Cowpea Accessions Research Articles

Genetic structure at the isocitrate dehydrogenase and malate dehydrogenase enzyme gene loci in cowpea (<i>Vigna unguiculata</i> (L) Walp) accessions

Genetic structure at the isocitrate dehydrogenase and malate dehydrogenase enzyme gene loci was studied in nine cowpea accessions by applying starch gel electrophoresis. Fifty-five individual plants were sampled from each accession and used for the study. Six loci were scored for the two enzyme systems. Mean observed and expected heterozygosities were 0.231 +0.03 and 0.653~0.02, respectively. F,s estimate withi; accessions ranged from -1.00 to 1.00 (mean of 0.488~0.04). F,s and FITestimates among accessions ranged from 0.046 to 0.886 (mean of 0.592+0.07) and from 0.05 to 0.844 (mean of 0.580~0.08). FSTestimates ranged from 0.028 to 0.190 (mean of 0.075~0.02). Mean geneflow and outcrossing rates ranged from 8.37 to 55.75 (mean of 24.14~5.51) and from 11.20 to 80.30 (mean of 37.84~7.51), respectively. The most diverse alleles were Idh2-4 (FST =0.530) and Idh2-5 (FST=0.32). The accessions differed in their relative proportions of alleles and genotypes. The cowpea accessions studied showed evidence of evolutionary changes at the enzyme gene loci. Overall gene flow among the accessions indicates sufficient geneflow to prevent local differentiation. A mixed mating system was observed. Original scientific paper. Received 7 Mar 03; revised 16 Apr 03.

Genetic diversity in cowpea [Vigna unguiculata (L.) Walp.] as revealed by RAPD markers

The present study, using RAPD analysis, was undertaken to characterize genetic variation in domesticated cowpea and its wild progenitor, as well as their relationships. The materials used consisted of 26 domesticated accessions, including accessions from each of the five cultivar-group, and 30 wild/weedy accessions, including accessions from West, East and southern Africa. A total of 28 primers generated 202 RAPD bands. One hundred and eight bands were polymorphic among the domesticated compared to 181 among wild/weedy cowpea accessions. Wild accessions were more diverse in East Africa, which is the likely area of origin of V. unguiculata var. spontanea. Var. spontanea is supposed to have spread westward and southward, with a loss of variability, loss counterbalanceed in southern Africa by introgressions with local perennial subspecies. Although the variabilty of domesticated cowpea was the highest ever recorded, cultivar-groups were poorly resolved, and several results obtained with isozyme data were not confirmed here. However primitive cultivars were more diverse than evolved cultivars, which still suggests two consecutive bottlenecks within domesticated cowpea evolution. As isozymes and AFLP markers, although with a larger number of markers, RAPD data confirmed the single domestication hypothesis, the gap between wild and domesticated cowpea, and the widespread introgression phenomena between wild and domesticated cowpea.

Seed weight and protein and tannin contents of 32 cowpea accessions in Ghana

AbstractCowpea seeds of 32 accessions had 100‐seed weight of 9.0 to 15.3 g and protein and tannin contents 16.4 to 27.3% and 0.12 to 2.38, respectively. Seeds that had the highest tannin levels were either brown mottled, dark mottled, brown or flesh‐coloured. There was a significant negative correlation between 100‐seed weight and protein content. Copyright © 2004 Whurr Publishers Ltd

Resistance in accessions of cowpea to the coreid pod-bug Clavigralla tomentosicollis (Hemiptera: Coreidae).

Laboratory and screenhouse experiments were conducted to identify antibiosis and tolerance in four wild accessions of cowpea, Vigna unguiculata subsp. dekindtiana, to Clavigralla tomentosicollis Stål, the most damaging pod-sucking bug on cultivated Vigna genotypes in West Africa. The wild accession TVnu 151 showed antibiosis resistance, causing >50% mortality of the nymphs within 3 d of placing them on pods. Nymphs died more quickly on TVnu 151 than on TVnu 72, the wild and resistant control of the V. vexillata species which affected the weights of surviving insects to a much greater degree than TVnu 151. The three other accessions of the subspecies dekindtiana (TVnu 369, TVnu 517, and TVnu 707) did not cause significant mortality to the bugs, but rather extended their developmental time, with surviving adults showing lower weights and slower oviposition rates than those on the susceptible control IT84S-2246. Wild accessions affected male and female bugs differently, resulting in differential survival. This was reflected by the sex ratio which was male biased on the wild accessions (1:0.3-1:0.9), and female biased on IT84S-2246 (1:1.5). No evidence of tolerance was found in the four accessions of V. unguiculata subsp. dekindtiana. Overall, seed traits seemed to be the major resistance component in these wild accessions.

