Combined effects of high relative humidity and K+ supply mitigates damage caused by salt stress on growth, photosynthesis and ion homeostasis in J. curcas plants

• High relative humidity mitigates toxic effects caused by salt stress.
• High relative humidity and adequate K+ supply enhance salt-tolerance of J. curcas.
• High affinity and selectivity for K+ over Na+ are crucial for salt-tolerance of species.
• Low humidity induces loss in the photosynthetic activity by stomatal limitations.
• The Na+ is the main limiting in the K+ accumulation independently of relative humidity.

This study evaluated the capacity of high relative humidity associated with K+ supply in mitigating harmful effects caused by salt stress on physiological parameters of Jatropha curcas plants. The plants were subjected to four different treatments (K0Na0, K0Na1, K1Na0 and K1Na1) and two different levels of humidity, low (40%) and high (80%), in chamber growth conditions. The plants’ growth was higher under conditions of high relative humidity when compared to low humidity in all the treatments studied. The CO2 assimilation rate, transpiration and stomatal conductance were stimulated by high humidity independently of treatments of K+ and Na+. The Na+ concentration in the shoots was more elevated in plants treated with NaCl when compared to NaCl-free plants. On the other hand, the shoots’ K+ concentration was highly stimulated in the presence of K+ ions, especially in combination with high relative humidity (RH). The shoots’ Na+ transport rate was strongly stimulated by a combination of 50 mM NaCl + 40% RH when compared to high humidity. Conversely, the shoots’ K+ transport was significantly higher in plants when they were supplied with K+ and exposed to high relative humidity. In the roots, the transport rate of Na+ presented a similar pattern to that found in the shoot. However, surprisingly, K+ transport rate was increased in plants supplied with K+ and 40% RH in NaCl absence. The plants exposed to low relative humidity showed lower K+/Na+ ratios than plants exposed to 80% RH both in shoots and roots. In addition, the shoots and roots selectivity (K,Na) was strongly stimulated in plants exposed to high humidity when compared to low humidity. In conclusion, the combined effects of high relative humidity and a supply of K+ were able to improve the growth, the leaf gas exchange and ionic homeostasis of J. curcas plants.