Proline accumulation in canola leaf discs subjected to osmotic stress is related to the loss of chlorophylls and to the decrease of mitochondrial activity

Many stress studies use simplified experimental models like leaf discs, but the validity of such approaches remains a matter of debate, especially whe n attempts are made to obtain suitable biotests. We report here the analysi s of the resistance of canola leaf discs to osmotic stress, as related to t he accumulation of proline, For that purpose, the explanted tissues were in cubated for 24 h under continuous light and at 28 degreesC on polyethylene glycol solutions of external potentials (IIext) ranging from -0.1 to -8.0 M Pa, Proline content, mater deficit and chlorophyll content were quantified, The cell membrane stability, which is a structural component of viability, was assessed according to electrolyte leakage, Biomembrane oxidative damag e was measured through the production of malondialdehyde and the mitochondr ial activity was quantified by assessing the ability of the tissues to redu ce 2,3,5-triphenyltetrazolium chloride. Although the water deficit of the t issues reached high values (i.e. up to 70%), the cell membrane stability re mained high. Furthermore, the oxidative damage to biomembranes was quite lo w, even in highly dehydrated tissues. In contrast, osmotic stress induced a significant decrease in the chlorophyll content and a strong accumulation of proline, These two processes each reached a maximum at a IIext of -3.0 M Pa, although the tissues appeared to be viable at even lower IIext suggesti ng they are not responsible for primary resistance. The mitochondrial activ ity was strongly decreased in the stressed leaf discs, although this activi ty was at least partially restored in tissues incubated for 24 h on a non-s tressing medium. It appears that the cell components of the osmotic stress resistance assessed observed in this study do not reflect the modes of resi stance of an intact plant.