Is High pH the Key Factor of Alkali Stress on Plant Growth and Physiology? A Case Study with Wheat (Triticum aestivum L.) Seedlings

Salinity and alkalinity stress are two major constraints on plant growth and crop production, limiting sustainable agricultural production. Wheat is a vital cereal crop. It is very important to ensure food security; however, its growth and yield are usually adversely affected by salinity and alkalinity stress. To investigate the differential effects of neutral and alkaline salt stress on the seedling growth of wheat, we set wheat hydroponic culture experiment: CK, neutral salt (NaCl:Na2SO4 = 9:1 pH = 6.5), neutral salt with high pH value (NaCl:Na2SO4 = 9:1 pH = 8.9), alkaline salt (NaHCO3:Na2CO3 = 9:1 pH = 8.9), all treatments at the same Na+ concentration. The results indicated alkaline salt inhibited seedling growth more than neutral salt and neutral salt with high pH value. The results showed that the salt and alkali stresses decreased chlorophyll contents in leaves of wheat seedlings, inhibited photosynthesis and induced osmotic stress, oxidative stress and ion toxicity to wheat seedlings and finally inhibited the growth of wheat seedlings, while the alkaline salt caused a stronger injurious effect on wheat seedlings than the neutral salt, neutral salt with high pH value. Our study results demonstrated that alkaline salt inhibited wheat seedlings more significantly than neutral salt and neutral salt with high pH value. And, the main factor affected seedling growth is not pH alone.