Variation in traits associated with chilling tolerance during emergence in cowpea germplasm

Warm-season annuals are sensitive to chilling temperatures. For cowpea, chilling tolerance during emergence has been associated with two independent seed traits: a specific dehydrin protein and slow electrolyte leakage in chilling conditions as a measure of membrane integrity. We screened F 2 and backcross populations for the presence of the dehydrin protein in seed using immunoblot analysis. Segregation of the dehydrin protein indicated a single dominant gene controls its expression. We screened four sets of cowpea accessions with diverse origins for the two traits that had been associated with chilling tolerance and the presence of hard seed that did not readily imbibe water. The hard seed trait also may enhance chilling tolerance since slower imbibition of water at low temperature can reduce chilling injury. We tested the hypothesis that the extent of these three chilling-tolerance traits may vary for germplasm populations of subtropical and tropical origins because tropical environments exhibit little chilling. The dehydrin protein was present in many landraces (59% out of 140) that evolved in subtropical zones around the Mediterranean sea where soils often are cool during emergence. However, the dehydrin protein also was present in most of the tropical accessions (88% out of 59), which suggests that it may have an additional adaptive role during seed maturation and/or germination since chilling tolerance would not be of significance in the tropics. Only three out of 61 US cultivars, which had been developed in subtropical zones where soils can be cool, contained the dehydrin protein. Only four out of 40 lines in a population of tropical origin that had been selected for chilling tolerance during vegetative and reproductive growth contained the dehydrin protein in their seed. Genotypes were identified that had similar slow electrolyte leakage from seeds at low temperature as the chilling tolerant line ‘UCR 1393-2-11’, but none of these accessions also contained the dehydrin protein as does ‘UCR 1393-2-11’. Frequency of hard seeds was high among Mediterranean landraces (48%) and US cultivars (21%) but low among tropical accessions (5%). We describe genotypes with traits associated with chilling tolerance during seedling emergence that could be useful in cowpea breeding.

GENETIC DIVERSITY IN COWPEA (Vigna unguiculata) USING ISOZYME ELECTROPHORESIS

Accessions of cultivated cowpea (V. unguiculata ssp. unguiculata cv. gr. unguiculata, cv. gr. sesquipedalis and cv. gr. biflora) from different countries were evaluated for isozyme variability. Zymograms were obtained by polyacrylamide gel electrophoresis using the PhastSystem and eight enzyme systems were studied. Cowpea accessions studied were characterised by very low genetic diversity. Accessions were monomorphic for almost all loci tested except for the esterase enzyme system. Comparative analysis of esterase zymograms made it possible to identify some of the accessions studied. However, the cultivated groups biflora and sesquipedalis could not be distinguished from one another or from the cv. gr. unguiculata.

The nutritional ecology of Maruca vitrata Fabricius I. Development on flowers of wild and cultivated Vigna species

Growth, development and fecundity of Maruca vitrata Fabricius (Lepidoptera: Crambidae) were studied on artificial diet and flowers of two accessions each of three wild Vigna species, namely V. vexillata (TVnu 594 and TVnu 72), V. oblongifolia (TVnu 957 and TVnu 42) and V. unguiculata ssp. dekindtiana (TVnu 979 and TVnu 863) and six accessions of cowpea, V. unguiculata ssp. unguiculata (Moussa Local, VICAM-1, TVu 13731, IT84S-2246, IT89KD-457 and IT91K-180). The percentage survival of the larvae, larval weights as well as growth and development indices were significantly higher on the artificial diet, flowers of cowpea cultivars and those of V. unguiculata ssp. dekindtiana compared to those of V. vexillata and V. oblongifolia. The fecundity of females from larvae reared on artificial diet and flowers of cowpea was also significantly higher than that of females from the wild Vigna genotypes. All variables were included in principal component analysis. Two components accounted for 81.79%. On the basis of this analysis, the diets appeared to fall into four categories based on their suitability for supporting development of M. vitrata: unsuitable (TVnu 42 and TVnu 957), fairly suitable (TVnu 72, TVnu 594, TVnu 979, Moussa Local and TVnu 863), suitable (IT89KD-457, IT91K-180, VICAM-1, TVu 13731 and IT84S-2246), and very suitable (artificial diet). The importance of these findings in the development of host plant resistance of cowpea to M. vitrata is discussed.

