Morpho-physiological and molecular responses of an ecologically important mangrove Kandelia candel (L.) Druce under salinity stress

Mangroves are ecologically important model plants for understanding their tolerance mechanisms to stresses. This research investigates the growth, survival performance, photosynthetic functioning, physiological, and molecular responses of Kandelia candel (L.) Druce under varying salinity regimes (10, 20, 30, 40, and 50 PPT NaCl) at two time points (15 and 24 DAT). The salinity-induced growth variations indicate early responses in seedlings, which begin at 10–20 PPT. Most leaf and stem-specific morphological parameters showed a decreasing trend with increasing salinity, leading to leaf shedding beyond 20 PPT. On the other hand, the root system showed better response, and the root-specific parameters increased up to 10–20 PPT. The salinity tolerance level of K. candel hardly reaches the seawater salinity range (~ 35 PPT). Beyond this, salinity hampers growth, ultimately leading to plant mortality at 50 PPT. The concentration of the photosystem pigments was higher in freshwater-grown plants, emphasising the facultative halophytic nature of K. candel. The photosynthetic performance completely ceased, initiating the progressive death of the above-ground plant tissues at 20 PPT, leading to root hydraulic failure and plant death at higher salinities. This study further explores the expression of key genes crucial for proper photosynthesis and water uptake. The RT-qPCR results show that the genes encoding photosystem proteins (PsaA, PsaB, and PsbA) and membrane transporters (AKT1, NHX, NIP, and TIP) are downregulated. In contrast, other photosynthetic genes (RbcS, RbcL, Ycf3, and LHCB) are upregulated under salinity stress. These results provide an understanding of the physiological and molecular basis of mangrove ecological responses to salinity stress.