Soil Water and Salt Affect Cotton Photosynthesis, Yield and Fiber Quality in Coastal Saline Soil

A two-year field experiment was conducted by setting various conditions for soil water and salt with different groundwater depths (0.6, 1.0, 1.4, 1.8, 2.2, 2.6 m in 2013 and 0.4, 0.8, 1.2, 1.6, 2.0, 2.4 m in 2014). Results showed that the leaf area per plant, chlorophyll content, net photosynthesis, total biomass per plant, cotton yield and fiber quality all came to the higher values in the groundwater depths of 1.4 m where the soil exhibited normal soil每relative water content with high salt and 1.8 m where the soil displayed mild drought with moderate salt in arid year of 2013, and in the optimal groundwater depths of 1.6 m and 2.0 m where the soil all showed normal soil每relative water content with low salt. Therefore, the groundwater depths of 1.4 m and 1.8 m (1.87 m for fitting) in 2013, 1.6 m and 2.0 m (1.73 m for fitting) in 2014 were regarded as the optimal groundwater depths. Otherwise, non每stomatal restriction which led to the reduction in net photosynthetic rate occurred in the treatment of 0.6 m in 2013 and 0.4 m in 2014. Compared to the average value of seed cotton yield in the optimal groundwater depths, the seed cotton yield was dropped by 73.9%, 21.4% and 71.4%, 21.4% under groundwater depths of 0.6 m, 2.6 m in 2013 and 0.4 m, 2.4 m in 2014, respectively. Reduced boll number played a critical role to decrease seed cotton yield. The decrease of fiber quality was mainly caused by fiber strength. In summary, the favorable soil每relative water content for 0-40 cm soil depth was 61.91%每72.14% in 2013 and 70.09%每73.66% in 2014; similarly, the favorable electrical conductivity of a 1:5 distilled water for 0每40 cm soil depth was recorded as 0.81每1.08 dS m每1 in 2013, while 0.37每0.42 dS m每1 in 2014.