertaken tofind out the extent of pollination occurring naturally inthe plantation without any insecticide application eventhough such studies are already reported from insecticidetreated cashew (Sundararaju, 2000, 2003 and 2004) andsubsequently exact patterns of deposit/adherence ofpollen grains on each stigma were also assessed in detail.As pollination by gravitational fall of pollen grains fromupper to lower panicle was reported in mango(Maheshwari, 1934), confirmative studies on similar lineswere made. The relationship between extend ofpollination and final fruitset was worked out in order tofind out its impact on enhancement of the productivityof cashew.

Observations were made for three cropping(flowering) seasons of 2003-04 to 2005-06 on extent ofpollination on different varieties (Vengurla-1, Vengurla-4, NRCC- Selection-2, Bhaskara, Ullal-3, Dhana, andVRI-3) grown without any insecticide application atPuttur (coastal Karnataka) and on VRI-3 at Vridhachalam(coastal Tamil Nadu) by collecting hermaphrodite flowers(minimum of 100/variety) at random from differentpanicles and examined under stereomicroscope on theday of anthesis at the evening hours to estimate the extentof pollination based on the pollen deposit on respective

Studies on extent of pollination and fruit set in cashew

(Manuscript Received: 20-12-10, Revised: 24-02-11, Accepted: 08-04-11)

Keywords: Cashew varieties, hermaphrodite flowers, geitonogamous and xenogamous pollination, pollen grains, gravitational fall, fruitset

stigma of hermaphrodite flower (Sundararaju, 2000). Forthis purpose flowers were carefully collected with forcepswithout touching the stigma with the anthers of adjacentflowers and secured it in erect position in a flower rack.The flower rack was designed by covering open portionof 10 mm Petri dish (only top of the Petri dish was used)with mosquito net cloth. In the mosquito net cloth, regularspacing was created by cutting mesh threads in order tosecure the individual flower in erect position withouttouching the adjacent flower (Fig. 1) and water was filledto keep the collected flowers in turgid condition. Afteraccommodating 25 -30 flowers/ rack, the flower rack wasclosed by another top of Petri dish and secured tight withrubber band till the flowers were observed undermicroscope. At the time of microscopic observation, thepattern of pollen grains deposited was recorded and

Fig.1. Flower rack and forceps used for collection of flowers

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categorized as single or cluster of two or more than twopollen grains out of total pollen grains deposited on eachstigma.

Whole panicles of VRI-3 and Vengurla-4 varietieswere caged for 60 days individually just before onset offlowering to exclude insect visitors and gently shakenevery day at 14.00 h. and the final fruit set was recordedon 60th day after removing the cages. The gentle shakingwas done to simulate wind movement inducinggravitation fall of pollen grains. Simultaneously fruitsetwas also recorded from respective tagged uncaged panicleas natural pollination. Gravitational fall of pollen grainsfrom long stamen was estimated in the laboratorycondition on above varieties, for which bothhermaphrodite and male flowers were collectedimmediately after anthesis (before dehiscing of anthersof long stamen) and kept in erect position separately. Inthis case, the same flower rack was designed by coveringthe Petri dish with filter paper instead of mosquito netcloth and hole to accommodate the individual flowers inerect position was made without touching the adjacentflower and water was kept in the Petri dish to keep flowersin turgid condition for 24 hrs. (Fig. 2). The wholearrangement was kept without any disturbance andobservation on gravitation fall of pollen grains from erectlong stamen was recorded after 24 h.

The extent of pollination assessed by observingdeposit of pollen grains on the stigma of hermaphroditeflowers during seasons of 2003-04 to 2005-06 waspresented in Table1. The percentage of pollination rangedfrom 27.0 to 49.5 on seven varieties (Vengurla-1,

Table 1. Extent of pollination in different varieties of cashew recorded atPuttur and Vridhachalam during 2003-04 to 2005-06 flowering season

Variety Extent of pollination Deposit of pollen grains /(%)# stigma+

2003- 2004- 2005- No.±S.E % deposited04 05 06 as 1-2 grains

out of totalpollen grains

Vengurla-1* 37.0 40.8 48.3 2.72±0.41 66.0 (1-15)Vengurla-4* 49.0 46.7 38.8 2.46±0.33 65.7 (1-8)Bhaskara* 42.3 44.4 46.4 2.49±0.28 63.3 (1-10)NRCC Selection-2* 37.2 49.5 35.1 2.06±0.19 73.7 (1-6)Ullal-3* 28.7 36.2 27.0 3.09±0.49 66.7 (1-7)Dhana* 27.7 30.3 36.2 1.90±0.35 84.9 (1-10)Vridhachalam-3* 28.4 41.7 49.4 2.90±0.39 66.7 (1-9)Vridhachalam-3** 16.9 18.8 N.R N.R N.R

