Abstract: 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.

Key words: Apple quality, Meteorological factors, Multiple statistical analysis, Linear program, Climate divisions