Abstract

Salt tolerance threshold is a critical parameter for establishing plant salt tolerance, which is generally known as the relative yield response to increase root area salinity. In this study, we focused on relative growth rate (RGR) and salt tolerance index (STI) in Andrographis paniculata (AP). For this purpose, after 30 days from culturing, the seedlings were placed in different salinity levels (0, 4, 8, 12, and 16 dSm^-1) on Hoagland medium and were compared at various exposure times (0, 5, 10, 15, and 20 days). The results revealed that salinity, exposure times and their interaction had significant effect on the measured traits. The injurious effects of salinity on RGR during exposure time indicated two specific outcomes: (1) at five days salt exposure during which growth began to decrease due to low osmotic potential, and (2) at 20 days salt exposure which identified reduction rate of yield, which was attributed to toxicity of specific ion. Although, salinity effects on STI were the same among accessions at 5 and 10 days salt exposure; it affected STI at 15 and 20 days, significantly. In view that all the growth indices decreased in sequence of increasing salinity levels, 12 dSm^-1 salinity and 15 days stress could be concluded as the extreme salinity level and exposure time.

Key words: Andrographis paniculata, salinity, salt tolerance threshold, exposure times, salt tolerance index.