# Based on observation of minimum of 100 bisexual flowers/variety through stere-oscopic dissection microscope; * Recorded at Puttur; **Recorded at Vridhachalam+ Recorded during 2005-06; N.R- Not recorded; Figures in brackets are range ofpollen grains deposited per stigma

Table 2. Extent of pollination on VRI-1 cashew variety recorded during 2007-08 and 2008-09

Flower Extent of Mean± S.E Range (pollens/types pollination (%)# (pollens/stigma) stigma)

2007-08 2008-09 2007-08 2008-09 2007-08 2008-09

Before DA 16.2 46.8 2.69± 2.14± 1-9 1-7(11.00- 0.58 0.3912.00 hr)*

After DA 36.8 40.7 2.93± 4.00± 1-7 1-17(12.00- 0.37 1.4213.00 hr)*

Evening 46.6 54.6 3.26± 3.48± 1-15 1-15collected 0.48 0.47(17.00 hr)**

# Based on observation of minimum of 100 bisexual flowers/variety through stere-oscopic dissection microscope; *Flowers visited by Apis cerana indica;** Flowers collected at random; DA- dehiscing of anther of long stamen

Fig.2. Full components of flower rack used for gravitational fall

To assess of the final fruit set, hand pollinationwas done in Bhaskara variety during 2006-07. For thispurpose, the bisexual flowers were manually pollinatedevery day and such flowers were tagged with the help ofsewing thread. The final fruit set was assessed between35 and 40 days of pollination and by the time the nutswere in matured green nut stage. During the month ofDecember 2006, a total of 464 flowers were manuallypollinated within a period of 14 days. Finally, a correlationwas worked out between number of flowers pollinatedand number of green nuts (final fruit set) recorded.

Vengurla-4, NRCC- Selection-2, Bhaskara, Ullal-3, Dhana,and VRI-3) without any insecticide application and it wassynonymous with earlier observation reported frominsecticide sprayed fields (Sundararaju, 2000, 2003 and2004). The confirmative studies on VRI-1 also indicatedmaximum of pollination of 54.6 percent (Table 2). Eventhough, range of pollination of 25.0 to 72.0 percent was

reported by Reddy (1993) in the east coast (AndhraPradesh), in the present studies, it could be observed in arange of 16.9 to 18.8 percent at similar major cashew

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region (Vridhachalam) of coastal Tamil Nadu (Table 1)and it may be due to pollination deficit (since it is lessthan 25.0%). However, lowest degree of naturalpollination of 18, 26 and 28 percent in Malaysia, Jamaicaand Kenya respectively was documented by Free (1993).But in Tanzania, Australia and Brazil, 78.0, 98.0 and88.0 percent pollination respectively was reported byNorthwood (1966), Heard et al. (1990) and Freitas andPaxton (1996). Considering above reports, the extent ofpollination occurring in the west coast of India appearedto be within the expected range of pollination.

The mean number of pollen grains deposited oneach stigma on different varieties also varied from 1.9 to3.1 (range: 1-15 pollen grains/stigma) and 63.3 to 84.9percent of pollen grains were deposited either as singlyor cluster of two pollen grains out of total pollen grainsdeposited on each stigma. It was again verified onVRI -1 variety during 2007-08 and 2008-09 seasons and47.1 to 74.4 per cent of pollen grains were deposited inthe form of single pollen grain and 13.22 to 29.2 per centof pollen grains were deposited in a clump of two pollengrains (Table 1 and Table 3). Report of deposit of singlepollen grain on each stigma and mean of 2.2 to 3.1 pollengrains per stigma was indicated in the document of Free

final fruit set on VRI-3 was observed from 20.8 and 64.6percent of panicles of caged and uncaged panicles of VRI-3 with 0.29 nut/panicle and 0.65 nut/panicle respectively;whereas on Vengurla-4, fruit set was observed on allpanicles of both caged and uncaged with 1.0 and 1.8 nuts/panicle respectively (Table 4). Thus it confirms that incashew, geitonogamous pollination occurs besides regular

Table 3. Pattern of pollen grain deposit on stigma out of total pollen grainsdeposited per stigma onVRI-1 cashew variety

Flower types % of pollen grain deposited as

2007-08 2008-09

Single Clump Clump Single Clump Clumpof 2 of > 2 of 2 of > 2

Before DA (11.00-12.00 hr)* 74.4 18.6 7.0 65.2 17.4 17.4After DA(12.00-13.00 hr)* 56.2 29.2 14.6 57.4 16.2 32.4Evening collected (17.00 hr)** 47.1 15.7 37.2 62.8 13.2 24.0

*Flowers visited by Apis cerana indica; **Flowers collected at random;DA- dehiscing of anther of long stamen

Table 4. Extent of fruitset recorded under experiment on gravitational fall ofpollen grains

Variety Caged panicles Uncaged panicles(gravitational fall) (natural pollination)

% panicles Mean no. nuts % panicles Mean no. nutshaving fruitset / panicle having fruitset / panicle