Variability in some quantitative characters of cowpea (<i>Vigna unguiculata</i> (L.) Walp) landraces in Ghana

Forty-five cowpea accessions randomly selected from cowpea germplasm from four major cowpea-producing regions of Ghana, namely Upper East, Upper West, Northern and Eastern regions, were evaluated to study variability in some quantitative characters. Variability within regions was not significant, but there were significant differences among the regions for all the characters studied. The accessions from Northern Region were the first to flower and mature, while those from Upper East Region were the last to flower and mature. The number of pods per plant and seed yield per plant had rather large and similar ranges. A large proportion of the observed variability in pod and seed character was due to genetic differences, especially for 100-seed weight. However, the lowest variability among accessions, except for the 100-seed weight, was recorded for the Eastern Region. Accessions from Upper East Region had the largest variability for most of the characters. This identifies the Upper East Region as a potential source of material for cowpea improvement. Variabilité en quelques caractères quantitatifs des variétés des dolique (Vigna unguiculata (L.) Walp) au Ghana Résumé Quarante-cinq accessions de dolique sélectionnées au hasard de germeplasme de dolique de quatre principales régions productrices de dolique au Ghana, à savoir, les regions d'Upper East, Upper West, Northern et Eastern étaient évaluées pour étudier la variabilité en quelques caractères quantitatifs. La variabilité à l'intérieur des régions n'était pas considérable, mais il y avait des différences considérables parmi les régions pour tous les caractères étudiés. Les accesions de Northern Region étaient les premières à fleurir et mûrir, alors que celles d'Upper East Region étaient les dernières à fleurir et mûrir. Le nombre de cosse par plante et le rendement de graine par plante avaient plutôt des variations grandes et semblables. Une grande partie de la variabilité observée dans les caractères de cosse et de graine était provoquée par des différences génétiques surtout pour le poids de 100-graine. Toutefois, la plus basse variabilité parmi les accessions, à l'exception du poids de 100-graine, était enregistrée pour Eastern Region. Les accessions d'Upper East Region avaient la plus grande variabilité pour la plupart des caractères. Ceci identifie Upper East Region comme une source potentielle de matiriel pour l'amélioration de dolique. Ghana Jnl agric. Sci. Vol.33(2) 2004: 165-170

Identification of suitable seed regeneration sites for photoperiod‐sensitive cowpea germplasm

To determine an appropriate location to regenerate photoperiod‐sensitive cowpea [Vigna unguiculata (L.) Walp.], seed production and virus contamination data were measured at Isabela, PR, St. Croix, VI, and Griffin, GA for six photoperiod‐sensitive cowpea accessions. The results indicate that Isabela was best suited for seed regeneration of photoperiod‐sensitive cowpea. Seed production at Isabela was equal to or greater than that at either of the other two locations for these accessions. Virus infection of plants and of seeds was high at Griffin. However, little virus infection occurred at Isabela and at St. Croix. The systematic approach used in these tests can be used as a model for curators in the National Plant Germplasm System and others to determine the best environment in which to regenerate germplasm.

Genetic relationships among subspecies of Vigna unguiculata (L.) Walp. based on allozyme variation

The cowpea [Vigna unguiculata (L.) Walp.] is a morphologically and genetically variable species composed of wild perennial, wild annual, and cultivated forms that are mainly used for edible seeds and pods. In this study, genetic variation in 199 germplasm accessions of wild and cultivated cowpea was evaluated using an allozyme analysis. The results from this survey showed that wild cowpea exhibits genetic variation perfectly fitted with the existing morphological classification. The cowpea gene-pool is characterized by its unusually large size. It encompasses taxa (ranked as subspecies) that could be considered as different species considering the high genetic distances observed between accessions belonging to different taxa. These subspecies can be classified into three groups characterized by their breeding systems: perennial outcrossers, perennial out-inbreds, and inbred annuals. Allozyme data confirm this grouping. Perennial outcrossers look primitive and are more remote from each other and from perennial out-inbreds. Within this large gene-pool, mainly made of perennial taxa, cultivated cowpeas (ssp. unguiculata var. unguiculata) form a genetically coherent group and are closely related to annual cowpeas (ssp. unguiculata var. spontanea) which may include the most likely progenitor of cultivated cowpeas.

