The synergistic effects of sodium and potassium on the xerophyte Apocynum venetum in response to drought stress.

Apocynum venetum is an eco-economic plant species with high adaptability to saline and arid environments. Our previous work has found that A. venetum could absorb large amount of Na and maintain high K level under saline conditions. To investigate whether K and Na could simultaneously enhance drought resistance in A. venetum, seedlings were exposed to osmotic stress (-0.2 MPa) in the presence or absence of additional 25 mM NaCl under low (0.01 mM) and normal (2.5 mM) K supplying conditions, respectively. The results showed that A. venetum should be considered as a typical K-efficient species since its growth was unimpaired and possessed a strong K uptake and prominent K utilization efficiency under K deficiency condition. Leaf K concentration remained stable or was even significantly increased under osmotic stress in the presence or absence of NaCl, compared with that under control condition, regardless of whether the K supply was sufficient or not, and the contribution of K to leaf osmotic potential consistently exceeded 37%, indicating K is the uppermost contributor to osmotic adjustment of A. venetum. Under osmotic stress, the addition of 25 mM NaCl significantly increase Na accumulation in leaves and the contribution of Na to osmotic adjustment, thus improving the relative water content, concomitantly, promoting the photosynthetic activity resulting in an enhancement of overall plant growth. These findings suggested that, K and Na simultaneously play crucial roles in the osmotic adjustment and the maintenance of water status and photosynthetic activity, which is beneficial for A. venetum to cope with drought stress.