Is foliar Cl- concentration the cause of photosynthetic decline in grapevine during mild salinity?

Moderate levels of Cl- have been associated with grapevine salt tolerance. The hypothesis to be tested in this work is: photosynthesis in grapevine is negatively correlated with foliar Cl- concentration. To further test this hypothesis, multiple mild salinity experiments on four different Vitis genotypes (Cabernet-Sauvignon, Riparia Gloire, Ramsey and SC2) were conducted and photosynthesis, ion concentrations and gene expression responses were quantified. The salt-tolerant rootstock Ramsey had greater Cl- exclusion capabilities than V. vinifera cultivars both during rooted cutting greenhouse experiments and three years of field-grafted experiments; SC2 also excluded Cl-. Differential gene expression indicated that salinity affected transcript abundance more in salt-sensitive genotypes (97.7 % of DEGs in the dataset), especially chloroplast-related transcripts. The transcript abundances of known anion transporters were determined and a family of putative B transporters was associated with the Cl- exclusion phenotype. Photosynthesis and growth were maintained in Ramsey and SC2 under mild salinity. However, photosynthesis declined in Cabernet-Sauvignon with isosmotic 20 mM salt concentrations of NaCl, KCl or NaNO3, independent of the salt type. While foliar Cl- concentrations did correlate with salt tolerance during control and NaCl conditions, it was not found to be the cause of photosynthetic decline in Vitis during mild salinity.