Variable Photosynthetic Responses of Pinus resinosa Seedlings to Herbicides

Abstract

HERBICIDES have shown variable toxicity to young gymnosperms; the susceptibility of these plants greatly varies according to the chemicals and dosage used, the species, and the age of the plants^1–4. Among the more obvious deleterious responses of young Pinus plants to certain herbicides are twisting and curling of cotyledons, distortion of old needles, chlorosis, and subsequent plant mortality. The incorporation of herbicides by the soil usually causes greater injury to plants than does the application of them to the soil surface^3,4. Comparisons of cross-sections of needles from young control Pinus resinosa and herbicide-treated plants showed marked effects on needle anatomy as a result of treatment⁵. Needles from control plants were approximately semicircular in outline. Their mesophyll cells were closely packed and infolded, and they contained large chloro-plasts. In contrast, needles of plants treated with atrazine, simazine or prometryne were distorted and chlorotic. Their mesophyll cells were often rounded and the infolding characteristic of normal mesophyll tissue no longer occurred. Frequently the mesophyll cells became vacuolated or nearly empty and sometimes large gaps were present among them⁵. These experiments have now been extended in order to evaluate the effects of five structurally different herbicides on photosynthesis of 3–0 Pinus resinosa Ait. seedlings.