Torrubiella pruinosa , a teliomorph of an entomopathogenic fungus Hirsutella versicolor of mango hopper ( Idioscopus clypealis ) from India

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Assessment of aphid infestation levelsin some cultivars of mustard withvarying defensive traitsFarha Rehmana, Fareed A. Khana & Shoeba B. Anisb

a Environmental Botany Research Laboratory, Department ofBotany, Aligarh Muslim University, Aligarh, Indiab Department of Zoology, Aligarh Muslim University, Aligarh, IndiaPublished online: 09 Dec 2013.

To cite this article: Farha Rehman, Fareed A. Khan & Shoeba B. Anis , Archives Of PhytopathologyAnd Plant Protection (2013): Assessment of aphid infestation levels in some cultivars ofmustard with varying defensive traits, Archives Of Phytopathology And Plant Protection, DOI:10.1080/03235408.2013.860724

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Assessment of aphid infestation levels in some cultivars of mustardwith varying defensive traits

Farha Rehmana*, Fareed A. Khana* and Shoeba B. Anisb

aEnvironmental Botany Research Laboratory, Department of Botany, Aligarh Muslim University,Aligarh, India; bDepartment of Zoology, Aligarh Muslim University, Aligarh, India

(Received 23 October 2013; accepted 26 October 2013)

Phloem-sucking mustard aphid (Lipaphis erysimi) is a major pest of mustard(Brassica juncea). Pot experiment with randomised block design was conducted withfive replicates of each of five cultivars (Alankar, Pusa Jai Kisan, Rohini, Sakha andVaruna) of the mustard for their degree of inherited resistance and/or susceptibility tothe mustard aphid infestation. Forty-five days old (from date of sowing) pot-grownplants of all selected cultivars of mustard were exposed to 40 adult mustard aphids.The aphid-infested plants were kept in specially designed net houses of fine mesh toprotect from predators and/or migration of aphids from one to other host. The aphidpopulation and some growth attributes of the selected cultivars of mustard wererecorded 15 and 30 days later (i.e. at 60 and 75 days after sowing). The aphidpopulation multiplied more rapidly on Rohini than other four cultivars. CultivarAlankar resisted most and supported to least number of aphid’s off-springs.Statistically analysed growth attributes (fresh plant mass, dry plant mass, protein,chlorophyll and carotenoid contents), resistance attribute (proline) and populationdemography of aphids revealed that some inherited characteristics of avoidance,antibiosis and repellence to herbivores helped cultivar Alankar to excel despite equaldegree of aphid attacks as on other cultivars. Cultivar Rohini for the want of suchresisting factors remained vulnerable to aphid herbivory. These two cultivars(Alankar and Rohini) form good research material for comparative studies on plantdefences to herbivory and a tri- trophic resistance through volatile chemicalsignalling.

Keywords: Brassica juncea; Lipaphis erysimi; growth analysis; proline; protein

Introduction

Mustard is a major oil crop and accounts for 10% of the total world oil production(Downey & Rimmer 1993). Mustard crop has gained wider acceptance among farmersfor its adaptability to irrigated and rain-fed areas as well. Brassica juncea is the majoroil seed crop in entire Indian sub-continent (Prakash 1980). Infestation of mustard cropwith insect pest is an important qualitative and quantitative yield-limiting factor (Singh2010). Among all growth stages (vegetative to seed setting), the flowering is the mostvulnerable stage of crop to aphid infestation (Singh 2010). Lipaphis erysimi (Kalt.) is avoracious phloem sap-sucking and mustard crop-destructive pest. The nymphs andadults of the mustard aphids live on the phloem sap from vegetative to flowering andseed-setting stages of the crop. Severe aphid infestation leads to excessive drain of

*Corresponding authors. Email: [email protected] (F.A. Khan), [email protected](F. Rehman)

© 2013 Taylor & Francis

Archives of Phytopathology and Plant Protection, 2013http://dx.doi.org/10.1080/03235408.2013.860724

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photosynthates from the hosts by their stylet at the cost of key growth function and cropyield (Mandal et al. 1994; Shahjahan 1994; Swati 2005; Bak et al. 2013; Louis & Shah2013) Owing to the preferences of B. juncea by Indian growers, several cultivars withvariable yield and/or disease resistance attributes have been developed for differentclimatic regimes.

