EVIDENCE THAT WOUND GUM, NOT LIGNIN, IS DEPOSITED IN INFECTION-RESISTANT INJURIES OF CITRUS PEEL

Injury sites from wounded pericarp of Citrus sinensis or C. limon that have developed resistance to infection by Penicillium digitatum characteristically react positively with phloroglucinol/HCI. Because of this, lignification has been proposed as the mechanism of resistance.
However, the histochemical reaction of the resistant-wound site in the Mäule test is always negative.
In dicots, lignin always reacts positively in the Mäule test.
These results exactly match those described for wound gum.
Further, ethanolic extracts from resistant wounds contain several induced, low molecular weight aldehydes; the induced aldehydes form red products with phloroglucinol/HCI and offer an alternative explanation of histochemical reactions.
Antifungal activity is associated with three of these compounds using the Cladosporium/TLC bioassay.
The predominate compound has been purified and identified as 3- [4-hydroxy-3-(3-methyl-2-butenyl)phenyl]-2-(E)-propenal by ¹H and ¹³C NMR and MS. Results suggest that the positive reaction of the resistant wound site with phloroglucinol/HCI should be interpreted as localization of induced antifungal compounds rather than lignin.

Physiological Basis of Postharvest Technologies

R.R. Stange Jr, S.A. Midland, J.J. Sims, J.W. Eckert

https://doi.org/10.17660/ActaHortic.1993.343.85