VRI-3* 20.8 0.29 (0-3) 64.6 0.65 (0-3)

Vengurla-4** 100.0 1.0 (1) 100.0 1.8 (1-3)

* Mean of 48 panicles;** Mean of five panicles; Figures in brackets is range ofnuts/panicle

(1993). In spite of sticky clumped nature of pollen grains,deposit of the pollen grains as single pollen grain couldoccur probably due to dispersal of pollen grains aidedthrough flight activities of various bees visiting theflowers. The extent of pollination recorded on VRI-1 aftervisit of honey bees (Apis cerana indica) clearly confirmsthat deposit of pollen grains on 16.2 to 46.8 per cent ofhermaphrodite flowers before dehiscence of anther oflong stamen was due to flight activities of bees resultingxenogamous or geitonogamous pollination. However, inspite of visit of all hermaphrodite flowers by honey bees,46.8 percent flowers only had pollen deposit (Table 2).

When the panicles of VRI-3 and Vengurla-4varieties were caged individually to exclude insectvisitors and gently shaken on every day at 14.00 hrs., the

Table 5. Gravitational fall of pollen grains among cashew varieties

Name of the % flowers had fall of pollenvariety grains from long stamen among#

Male flowers Bisexual flowers

NRCC Selection-2 0.0 18.5Ullal-3 0.0 26.9Bhaskara 0.0 9.4Vengurla-1 0.0 9.7Vengurla-4 0.0 3.9Dhana 0.0 10.8VRI-3 0.0 3.3

# Based on observation of minimum of 100 bisexual flowers/variety

insect pollination under natural condition. Whengravitational fall of pollen grains from long stamen wasestimated in the laboratory condition by keeping thenewly opened flowers in erect position for 24 hrs., it wasobserved only from hermaphrodite flowers with a rangeof 3.3 to 26.9 percent among seven varieties (Vengurla-1, Vengurla-4, Ullal-3, NRCC Sel.2, Bhaskara, Dhanaand VRI-3) (Table 5). Among male flowers of samevarieties, no such gravitational fall of pollen grains fromlong stamen could be noticed. The existence ofgeitonogamous pollination through gravitational fall ofpollen grains is in similarity with pollination of mango(Maheshwari, 1934).

The final fruit set was observed to a tune of 24.6percent on Bhaskara variety by hand pollination, whereasamong naturally pollinated flowers, the final fruit set toa tune of 10.1 percent was observed. When the correlationwas worked out between number of flowers pollinated/day within a period of 14 days and number of final fruitset observed from manually pollinated flowers/day, it was

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Directorate of Cashew ResearchPuttur (D.K.), Karnataka-574 202

D. Sundararaju

highly significant with positive correlation (r = 0.77,significant at 1 %). Thus, this study has given vitalindication that by enhancing the level of pollination, theproductivity of cashew can be enhanced, for which,adequate active insect pollinators will help rapid transferof pollens immediately after anthesis as suggested byWunnachit et al. (1992) and therefore, suitable methodsneed to be devised for enhancement of pollination.

Acknowledgement

The author thanks the Director, Directorate ofCashew Research, Puttur for providing the facilities. Healso thanks Shri. Laxmipathi, Directorate of CashewResearch for the technical assistance.

References

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Frietas, B.M. and Paxton, R.J.1996. The role of wind and insects incashew (Anacardium occidentale) pollination in NE Brazil.J. of Agric. Sci. Cambridge 126: 319-326.

Heard, T.A., Vithanage, V. and Chacko, E.K.1990. Pollination biologyof cashew in Northern territory of Australia. Aust.J. Agric.Res. 41:1101-1114.

Maheshwari, P.1934. The Indian mango. Current Sci. 3:97-98.

Northwood, P.J. 1966. Some observations on flowering and fruit-setting in cashew Anacardium occidentale L. Tropical Agric.(Trinidad) 43:35-42.

Reddy, E.U.B. 1993. Pollination studies of cashew in India: anoverview. pp.321- 324. In: Proc. Int. Symp. Polln. Trop.,(Eds.). Veeresh, G.K., Uma Shankar, R. and Ganeshaiah, K.N.,UAS, Bangalore.

Sundararaju, D. 2000. Foraging behaviour of pollinators on cashew.The Cashew 14 (4):17-20.

Sundararaju, D. 2003. Occurrence of bee fauna and extent ofpollination in insecticide sprayed ecosystem of cashew. J.Palynol. 39:121-125.

Sundararaju, D. 2004. Evaluation of promising new insecticides inlarge plots for management of tea mosquito bug on cashew.J. Plantn. Crops 32 (Suppl.):285-288.

Wunnachit, W., Pattison, S.J, Giles, L., Millington, A.J., Sedgley, M.1992. Pollen tube growth and genotype compatibility incashew in relation to yield. J. Hort. Sci. 67: 67-75.