Salt tolerance of potato (Solanum tuberosum L. var. Desirée) plants depends on light intensity and air humidity

Potato (Solanum tuberosum L. var. Desireé) plants were grown in a hydroponic system at high (85% relative humidity) and low air humidity (60% relative humidity), respectively. After 6 weeks of growth, 300 mM NaCl was added to the nutrient solution. While the plants growing at low air humidity became irreversibly damaged within 5 days, the plants which were growing under high air humidity survived this treatment for at least 3 weeks, and were able to recover after retransfer to control medium. A comparison of leaf ion contents, osmotic potential and gas exchange rates indicates that the rate of Na+ accumulation is very low at high air humidity, because the water-use efficiency had increased more. Chlorophyll fluorescence measurements indicate that under high air humidity, a high non-photochemical quenching of the variable chlorophyll fluorescence (NPQ) is built up and induces down-regulation of photosystem-II activity during the salt treatment. Since the rate of electron transport is decreased by this way, oxidative stress is barely detectable during the treatment. The effects of NaCl in high air humidity were compared with those of KCl, which is taken up more rapidly, and of polyethyleneglycol which is not taken up. The results indicate that the rate of ion accumulation and the time-course of osmotic changes within the plant are decisive for the onset of damages. In summary, plant growth at high air humidity and at low light can be used as a tool to study NaCl-induced stress in glycophytes under laboratory conditions, without concomitant metabolic and genetic alterations induced by oxidative stress.