The presence of Puccinia recondita Rob. ex Desm. f. sp. tritici, leaf rust, alltered the water balance of tillering wheat plants, variety M-1. At two temperature levels, 22 and 35° C, and three light intensities, 0 - 1.000 - 2.000 foot-eandles, infected plants transpired more than rust free plants, and their absorption rate was consistently below their transpiration rate. It appeared that the negative water balance occurring under high temperature and Iight intensity tended to be compensated under darkness by a higher absorption rate and a lower transpiration rate. The data allows to assume that the effect of the pathogen on the water balance of the infected host must be one of the main variables that determine lower yields.

1 Ingeniero Agrónomo, M. S., Proyecto Trigo, Estación Experimental La Platina, Instituto de Investigaciones Agropecuarias; Profesor Cátedra Investigación Agrícola, Facultad de Agronomía, Universidad Católica de Valparaíso. Actualmente Rockefeller Fellow, Purdue University, Lafayette, Indiana 47907, USA.
2 Assistant Professor of Agronomy, Department of Agronomy, The University of Georgia, Athens, Georgia 30601, USA. Anteriormente, Instructor, Department of Agronomy, Purdue University