Leaf structure and ultrastructure changes induced by heat stress and drought during seed filling in field-grown soybean and their relationship with grain yield oleh CONSTANZA S. CARRERA, STELLA M. SOLÍS, MARÍA S. FERRUCCI, CLAUDIA C.R. VEGA, BEATRIZ G. GALATI, VERÓNICA ERGO, FERNANDO H. ANDRADE, RAMIRO H. LASCANO