Impacts of Salinity and Nitrogen on the Photosynthetic Rate and Growth of Sunflowers (Helianthus annuus L.)

Understanding the interactions between salinity and fertilizers is of significant importance for enhancing crop yield and fertilizeruse efficiency. In this study a complete block design experiment was performed in the Hetao Irrigation District of Inner Mongolia, China, to evaluate the effects of interactions between soil salinity and nitrogen (N) application rate on sunflower photosynthesis and growth and to determine the optimum N application rate for sunflower growth in the district. Four levels of soil salinity expressed as electrical conductivity (0.33–0.60, 0.60–1.22, 1.2–2.44, and 2.44–3.95 dS m−1) and three application rates of N fertilization (90, 135, and 180 kg ha−1) were applied to 36 micro-plots. Soil salinity inhibited the photosynthetic rate, stomatal conductance, transpiration rate, plant height, leaf area, and aboveground dry matter of sunflowers. The intercellular CO2 concentration first decreased and then increased with increasing soil salinity in the seedling stage, and the instantaneous leaf water-use efficiency fluctuated with soil salinity. The stomatal and non-stomatal limitations of sunflowers alternated in the seedling stage; however, in the bud, blooming, and mature stages, the stomatal limitation was prevalent when the salinity level was lower than 2.44 dS m−1, whereas the non-stomatal limitation was predominant above the salinity level. The application of N fertilizer alleviated the adverse effects of salinity on sunflower photosynthesis and growth to some extent. During some key growth periods, such as the seedling and bud stages, a moderate N application rate (135 kg ha−1) resulted in the maximum photosynthetic rate and yielded the maximum dry matter. We suggest a moderate N application rate (135 kg ha−1) for the Hetao Irrigation District and other sunflower-growing areas with similar ecological conditions.