Trichomes of Nama (Hydrophyllaceae) That Produce Insect-active Compunds

Nama hispidum. N. lobbii, N. rothrockii, and N. xylopodum have two basic types of trichomes on the adaxial and abaxial surfaces: glandular and nonglandular. Nama hispidum and N. xylopodum have (1) short semierect or intermediate-length acicular trichomes that often recurve toward the leaf surface and (2) short-stalked capitate glands. The larger acicular trichomes have micropapillae. Nama lobbii has long filiform trichomes and sessile capitate glands. Nama rothrockii has erect, smooth subulate trichomes and long-stalked capitate glands. Morphological diversity of trichomes in Nama and their possible functional significance as a predator defense are discussed.

Leaf surface structures such as hairs and nettles act as physical barriers to reduce damage caused by the attack of insects. The density of pubescence (vestiture) is correlated with a lower incidence of insect predation in soybean, French bean, cotton and other crops (Stipanovic 1983). These structures may act as spears to impale or elevate eggs and nymphs from the leaf surface, thus facilitating desiccation and increasing exposure to parasites, predators, and pathogens (Juniper and Southwood 1986). Glandular trichomes with enlarged terminal heads may release protective compounds upon contact with herbivores and other animals (Kelsey et al. 1984). Plants thus protected by trichomes and chemicals effectively deter insect herbivores from attacking the leaf and stem surfaces (Lin et al. 1987;Duffey 1986;Khan et aL 1986;Pillemer and Tingey 1976).
Some plants in the genus Nama contain insect antijuvenile hormones and/or juvenile hormone mimics. Binder et aL (1991) report antijuvenile hormones from N. hispidum Gray and N. lobbii Gray, antijuvenile hormones and juvenile. hormone activity from N. sandwicense Gray, juvenile hormone activity from N. rothrockii Gray, and neither antijuvenile hormone nor juvenile hormone activity from N. demissum Gray, N. densum Lemmon, N. jamaicense L., N. stevensii Hitchcock, and N. xylopodum (Woot. & Standi.) C.L. Hitchcock. Insect antijuvenile hormones occur in Nama hispidum, N. lobbii, and N. sandwicense and have been detected in the glandular trichomes of N. hispidum var. revolutum Jepson (Binder et al. 1991). These antijuvenile hormones, precocene II and precocene I (6,7-methoxy-2,2-dimethylchromene, 7 -methoxy-2,2-dimethylchromene) affect the growth, development, and physiology of insects (Bowers 1985) and, therefore, may provide substantial protection against herbivorous insects. Presence of antijuvenile hormones in glandular trichomes of N. hispidum (Binder et al. 1991) indicates that a phytochemical defense in some species of Nama may occur in the leaf surface structures. This study examines the trichome types of four species of Nama: N. hispidum and N lobbii, which contain insect antijuvenile hormones, N. rothrockii, with juvenile hormone mimics, and N. xylopodum, which lacks both types of compounds, to determine if there are differences in trichome type that correspond to the type of or lack of insect-active phytochemicals from the different Namas.

Leaf Preparation for Scanning Electron Microscopy
Air-dried leaves were fixed in formaldehyde:glutaraldehyde (4:1) in 100 mM sodium phosphate buffer, pH 7 .2. Leaves were processed through an ethanol and Freon TF (Van Waters and Rogers, San Mateo, CA) dehydration series, critical point dried using liquid carbon dioxide, and sputter coated with gold (Postek et al. 1980). Scanning electron micrographs were taken of the adaxial and abaxial leaf surfaces on an International Scientific Instrument DS 130. Leaf surface structures are classified following the nomenclature of Theobald et aL (1979).

Measurements
Only the mean values and standard errors were cited in the results for all dimensions, which are based on ten measurements.
By contrast, the abaxial surface was nearly covered by the long, closely interwoven, filiform trichomes (Fig.  2B). Length and spacing among the filiform trichomes could not be determined. Glands on the abaxial surface were nearly completely obscured by the mat of filiform trichomes. Stomata were visible on the adaxial surface; on the abaxial surface stomata were mostly blocked from view by the long, tangled, nonglandular trichomes.

DISCUSSION
All leaves examined have nonglandular and glandular trichomes on the adaxial and abaxial· leaf surfaces. The nonglandular trichome structure, however, is variable: those of N. lobbii are long, narrow and flexible and are tangled and interwoven into a dense sheet on the abaxial surface; those of N. rothrockii are cone-shaped, erect, and stiff; those of N. hispidum and N. xylopodum are semierect, intermediate-length, and recurve toward the leaf surface. All four species have multicellular glandular trichomes; those of N. lobbii are short and sessile; those of N. rothrockii are long and clavate; those of N. xylopodum and N. hispidum are narrow and capitate. Trichomes are known to help protect plants from insect herbivory (Kahn et al. 1986;Norris and Kogen 1980) and these leaf structures may have a similar contribution to the defense against herbivores in the genus Nama. Because of a lack of information about the insects associated with plants in Nama, defining a role for trichomes in the defense against insect predators must wait until further studies are completed.
Adaxial and abaxial surfaces in each species are morphologically different. Typically, the abaxial leaf surface is more densely populated with trichomes. The densely tangled abaxial trichomes of N. lobbii nearly cover the leaf surface and this feature may prevent or deter insect herbivore attack as it does in other plant genera (Norris and Kogan 1980). Similarly, the long nonglandular trichomes of N. xylopodum may protect the plant from attack as do similar structures associated with leaves of Phaseolus vulgaris L. (Pillemer and Tingey 1976). The well-protected abaxial surface may influence oviposition by certain types of insects such as moths, some of which are known to preferentially oviposit on the underside of leaves (Jackson et al. 1983, Navasero andRamaswamy 1991).
Phytochemicals may account for some differences in leaf structures in Nama. Nama hispidum and N. lobbii produce antijuvenile hormones while N. rothrockii produces juvenile hormone mimics (Binder et al. 1991) and these phytochemicals may have an important role in defense against herbivores. Precocene II is recognized as a toxicant and mediator of insect behavior (Binder and Bowers 1991, 1992, 1993 and its presence in the trichomes of N. hispidum may assist the delivery of this compound at the appropriate time or to the best location for maximum protection from insects. For species of Nama examined in this study, however, there is no apparent relation between type of trichome and chemical defenses. Nama hispidum and N. xylopodum have similar trichomes, but N. hispidum and N. lobbii produce precocene. Moreover, N. lobbii has trichomes distinct in structure from both N. hispidum and N. xylopodum. Nama rothrockii produces ALISO juvenile hormone mimics and has trichomes that are distinct from those of the above species. Trichome diversity in Nama may be species specific and not chemistry specific although further studies are needed on additional Nama species to delineate relationships of chemical biosynthesis of insect-active compounds and trichome types in Nama.