Chlorophyll accumulation is enhanced by osmotic stress in graminaceous chlorophyllic cells

We have developed a new chlorophyllic cell line ('TADH-XO') from the highly water stress tolerant grass Bouteloua gracilis (blue grama). When grown under normal (non-stress) conditions, this new cell line accumulates higher levels of chlorophyll (up to 368.1 μg total chlorophyll g^-1 FW) than a previously obtained cell line ('TIANSJ98'). Both cell lines are capable of developing substantially higher amounts of chlorophyll when subjected to osmotic stress. In order to explain these changes in the chlorophyll kinetics of the chlorophyllic cells, we analyzed the following population variables in cells subjected to polyethylene glycol 8000-induced osmotic stress: growth, viability, chlorophyll (total, 'a' and 'b'), cell size, percentage of green cells and chloroplast (number and size). Although previous studies in some chlorophyllic cells of dicots have already reported that chlorophyll increases under saline stress, in this report we show that, at least in this graminaceous cell line, the increase in chlorophyll is an immediate and proportional response to the osmotic stress applied and not the result of a progressive adaptation process. Consistent with previous studies, the increase in chlorophyll accumulation could be the result of chloroplast development (increased thylakoid number per chloroplast). On the basis of our results, the increases in chlorophyll accumulation previously observed in salt-adapted dicot cells may be the result of the osmotic shock (water deficit), rather than the ionic effect of salt on the physiology of chlorophyllic cells of dicots. Under the cell population experimental approach we followed, our study provides important insights related to the physiological behavior of chlorophyllic cells subjected to osmotic stress.