Omeprazole Promotes Chloride Exclusion and Induces Salt Tolerance in Greenhouse Basil

The role of small bioactive molecules (&lt
500 Da) in mechanisms improving resource use efficiency in plants under stress conditions draws increasing interest. One such molecule is omeprazole (OMP), a benzimidazole derivative and inhibitor of animal proton pumps shown to improve nitrate uptake and exclusion of toxic ions, especially of chloride from the cytosol of salt-stressed leaves. Currently, OMP was applied as substrate drench at two rates (0 or 10 &mu
M) on hydroponic basil (Ocimum basilicum L. cv. Genovese) grown under decreasing NO3&minus
Cl&minus
ratio (80:20, 60:40, 40:60, or 20:80). Chloride concentration and stomatal resistance increased while transpiration, net CO2 assimilation rate and beneficial ions (NO3&minus
PO43&minus
and SO42&minus
) decreased with reduced NO3&minus
ratio under the 0 &mu
M OMP treatment. The negative effects of chloride were not only mitigated by the 10 &mu
M OMP application in all treatments, with the exception of 20:80 NO3&minus
but plant growth at 80:20, 60:40, and 40:60 NO3&minus
ratios receiving OMP application showed maximum fresh yield (+13%, 24%, and 22%, respectively), shoot (+10%, 25%, and 21%, respectively) and root (+32%, 76%, and 75%, respectively) biomass compared to the corresponding untreated treatments. OMP was not directly involved in ion homeostasis and compartmentalization of vacuolar or apoplastic chloride. However, it was active in limiting chloride loading into the shoot, as manifested by the lower chloride concentration in the 80:20, 60:40, and 40:60 NO3&minus
treatments compared to the respective controls (&minus
41%, &minus
37%, and &minus
24%), favoring instead that of nitrate and potassium while also boosting photosynthetic activity. Despite its unequivocally beneficial effect on plants, the large-scale application of OMP is currently limited by the molecule&rsquo
s high cost. However, further studies are warranted to unravel the molecular mechanisms of OMP-induced reduction of chloride loading to shoot and improved salt tolerance.