Variability studies in some qualitative characters of cowpea (<i>Vigna unguiculata</i>) (L.) Walp) accessions from four cowpea-growing regions of Ghana

Forty-five cowpea accessions, randomly selected from cowpea germplasm from four cowpea-growing regions Of Ghana, namely, Upper East, Upper West, Northern, and Eastern Regions, were characterized and evaluated to determine the range of variability in qualitative characters in the samples. Variability was observed in growth habit, twining tendency, plant and pod pigmentation, and raceme position. Accessions from the Upper West, Northern and Eastern Regions showed three types of growth habit: prostrate, semi-prostrate and intermediate, with the semi-prostrate type in the majority, whilst and Upper East Region showed only the semi-prostrate type. Accessions from the Northern and Eastern Regions showed three types of twining tendency, namely, no twining, slight, and intermediate twining, whilst the Upper East Region accessions showed only none twining types. Majority of the accessions showed the no twining type. All the regions showed great variability in plant and immature pod pigmentation in the cowpea accessions. Two types of flower colour were observed, purple and white. With the exception of the Upper East Region accessions which showed only white flower colour, accessions from all the other regions showed both white and purple colours with the accessions with purple colour in the majority. In most of the accessions from the Upper West, Northern and Eastern Regions, the racemes were held above the canopy. Ghana Jnl.agric. Sci Vol.32(1), 1999: 3-10

Heterosis in cowpealandraces from Ghana

Four crosses involving five local accessions of cowpea were evaluated to determine the amount of heterosis for maturity date, seed yield, and components of yield. Heterosis for seed yield plant-1 was greater than that for other characters with a high-parent heterosis of 2.4 to 72.1 per cent. Number of pods plant-1 was the most heterotic of the yield components. The cross between high-yielding parents produced the highest heterosis above the mid-parent and high parent for seed yield plant-1. Seed yield may be improved by crossing among local landraces of cowpea through improving the number of pods plant-1. Ghana Jnl.agric. Sci Vol.32(1), 1999: 27-30

Morphological study of cultivated cowpea Vigna unguiculata (L.) Walp. Importance of ovule number and definition of cv gr Melanophthalmus

More than 400 cultivated cowpea accessions were studied using 29 morphophysiological characters. The analysis of data contrasts evolved cultivars and more primitive cultivars (and wild forms) according to seed size on the one hand and early and late flowering (under inductive conditions) on the other hand. The latter character is markedly correlated with ovule number. Photosensitive and early-flowering photoindependent cultivars have 11-17 ovules per pod and late flowering photoindependent cultivars have 16-25 ovules per pod. The analysis leads to assigning the accessions studied to the following different cultigroups: cv gr Unguiculata, cv gr Sesquipedalis, cv gr Biflora and cv gr Melanophthalmus, the latter newly separated from cv gr Unguiculata. (O InraElsevier, Paris.)

Seed protein electrophoretic characterization of cowpea (Vigna unguiculata) germplasm from IITA gene bank

The total seed protein, globulin and albumin fractions of 20 cowpea (V. unguiculata) accessions from IITA gene bank were investigated by SDS-polyacrylamide gel electrophoresis. Total seed protein extracts were prepared from the defatted meal by homogenisation and centrifugation in Tris-glycine buffer. The globulins and albumins were obtained from the total protein extract by exhaustive dialysis against sodium acetate buffer. Two main total seed protein electrophoretic patterns were observed with respect to the 39 and 20 kD subunits, which were present in six out of the twenty accessions analysed. While there was no correlation between seed colour and total seed protein banding pattern, the six insect-resistant cultivars were characterized by the presence of the 39 and 20 kD subunits. The globulins were the predominant class of the total seed proteins and consisted mainly of 64, 58, 56 and 14 kD subunits which make up CP1 and CP2, the major globulins. The albumins in all accessions were a heterogeneous protein fraction consisting of both high and low molecular weight subunits. It was suggested that the insect-resistant cultivars may be genetically related and that the 39 and 20 kD subunits may be involved in the insect resistance mechanism.

