Using excised leaves to screen lucerne for salt tolerance: physiological and cytological evidence.

Salinity affects many physiological processes at all levels of plant structural organization. Being a physiologically and genetically complex trait, salinity tolerance implies a coordinated contribution of multiple mechanisms, making plant screening for salt tolerance extremely difficult. In this work, we show how the use of excised leaves can fulfill that task. We argue that, by adding NaCl directly to the transpiration stream, the protective effects of several mechanisms regulating Na(+) delivery to the shoot are eliminated, enhancing PSII exposure to salinity treatment and resulting in a significant decline in leaf photochemistry (Fv/Fm characteristics). We suggest that measuring Fv/Fm characteristics on excised salt-treated leaves provides an opportunity to evaluate the efficiency of vacuolar Na(+) compartmentation, arguably the most important feature for salt tolerance. We also explain the observed decline in Fv/Fm values as salt-induced structural damage to chloroplasts caused by oxidative stress.