CARBON BALANCE, TRANSPIRATION, AND BIOMASS PARTITIONING OF GLYPHOSATE-TREATED WHEAT (TRITICUM-AESTIVUM) PLANTS

Whole plant studies were conducted to examine the effects of glyphosat e on components of carbon balance, transpiration, and biomass partitio ning of wheat plants grown in Olton sandy clay loam soil and in a well -aerated fritted clay medium under controlled environmental conditions . Well-irrigated plants were transferred from a nursery room into a te st chamber about 48 d after planting. Two to five days later, 12 to 42 ml of a glyphosate solution with a concentration of 480 mg ai L-1 wer e sprayed until full coverage of the foliage. Environmental conditions in the chamber were air temperature 25 C, dew point 18 C, windspeed 1 .1 m s-1, and PPFD 1500 mmol m-2 s-1 (at the top of the foliage) for 1 2 h daily. Glyphosate treatment resulted in destruction of the root sy stem, as determined at the end of the tests and at the start of tests using companion plants. Plants grown in soil lost 0.53 kg ke of the in itial root mass, while this loss was 0.38 kg kg-1 in plants grown in f ritted clay. Glyphosate treatment rapidly inhibited daily rates of gro ss carbon uptake and transpiration of wheat plants grown in both media . Effects occurred more than twice as rapidly in plants grown in soil as in fritted clay. Similarity in the patterns of inhibition of gross carbon uptake and transpiration suggests that glyphosate may also affe ct leaf stomata. After applying glyphosate, daily rates of carbon loss increased for 3 d in soil-grown plants but remained almost constant f or 10 d in plants grown in fritted clay; thereafter, the rates of carb on loss declined. The early increase or the constancy of carbon loss o bserved after applying glyphosate was related to catabolic processes o ccurring in roots.