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Abstract. The pathway of a systemic electrical signal possibly linking wounding and the systemic synthesis of proteinase inhibitor was investigated in tomato (Lycopersicon esculentum Mill. cv. Moneymaker) plants. Heat, causing wounding to a cotyledon, was used to induce both a travelling electrical signal and systemic proteinase inhibitor activity. Intracellular recordings of changes in the membrane potential of different cell types were measured in the petiole of leaf 1, the first true leaf, and impaled cells were identified by injection of fluorescent dye (Lucifer Yellow CH). No difference was found between the membrane potentials of the different cell types; the mean membrane potential of all the cell types was -148 _+ 3 mV. Only sieve-tube elements and companion cells produced large (79 _ 3.3 mV) action-potential-like depolarisations following wounding, although smaller (23 + 1.6 mV) depolarisations were observed in other cell types. It was concluded that the electrical signal possibly linking a wound stimulus in a cotyledon with the induction of systemic proteinase inhibitor synthesis was propagated in the sieve-tube element/companion cell complex.

Nitrogen fixation by Vigna sinesis nodulated effectively either by Rhizobium strain QA323 or by strain CB441 is little restricted by applications at sowing of ammonium nitrate up to 24 mg nitrogen per plant. The growth patterns of these two associations are differentially affected by nitrogen level, and are both considerably different from that of unnodulated plants given combined nitrogen. Nitrogen fixation by V. sinensis-strain SU318 may be stimulated by small doses of combined nitrogen at sowing, but for Vicia atvopurpurea all the combined nitrogen levels used in these experiments depressed fixation. Primary root nodulation of V. atropurpurea by the effective Rhizobium strain V27E and the ineffective strain NA6, and of V. sinensis by the effective Rhizobium strain SU318, is influenced by the form and amount of the nitrogen compound applied (ammonium nitrate, potassium nitrate, ammonium sulphate, or urea), and that of V. atropurpurea is also influenced by the Rhizobium strain. These forms of combined nitrogen restrict primary root nodulation on both hosts similarly except that urea has little effect on V. atuopurpurea. In both species many more nodules formed on the secondary roots than on the primary, but numbers of secondary root nodules are little affected by the combined nitrogen. Immersion of the first leaves of V. sinensis seedlings in solutions of combined nitrogen depressed nodulation, but urea slightly increased the dry weight of tops.

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