Effects of different ratios of mineral nitrogen form on Helianthus tuberosus seedlings physiology under salt stress

A sand cultural experiment was conducted to study the photosynthetic characteristics and ion absorption of Helianthus tuberosus seedlings under the coupling effects of nitrogen nutrition (NH4+-N/NO3--N ratio 4/1,1/1,and 1/4) and salt stress. Low concentration salt stress had less effect in inhibiting H. tuberosus growth,while high concentration salt stress inhibited the growth remarkably. Under the same salt concentrations,increasing proportion of NO3--N could mitigate the inhibition effect of salt stress,and increase the fresh and dry weights of H. tuberosus. With the increase of salt concentration,the Na+ concentration in seedling's shoots and roots increased significantly,while the K+ and Ca2+ concentrations decreased. Increasing the proportion of NO3--N promoted the selective absorption of K+ and Ca2+ and their translocation from roots to shoots,making shoots keep high ratios of K+/Na+ and Ca2+/Na+. Under the three ratios of NH4+-N/NO3--N and with increasing salt concentration,the leaf chlorophyll content,photosynthetic rate,transpiration rate,stomatal conductance,and water use efficiency all decreased significantly,while the intercellular CO2 concentration had a significant increase. With the increase of the proportion of NO3--N and under the same salt concentrations,the chlorophyll content,photosynthetic rate,transpiration rate,stomatal conductance,and water use efficiency increased significantly,while the intercellular CO2 concentration decreased significantly. Nitrogen nutrition at NH4+-N/NO3--N ratio of 1/4 had the best alleviation effect to salt stress,e.g.,under 100 mmol·L-1 NaCl stress,the chlorophyll content,photosynthetic rate,transpiration rate,stomatal conductance,and water use efficiency at NH4+-N/NO3--N ratio of 1/4 increased by 1.63,1.57,1.39,1.37,and 1.2 times,respectively,while the intercellular carbon dioxide concentration decreased by 17%,compared with those at the NH4+-N/NO3--N ratio of 4/1. Therefore,relative to NH4+-N,the increase of the proportion of NO3--N was more conducive to maintain the balance of mineral nutrients,promote the photosynthesis,and enhance the resistance of H. tuberosus to salt stress.