The aphid resistance ability of most cultivars depends on cultivar level variations in(a) genetic makeup, (b) biochemical profile and (c) chemical defence package. All thethree factors are inter-depended. The sequestering of volatile defence chemical (to wardoff attacking herbivore) is a complex biochemical process triggered by specific set ofgenes (Farha-Rehman, Khan, & Badruddin 2012; Farha-Rehman, Khan, & Anis 2012).The host plants, attacking herbivores and plant mustard predator have co-evolved for acontrolled partitioning of photosynthates (energy) from plants to herbivores and to carni-vores for just sustainable ecosystem functioning. On insect attack, the host plants withspecific genes switch on to the biochemical process for sequestering different blends ofvolatile chemicals to (a) ward off the herbivores and/or (b) signal-specific predator/ para-site for attacking herbivore or plant pests (De Moraes et al. 2001; Farha-Rehman et al.2010; Farha-Rehman, Khan, & Badruddin 2012; Farha-Rehman, Khan, & Anis 2012).The crop growers may avoid excessive use of pesticides on proper selection of ecosystemfriendly cultivars. In the present study, five common cultivars of B. juncea cvs Alankar,Pusa Jai Kisam, Sakha, Varuna and Rohini have been investigated for their inheritedtraits of relative resistance and vulnerability to the attack of a common mustard aphid (L.erysimi).

Materials and methods

Seeds of five cultivars of B. juncea L. (Czern. & Coss.) namely, cvs. Alankar, Pusa JaiKisan, Rohini, Sakha and Varuna were sown in 9-inch pots (five replicate for each vari-ety) filled with farmyard manure and soil (1:6 ratio). The pots were arranged in com-plete randomised block design in fine mesh net houses. Forty-five days old plants (fromdate of sowing) were infested with 40 adult aphids. The selected growth attributes (plantlength, fresh mass and dry mass, chlorophyll contents and carotenoid contents, proteincontent), resistance attribute (proline accumulation) and increase in aphid population (oneach cultivars) were recorded 15 and 30 days later (i.e. at 60 and 75 days after sowing,respectively). Control plants (without aphid infestations) were also maintained (data notshown in the results).

Aphid population

The aphid population multiplied at 60 and 75 days after sowing (DAS) was determined.The census of aphids was grid based. The total infected length of stem and inflorescencewas multiplied with the respective average (five replicates) number of aphids per inchof infected portion. The average number (five replicates) of aphids cm−2 of leaf (bothsides) was multiplied with the average leaf area of the plant.

Estimation of leaf pigments

The chlorophyll (a, b and total) and carotenoids contents were determined in fresh leaftissues following the method of Lichtenthaler and Buschman (2001). The opticaldensities (OD) of pigment solution were read on a spectrophotometer (UV-1700,Shimatzu, Japan) at wavelengths 645 and 663 nm for substituting in formulae of

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chlorophyll estimates and OD values at 480 and 510 nm for determination of carotenoidcontents.

Figure 3. Regression line showing the correlation coefficient between per cent variation in plantgrowth and biochemical parameters vs herbivore population. viz., (A) Plant length, (B) Plant dryweight, (C) Carotenoid content, (D) Protein content, (E) Plant fresh weight, (F) Total chlorophylland (G) Proline content at 60 DAS.

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Figure 4. Regression line showing the correlation coefficient between per cent variation in plantgrowth and biochemical parameters vs herbivore population. viz., (A) Plant length, (B) Plant drymass, (C) Carotenoid content, (D) Protein content, (E) Plant fresh weight, (F) Total chlorophylland (G) Proline content at 75 DAS.

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Estimation of protein and proline

The proline content in fresh leaf samples was estimated by the method of Bates et al.(1973). The absorbance of the toluene layer was read at 528 nm, on thespectrophotometer. The protein was estimated following the method of Lowry et al.(1951) in 50 mg of oven-dried leaf powder using folin phenol reagent. Absorbance ofthe protein solution was read at 660 nm.

Statistical analysis

The data were analysed statistically using SPSS 17.0 (Chicago, USA) for variance. Themeans were compared using Duncan Multiple Range Test (DMRT, α = 0.05).

