: any of various antimicrobial chemical substances produced by plants to combat infection by a pathogen (such as a fungus)
borrowed from German Phytoalexin, from phyto-phyto- + Alexin "complement (in immunology)," from Greek aléxein "to ward off, defend" + German -in-in entry 1; aléxein going back to Indo-European *h2lek-s- "ward off, defend" (whence also Sanskrit rákṣati "[s/he] defends"), suffixed form, with shifted ablaut vowel, of *h2elk- "ward off," hence Greek álalke "(s/he) warded off, turned away" (reduplicated aorist), alkḗ "defense, help," and perhaps Old English ealgian "to defend, protect" (going back to *algōjan-)
The term Phytoalexin was introduced by the plant pathologists Karl Otto Müller (1872-1958) and Hermann Börger in "Experimentelle Untersuchungen über die Phytophora-Resistenz der Kartoffel," Arbeiten der Biologischen Reichsanstalt für Land- und Forstwirtschaft, Band 23 (1940), p. 223. The term Alexin (replaced by Komplement in German and complement in English) was introduced by the German bacteriologist Hans Buchner (1850-1902) in "Kurze Uebersicht über die Entwickelung der Bacterienforschung seit Naegeli's Eingreifen in dieselbe," Münchener medicinische Wochenschrift, 38. Jahrgang, No. 25, June 23, 1891, p. 437. The Indo-European base *h2elk- "ward off" has been connected to the Germanic noun represented by Old English ealh "temple," Old Saxon alah, Gothic alhs, with its Baltic congeners Lithuanian al͂kas "sacred grove on a hill," Latvian ȩ̀lka kalns "temple hill"; however, the semantic relationship is strained and the derivational relationship arbitrary.