AbstractThe fate of cells in the endosperm of developing wheat kernels was investigated under normal conditions and upon premature slow drying on the cut ear. To follow the changes in membrane integrity and cellular ultrastructure, an electron spin resonance (ESR) spin probe technique and low temperature scanning electron microscopy (LTSEM) were used. ESR data indicated that during development, the relative amount of cells with intact membranes decreased and became almost zero at the stage of mass maturity, i.e. several days before the onset of maturation drying. This suggests that the death of starchy endosperm cells is a developmental phenomenon rather than one induced by water loss. Even at 8 days after anthesis (daa), early in the differentiation phase, a considerable proportion of endosperm cells had already lost plasma membrane integrity. Comparison of ESR data with LTSEM micrographs revealed that the loss of plasma membrane integrity occurred early in the starch accumulation process, from which it was concluded that the programmes of starch accumulation and developmental death are simultaneously switched on. When the differentiation into starchy endosperm was arrested by premature slow drying of kernels on the cut ear, meristematic cells (aleurone initials) acquired desiccation tolerance as assessed by the combination of the spin probe technique and LTSEM. In contrast, fast drying caused immediate death of these cells. Thus, meristematic endosperm cells have the competence to acquire desiccation tolerance. This occurs upon premature slow drying or during normal differentiation into aleurone cells, but is lost when the cells differentiate into starchy endosperm.