Salinity induced leaf anatomical responses and chemical composition in tetragonia decumbens Mill.: an underutilized edible halophyte in South Africa

Abstract

Tetragonia decumbens Mill. has recently been reported to withstand the adverse effect of salinity. However, its leaf anatomical responses are poorly understood since previous studies were focused on basic physiological and biochemical parameters. This study was designed to examine leaf micromorphological traits and internal leaf elemental compartmentalization using Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray (EDX) to elucidate relative salinity tolerance mechanisms. Salt concentrations were applied to six treatments by increasing the concentrations of NaCl in a nutrient solution. The control treatment (0 mM) was irrigated solely by the nutritional solution, whereas other treatments contained graded NaCl concentrations (50, 100, 150, 200, and 250 mM). Micromorphological examination of the adaxial layer of the epidermis revealed distinctive glandular peltate trichomes among treatments. In control plants, the trichomes were flaccid and not easily detectable which resulted in low trichome density. While plants treated with salinity had uprooted trichomes which were modified to dish-like structures as salinity increases, with a more pronounce visibility at the highest salinity treatment (250 mM). On the contrary, increasing salinity reduced the stomatal density as well as stomatal opening with a more pronounced effect at 250 mM. Furthermore, the EDX revealed the presence of important elements such as potassium (K), calcium (Ca), magnesium (Mg), sodium (Na) and chlorine (Cl) which are responsible for salt tolerance in many species. Na increased with increasing saline treatment up to 100 mM then declined drastically. The lowest Na was detected in control plants which were comparable to plants irrigated with 150, 200 and 250 mM respectively. Likewise, the lowest chlorine content was detected in control plants while it increased in saline treated plants up to 150 mM and declined in plants irrigated with 200 and 250 mM respectively. When assessing K quantification, saline treatment drastically reduced K content with increasing saline irrigation. Contrariwise, the Mg content increased with saline irrigation up to 100 mM and was not detected in plants irrigated with 200 and 250 mM respectively. Interestingly, Ca was only detected in plants irrigated with 150, 200 and 250 mM respectively. These findings validate that T. decumbens can tolerate salinity by modulating anatomical features such as trichomes, control of stomatal aperture and effectively managing ion toxicity with increasing salinity.