New methods of insect pest management include the application and development of the existing natural resistance mechanisms, which protect plants against insect feeding. Aphids (Homoptera) have the exceptionally modified mouthparts, the piercing-sucking type, which enables feeding on plant sap directly from the sieve elements of the phloem tissue. Most aphid species are oligophagous; i. e. their host plant range is restricted to selected related plant species. The crucial step in host-plant recognition by these insects is the gustatory examination of the plant sap. Aphids do not possess external contact chemoreceptors, so this step requires stylet penetration of the plant and the subsequent sap sampling. The effect of chemical properties of a plant, and in fact the acceptability of a plant to an aphid, can be investigated directly by using the electronic monitoring of stylet penetration (EPG = Electrical Penetration Graph) technique. In this system the aphid and the plant are made parts of an electric circuit and any changes in electric characteristic of the circuit express aphid activities, such as: stylet penetration of peripheral plant tissues, feeding on phloem sap, salivation into phloem elements, e.t.c. The duration and frequency of these activities can be used as indicators of plant suitability. The existence and tissular localisation of factors determining the level of plant resistance against aphids can also be derived from these data.
The studies on the effect of heavy metals on plants have shown that they cause intensification of two types of unfavorable processes: ? inactivation of macromolecules and cellular structures, ? induction oxidative stress. All molecular, structural and metabolic changes on the level of molecules, tissues and organs lead to changes of plant morphology. One of the change is inhibition of plant growth, reflected as a reduction of its size, mass of either the whole plant or its part, organs or tissues. In response the plant activates processes restoring its homeostasis. In removing reversible changes a particular role is played by the processes of heavy metal detoxification as well as removal of active forms of oxygen.
Inheritance of partial leaf rust and stripe rust resistance of a Thatcher wheat 90RN2491, earlier reported to carry two doses of the gene pair Lr34-Yr18 and the reference line RL6058 (6*Thatcher/PI58548) for the Lr34-Yr18 gene pair was studied against predominant and highly virulent Indian races. Thatcher derivatives 90RN2491 and RL6058 were intercrossed as well as crossed with the leaf rust and stripe rust susceptible Indian cultivar WL711. The F1, F2 and F3 generations from these crosses were assessed for rust severity against leaf rust race 77-5 and stripe rust race 46S119. The F2 and F3 generations from the crosses of RL6058 and 90RN2491 with WL711, segregated 15 resistant : 1 susceptible (F2) and 7 homozygous resistant : 8 segregating : 1 homozygous susceptible (F3) ratios, respectively, both for leaf rust and stripe rust severity. Therefore, partial resistance against each of the leaf rust and stripe rust races in both RL6058 and 90RN2491 is ascribed to two independently inherited dominant genes. One of the two genes for leaf rust and stripe rust resistance in 90RN2491 and RL6058 is Lr34 and the linked gene Yr18, respectively. The second leaf rust resistance gene in both the Thatcher lines segregated independently of stripe rust resistance. Therefore, it is not Lr34 and it remains unidentified.
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