Results

The aphid population multiplied with varying degree on different cultivars despiteuniform inoculation (40 adult aphids per plant) and caused proportionate growthimpairment at 60 and 75 days after sowing (DAS). All five selected cultivars varied inextent of growth and biochemical responses (Figures 1(B–E), 2(A–E)). Cultivar Rohiniwas most sensitive and cultivar Alankar the least in terms of growth reductions (Figures1(B–E) and 2(A–E)) and increase in aphid population (Figure 1(A)) at 60 and 75 DAS,respectively. On the basis of relative sensitivity, all five selected cultivars may bearranged in decreasing order of sensitivity (highest to lowest) as Rohini > Sakha >Varuna > Pusa Jai Kisan > Alankar. The increase in aphid population reduced leaf areato a far greater extent than plant dry mass and fresh mass. On the basis of the percen-tage reductions (compared with control), various growth attributes of cultivar Rohinimay be arranged in the following order of sensitivity (highest to lowest): photosyntheticpigments > leaf area > plant dry mass > fresh plant mass > protein content > shootlength > root length. The pattern of proline accumulation was in contrary to othergrowth parameters (Figure 2(E)). The proline accumulation was highest in cultivarAlankar and lowest in Rohini (Figure 2(E)). The aphid population was minimum incultivar Alankar and maximum in Rohini (Figure 1(A)).

The correlation coefficients and linear regression lines between percentagereductions in dependent growth estimates (plant height, fresh and dry plant mass, leafarea, total chlorophyll content and protein contents) and independent variable (percent-age increase in aphid population) of all five selected cultivars showed high degree ofcorrelation and a steep straight line (Figure 3(A–E)). In contrary, reverse but signifi-cantly high degree of correlation exists between percentage increase in proline contentand percentage increase in aphid population (Figure 4(G)).

Discussion

Aphid infestation damages the cell membrane around the area of injury. The loss ofmembrane permeability results into the spill of cell water and vital contents (Walling2000; Louis & Shah 2013). In the present study, growth of plant (plant length, leaf area,fresh plant mass, dry plant mass, chlorophyll content and protein content) decreasedin proportion to the increase in aphid population in all five selected cultivars(Figure 1(A–E)). Proteins play a role in eliciting the response of resistant plants (Elzinga& Jander 2013). The growth attributes had a high degree of correlation with the aphid

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population (Figure 3(A)). Aphids are phloem suckers and their water as well as nutrientrequirements are fulfilled through the consumed phloem sap. The cultivars with highpopulation densities of aphids partitioned proportionately high quantities of phloem sap(water and photosynthates) to herbivorous aphids as is evident from correlation studies.On aphid attacks, the selected cultivars suffered from abundant availability of photosyn-thates for cell division and adequate water required for the cell turgidity during cellexpansion. The correlation coefficients and linear regression between percentagereduction in fresh plant mass (Figures 3(A), 4(A)) and percentage increase in aphidpopulation (Figure 1(A)) explicitly explains the loss of water from plant tissues viaphloem sap consuming aphids led to water stress in plant tissues (Khattab 2007; Sadeket al. 2013).

The correlation and straight linear regression between reduction in plant dry weight(Figures 3(B), 4(B)) and increase in aphid population established that growing aphidpopulation consumed excessive quantity of photosynthates and resulting carbon defi-ciency suppressed cell division. Walling (2000) reported that phloem sap herbivoryadversely affected the plant productivity (Bak et al. 2013; Sadek et al. 2013)

The presumption (as above) that phloem sap directly consumed by aphids causedwater stress in the plant tissues is further supported from statistically analysed relation-ship between accumulation of proline content (inversely proportional) (Figure 2(E)) andaphid population in varying cultivars. The proline in water-stressed conditions acts aswater-stress adjuster in plants (Khattab 2007).

In the present study, photosynthetic pigments, leaves and plant mass of all selectedcultivars were more severely affected than roots. It may be due to severe cell damagecaused by aphid in leaves, stem and inflorescence. The minute loss to roots may havebeen due altered carbon partitioning and loss of carbon in above ground plant parts.Aphid infestation reduced chlorophylls (Figure 2(A–D)), proteins (Figure 2(F)) andother growth attributes (Figure 1(B–E)) in all cultivars of B. juncea (El-Khawas &El-Khawas 2008). Insect infestations in wheat crop inhibited chlorophyll biosynthesis(Heng-Moss et al. 2003).

Usually, dose–response variables are in common practice of correlation studies. Theresponses of one species to varying doses of aphids establish relationship at individualspecies level only. The cultivars of any single species differ in the inherited traits forresistance, defensive strategies, growth and yield potentialities. The treatment of fivecultivars with single aphid inoculum level (40 aphids per plant) and correlation ofcultivar–response variables establish a community level response. Such studies andanalysis help in determining the relative resistance of cultivars to a given stress-likeaphid infestation. The response variations are attributable to variation in genetic traits ofeach cultivars to defence against a given species of aphid.

AcknowledgementWe acknowledge the Chairman, Department of Botany, Aligarh Muslim University for necessaryfacilities. University Grant Commission, New Delhi for financial assistance to F-R.

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