Potassium is indispensable for plant growth. Recently, a role of K+ channels has emerged in sensing and transducing stress and nutrient status. Tetraethylammonium (TEA+) is a specific blocker of K+ transport and affects K+ channel gene expression. Two barley varieties with contrasting salinity tolerance, and a chlorophyll b-less mutant, were grown either in the presence of TEA+ alone or combined with NaCl, at two different concentrations of external K+ and Ca2+, and were analyzed nine days after germination. Chlorophyll a transients monitored via JIP-tests were used to evaluate the state of the photosynthetic machinery. In contrast to reported responses to K+ deficiency, TEA+ inhibited shoot growth while inducing root growth and increasing photosynthetic performance. Both TEA+ and NaCl induced the appearance of negative K-bands in OJIP kinetics and an increase in PIABS, indicating a stimulation of photosynthesis by increased sink strength in the context of root to shoot signaling.