Studies on the interaction of fungicides and plants. 2

Stoichiometry in the detoxication of PMA with the thiol group

Abstract

In the previous paper, the author concluded that the detoxication of PMA by the rice leaf juice was due to the thiol group present in it. This paper deals with the stoichiometrical reaction of PMA with the known thiol compounds and thiol groups contained in the rice plant.
The thiol group was determined by the amperometric titration method, by which SH in the range of 0.03-0.6μM was determined and 96per cent was recovered from glutathione (GSH).
The SH contents of egg-albumin, yeast-extract, peptone, γ-globulin were 14.4, 1.23, 0.25, 0.1μM per gram respectively. The detoxication effect varied in this order.
In the rice juice, from which protein had been removed, and in the yeast-extract, SH diminished more rapidly than in GSH solution. The SH in the rice juice decreased more slowly during the storage at 5°C than in GSH solution.
The SH content was highest in the upper three leaves, about 300μM per 100g fresh weight, decreasing toward the lower leaves. The content was about 1/4 in the leaf still folded and below 1/10 in the leaf sheath, the stem and the root. The SH contents of these tissues varied in parallel with the protein contents. As the stage advanced from the vegetative growth to the reproductive, the SH content in the leaf decreased slowly but the SH became hardly extractable into the juice because of hardening of the tissues.
The juice-SH reacted stoichiometrically with PMA at one to one mole ratio. When both PMA and the juice were added to the spore suspension of Cochliobolus miyabeanus, the spore germination was not inhibited until PMA exceeded the equal mole to the juice-SH. The elongation of germ tube was not inhibited until PMA exceeded the half mole of the juice-SH.
When PMA dust is applied to the rice plant for the control of blast disease, the amount of PMA deposited on the rice leaves may be at most 1/100-1/1, 000, by mole, to that of SH present in leaves.
The author concludes that the effectiveness of PMA for the blast disease is mainly due to the PMA present on the leaf surface or penetrated into the cell membrane, but not due to the PMA penetrated and diffused in the cell sap, because the latter PMA will easily be detoxified by the excessive amount of SH.