Contents - Faperta

Physico-Chemical and Molecular Markers for Resistance to Insect Pests 165
leaves resulted in retarded growth rates, extended development period, and reduced survival
of H. zea and S. exigua ( Juvik et al., 1994). In the case of H. virescens resistance in
tobacco, larval weight gain is negatively correlated with nicotine content at various leaf
positions. Larvae survived better and grew faster on the bud leaves, which have lower
nicotine levels than the leaves further down the stalk where alkaloid levels are high
( Jackson, Johnson, and Stephenson, 2002). Luteolin-C-glycosides, maysin, and isomaysin,
which occur in maize silks, confer resistance to H. zea (Waiss et al., 1979). In soybean, pinitol
confers resistance to H. zea (Dougherty, 1976), while long chain fatty acids in sunfl ower
act as growth inhibitors for H. virescens and H. zea (Elliger et al., 1976). In potato, concentrations
of the glycoalkaloids, solanine and chaconine are associated with resistance to
E. fabae (Sanford et al., 1992).
Nonprotein Amino Acids
Nonprotein or unusual amino acids are found in a number of plant species, and afford
protection against herbivores. The protective effect is elicited through their structural analogy
to the commonly occurring essential amino acids. The harmful effects on insects occur
as these get incorporated into proteins that are toxic to insects. Among these, L-canavanine,
azetidine-2-caboxylic acid, 2,4-diamino butyric acid, minosine, and 3-hydroxyproline have
signifi cant growth inhibition effects on insects (Parmar and Walia, 2001). L-canavanine is
a structural homologue of L-arginine, and occurs in over 1,500 leguminous plant species.
Some of the nonprotein amino acids also act as enzyme inhibitors. Canaline, a hydrolytic
product of canavanine, inhibits pyridoxal phosphate-dependent enzymes by forming a
covalent bond. Minosine is a strong inhibitor of rat liver cytothionine synthetase and cytothionase,
and inhibits the growth of rust red fl our beetle, Tribolium castaneum (Herbst)
(Ishaaya et al., 1991).
Nutritional Factors
For a plant to serve as a host for an insect, it should be able to provide holistic nutrition to
support growth, development, and reproduction (Beck, 1972). It should also be suitable for
digestion, assimilation, and conversion into energy and structural compounds required by
the insects. Some of the nutrients such as amino acids, phospholipids, fatty acids, steroids,
and ascorbic acid also serve as phagostimulants for different insect species (Dadd, 1970).
Variation in the amounts of these compounds results in various degrees of host plant
acceptance and damage by the herbivorous insects. Imbalanced ascorbic acid content in
maize (Penny, Scott, and Guthrie, 1967), amino acid content in pea (Auclair, Maltais, and
Cartier, 1957), low quantities of glutamic acid and asparagine in rice (Sogawa and Pathak,
1970), low lysine content in sorghum (Singh and Jotwani, 1980), and high proline content in
cotton (Sharma and Agarwal, 1983a) are associated with resistance to insects. Total soluble
sugars in the pod wall have a signifi cant and negative correlation with pod damage by
H. armigera in pigeonpea. Pea varieties defi cient in certain amino acids are resistant to the
pea aphid, Acyrthosiphon pisum (Harris) (Auclair, 1963).
In cotton, higher amounts of reducing sugars are associated with resistance to the
leafhopper, A. biguttula biguttula (Singh and Agarwal, 1988), and amino acid content to
stem weevil, Pempherulus affi nis (Kalt.) (Parameswaran, 1983). Rice varieties resistant to
thrips, Stenchaetothrips biformis (Bagnall), have lower amounts of sugars and free amino
acids (Thayumananan et al., 1990), while low amounts of amino acids are associated with
resistance to brown planthopper, N. lugens (Sogawa and Pathak, 1970; Bharathi, 1989).