Ecophysiological adaptations and anti-nutritive status of sustainable cattle feed Haloxylon stocksii under saline conditions

Abstract

Haloxylon stocksii is a perennial halophytic shrub found in saline arid habitats. Ecophysiology of this xero-halophyte was studied to better understand its adaptations to survive under saline conditions (0–300 mM NaCl). Growth parameters remained unaffected at 100 mM NaCl compared to non-saline control and declined at 300 mM; root biomass however, increased in 100 mM NaCl. Shoot water potential, osmotic potential and turgor decreased under saline conditions. Gas exchange parameters such as rate of photosynthesis, rate of transpiration and stomatal conductance decreased in 300 mM NaCl with a decline in intercellular carbon dioxide concentration suggesting stomatal limitation of photosynthesis. Water use efficiency increased significantly in 300 mM NaCl compared to 0 and 100 mM NaCl. Chlorophyll content decreased progressively with increasing salinity while chlorophyll a/b ratio remained unaffected. Potential photochemical quantum yield of PSII (Fv/Fm) and the effective quantum yield [Y (II)] were not affected by salinity; however, there was a constitutive increase in non-photochemical quenching under saline conditions (100 and 300 mM) and decrease in relative electron transport rate in 300 mM NaCl as a protective measure to reduce oxidative stress at cellular level. Total soluble sugars increased by ≈25% in 300 mM NaCl possibly indicating a lower energy demand for growth. Anti-nutritive secondary metabolites such as polyphenols, tannins, flavonoids, saponins and nitrates decreased under saline conditions except for some increase in oxalates in 100 mM NaCl. This study indicates that Haloxylon stocksii is a moderately salt tolerant plant with adaptations to down- regulate its photosynthetic machinery to avoid oxidative stress and could be a palatable animal feed.