New sources of resistance of cowpea (Vigna unguiculata) toStriga gesnerioides, a parasitic angiosperm

Thirty-seven accessions of cowpea and yard-long bean were assessed for resistance toStriga gesnerioides. Cowpea plants were grown using anin vitro method, then inoculated with young seedlings ofS. gesnerioides produced from seed from three West African countries. Resistance was assessed by comparing the number and size ofS. gesnerioides tubercles on these accessions with those on a known susceptible cowpea, cv. Blackeye. Two cowpea landraces, APL-1 and 87-2, were completely resistant toS. gesnerioides from Burkina Faso, Mali and Cameroon and partially resistant toS. gesnerioides from Niger. Complete resistance was expressed either as a hypersensitive response of infected root tissues or as a severely retarded development of successful infections. All other accessions, including three samples of yard-long bean were susceptible toS. gesnerioides. The original 87-2 plants segregated for resistance and susceptibility. However, uniformly resistant progeny were obtained by producing seed from vegetatively propagated clones of single resistant 87-2 plants. Resistance of APL-1 and 87-2 toS. gesnerioides was confirmed in pot and field trials. Neither of these cowpeas were resistant toAlectra vogelii. Varieties APL-1 and 87-2 provide additional sources of resistance to most races ofS. gesnerioides, including a newly discovered virulent race from Benin.

Drought and Pot Size Effects on Transpiration Efficiency and Carbon Isotope Discrimination of Cowpea Accessions and Hybrids

Carbon isotope discrimination (Δ) has been proposed as a selection criterion for improving adaptation to water-limited environments because it provides a measure of seasonal transpiration efficiency ( WUE). In cowpea (Vigna unguiculata (L.) Walp.), consistent genotypic and drought-induced differences in Δ and WUE have been observed which were correlated as expected based on theory. Values of Δ and WUE for reciprocal hybrids grown under field conditions indicated nuclear inheritance for both characters. High WUE and low Δ were partially dominant under dry field pot conditions, whereas high Δ was partially dominant under natural wet soil conditions. Studies were conducted to test whether differences in rooting environment and xylem ABA levels are responsible for this change in dominance relations. Cowpea accessions and hybrids were grown in the field and subjected to wet or dry treatments with three different pot sizes. The experiment was conducted twice, giving similar results. The dry treatment resulted in decreases in Δ, and increases in WUE and ABA concentration in the xylem sap. Under drought, genotypes with higher WUE had higher xylem ABA, and the hybrids exhibited greater increases in ABA concentration in response to the dry treatment than either parent. Partial confounding was present in that the hybrids had substantial leaf area and water-use rate, and may have experienced greater soil drought in some conditions than some parents, with interacting effects of pot size. Plants in larger pots produced more biomass and leaf area but with no changes in xylem ABA. Concentration of ABA in the xylem sap was correlated with Δ and WUE for genotypic and drought treatment effects but not for pot size effects. Hybrids tended to have higher Δ and lower WUE in relation to mid-parent means when grown in large wet pots than in small dry ones. Changes in hybrid performance with respect to Δ and WUE were more consistent with changes in xylem ABA for drought treatment effects than for pot size effects. Another chemical signal might be involved in mediating pot size effects.

Carbon Isotope Discrimination and Gas Exchange of Cowpea Accessions and Hybrids

Carbon isotope composition may be useful for selection in plant breeding. Theory predicts a negative association between 13C discrimination (Δ) and the ratio of CO2 assimilation (A) to leaf diffusive conductance (g) in C3 plants. In cowpea [Vigna unguiculata (L.) Walp.] previous studies showed that the association between Δ and gas exchange efficiency (A/g) agreed with theory for drought‐induced effects, but not for genotypic effects when gas exchange was measured in the afternoon. Field studies were conducted for 2 yr to evaluate the association between Δ and A/g measured during different time periods within the day. Four cowpea genotypes were grown either on weekly irrigation or stored soil moisture in 1 yr, and under weekly irrigation, together with two hybrids between low and high Δ parents, in the second year. Cultivars CB46 and Prima had higher leaf Δ than genotypes UCR 237A and TVx 309‐1G with different irrigation treatments, sampling dates and years. The hybrids had values of Δ and gas exchange close to their midparent means. A negative correlation was observed between Δ and A/g for both drought‐induced and genotypic effects. Genotypic mean values of Δ were more strongly correlated with A/g when measured early in the morning than in the afternoon. Both A and leaf conductance to water vapor (gh) were significantly higher early in the morning and then decreased progressively towards the end of the day. Early morning measurements were most effective in detecting genotypic differences in A/g, and they were associated with genotypic differences in Δ, as expected, based on theory, but with some unexplained variation. Low Δ was associated with high photosynthetic rate for TVx 309‐1G, but low stomatal conductance for UCR 237A.

