Trichomes on Pigeonpea [Cajanus cajan (L.) Millsp.] and Two Wild Cajanus spp

thy, 1988; Peter et al., 1995). They could therefore provide a potential resistance mechanism against H. armiTrichomes have been modified in a number of crops to develop gera and other pests of pigeonpea. Bisen and Sheldrake insect-tolerant genotypes. Pigeonpea, Cajanus cajan (L.) Millsp., is often heavily damaged by insect pests, and trichomes provide a poten(1981) and Navasero and Ramaswamy (1991) studied tial insect resistance mechanism. The following study was conducted trichomes on C. cajan but no information is available to identify and characterize the distribution of trichomes on pigeonpea on the trichomes of noncultivated Cajanus spp. The and two wild species, C. platycarpus (Bentham) van der Maesen and following study was conducted to characterize and comC. scarabaeoides (L.) Thours. Three glandular (Types A, B, and E) pare the trichomes on C. cajan and two noncultivated and two nonglandular (Types C and D) trichome types were identified species, C. platycarpus and C. scarabaeoides. with light and electron microscopy. Types A, B, C, and D were found on leaves, pods, and calyxes of all three Cajanus spp., except for Type MATERIALS AND METHODS A, which was not found on pods and calyxes of most C. scarabaeoides accessions examined. Because of their small size and rarity, Type E The plant material used for this study was collected from trichomes were not considered in this study. Pods of C. scarabaeoides fieldor greenhouse-grown plants at the research station of were the most densely pubescent, followed by pods of C. cajan and the International Crops Research Institute for the Semi-Arid C. platycarpus. Trichome density on pods varied significantly among Tropics (ICRISAT), located near Hyderabad, India. pigeonpea genotypes and different accessions of C. scarabaeoides. Differences across seasons and in greenhouse versus field-grown Trichome Description plants were also significant. Leaves of C. platycarpus possessed the Trichomes on pods, leaves, and calyxes of three Cajanus fewest trichomes, while C. cajan and C. scarabaeoides had highly spp. were described from field-grown plant material from one pubescent leaves. The resistance of C. scarabaeoides pods to Helicogenotype of C. cajan (ICPL 87) and one accession each of verpa armigera (Hübner) larvae reported in an earlier study is due C. scarabaeoides (ICPW 82) and C. platycarpus (ICPW 68) to the high density of nonglandular trichomes. This wild species may during the 1994 cropping season. In this and all subsequent thus be an important source for developing insect resistant pigeonpea. observations, fully expanded leaves and full-grown, green pods were selected. The plant material was fixed and prepared for electron microscopy by the methodology described by Reddy P is an important grain legume of the semiet al. (1995). Electron micrographs of the samples were taken arid tropics and is attacked by more than 200 insect with a JEOL JSM 35 CF (Tokyo, Japan) scanning electron mispecies (Lateef and Reed, 1990). The most devastating croscope. pest is the pod borer Helicoverpa armigera, which causes The presence of trichomes on pods, leaves, and calyxes worldwide yield losses of more than $300 million annuwere determined by observing a minimum of 10 plants from ally (ICRISAT, 1992). More than 14 000 pigeonpea each Cajanus spp. with a light microscope. The density of pod and leaf trichomes was determined with an ocular measuring germplasm accessions have been screened in an attempt grid. Trichome density was measured on 20 pods (three obserto identify sources of insect resistance, but only low vations per pod) and 15 leaves (three observations per leaf) levels have been detected (Lateef, 1992; Sachan, 1992). collected from different plants of each Cajanus spp. Trichome As a result, the search for resistant sources has been density on leaves was calculated for the interveinal area of extended to include noncultivated Cajanus spp. Several upper and lower surfaces separately. The sampling unit for species were initially recognized as potential sources of trichomes on pods (locule area) of pigeonpea, C. platycarpus, resistance to major pod-damaging insect pests (ICRIand leaves of all three species was an area of 1.21 mm (Type SAT, 1980; Lateef et al., 1981), but little research has C) or 4.84 mm (Types A, B, D). The sampling unit for tribeen devoted to this topic recently. chomes on pods of C. scarabaeoides was an area of 0.13 mm Trichomes and trichome exudates on plant surfaces (Type C) or 3.24 mm (Types A, B, D) because of the high density of Type C trichomes and the smaller locule area on play an important role in the host selection process of this species. The mean of the three observations per pod or insect herbivores (Bernays and Chapman, 1994). The leaf was the experimental unit in the data analysis. type of trichomes and their orientation, density, and Trichome length was measured by gently pressing sticky, length have been correlated with reduced insect damage transparent tape to the pod surface. Trichomes adhered to in several crops (Jeffree, 1986; David and Easwaramoorthe sticky surface. The tape was then fixed to a glass slide and trichome length was measured under a microscope with an J. Romeis, Institute of Plant Sciences, Applied Entomology, ETH ocular micrometer. Trichomes were collected and measured Zenstrum/NW, Clausiusstrasse 25, 8092 Zürich, Switzerland; T.G. on pods from at least 10 different plants per Cajanus spp. In Shanower, Northern Plains Agricultural Research Lab., USDA/ARS, total, 120 Type A, C and D, and 14 Type B trichomes were 1500 N. Central Ave., Sidney MT 59270; A.J. Peter, Nagarjuna Agmeasured per species. ricultural Research and Development Institute, 28 P & T Colony, Secunderabad 500 009, Andhra Pradesh, India. Approved as Journal Article No. 1977 by the International Crops Research Institute for Variation in Trichome Density the Semi-Arid Tropics (ICRISAT). Received 27 March 1998. *CorreIntraspecific variation in trichome density was investigated sponding author ([email protected]). with pods from 12 pigeonpea genotypes (ICPL 87, ICPL 151, ICPL 269, ICPL 84052, ICPL 86012, ICPL 86015, ICPL 88034, Published in Crop Sci. 39:564–569 (1999).