Isozyme Diversity in the Cowpea Species Complex

The cowpea [Vigna unguiculata (L.) Walp] is an important crop in many countries of tropical Africa, Asia and South America. Compared with other leguminous crops, little is known about the population structure of the cowpea and the partitioning of genetic diversity between cultivated and wild cowpea. One hundred and twelve accessions of cultivated cowpea (V. unguiculata ssp. unguiculata) and 43 of wild cowpea (V. unguiculata ssp. dekindtiana) were evaluated for genetic diversity at 26 isozyme loci. Cultivated accessions were characterized by very low genetic diversity (Ht = 0.029), with only six polymorphic loci. Four of these polymorphic loci displayed one very common allele and one rare allele. Accessions possessing these rare alleles were dispersed throughout Africa and a geographical center of isozyme diversity was not evident. Cultivated groups biflora (the catjang) and sesquipedalis (the yard‐long bean) could not be differentiated from the cowpea. Wild cowpea, on the other hand, were highly diverse (Ht = 0.168), with 19 out of 26 loci polymorphic. Our data indicate introgression from wild to cultivated must be very rare. Six wild accessions displayed high identity with the cultivated cowpea. Wild cowpea may, therefore, be the progenitor of the cultivated cowpea. Because these wild accessions were dispersed throughout equatorial Africa, there is no isozyme evidence for a narrowly defined center of domestication.

Inheritance of Carbon Isotope Discrimination and Water‐Use Efficiency in Cowpea

Theory has been developed predicting an association between water‐use efficiency (WUE = total biomass/transpiration) and leaf discrimination against 13C (Δ), which could be used to indirectly select for WUE in C3 plants. Previous studies indicated variation in WUE and Δ among genotypes of cowpea [Vigna unguiculata (L.) Walp.] and due to drought. Moreover, a highly significant negative correlation between WUE and Δ was observed for both genotypic and drought effects, as expected based on theory. Present studies were conducted to investigate whether the inheritance of WUE and Δ is nuclear or maternal, and whether any dominance is present. Contrasting cowpea accessions and hybrids were grown over 2 yr in two outdoor pot experiments, subjected to wet or dry treatments, and under full irrigation in natural soil conditions in 1 yr. Highly significant differences in WUE were observed among cowpea parents and hybrids, and due to drought, which were strongly and negatively correlated with Δ, as expected based on theory. Data from reciprocal crosses indicated that both WUE and Δ are controlled by nuclear genes. High WUE and low Δ exhibited partial dominance under pot conditions. In contrast, high Δ was partially dominant for plants grown under natural soil conditions but in a similar aerial environment as in the pot studies. We speculate that differences in rooting conditions were responsible for the differences in extent of dominance for Δ of plants growing under pot conditions compared with natural soil conditions in a similar field aerial environment.

Protease inhibitors and lectins in cowpea

Wild and cultivated accessions of cowpea were analysed for trypsin and chymotrypsin inhibitors, and lectins in order to assess: their variability; their influence in the mechanism of Bruchid resistance; their evolution during species domestication. Wild Vigna vexillata showed significantly higher values ( P < 0·0001) for trypsin and chymotrypsin inhibitors and significantly lower values ( P < 0·025) for lectins as compared with cultivated accessions. This may imply that domestication operates an indirect selection for these characters. High degrees of correlation between chymotrypsin inhibitors and trypsin inhibitors ( r = 0·959) and significant correlation ( P < 0·01) between trypsin inhibitors and protein content were found. The high resistance to Bruchid and the high trypsin inhibitor content of V. vexillata suggest that, even where there is no direct relationship between Bruchid resistance and trypsin inhibitor content, protease inhibitors promote, or are a component of, the plant's defence mechanism.