Soil water and salt affect cotton (Gossypium hirsutum L.) photosynthesis, yield and fiber quality in coastal saline soil

To target the favorable conditions for cotton growth in coastal saline soil, a two year field experiment was conducted in 2013 and 2014 by setting various environments 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 (1) in relatively arid year of 2013, soil exhibited normal soil-relative water content with high salt and mild drought with moderate salt in the optimal groundwater depths of 1.4 m and of 1.8 m (1.87 m for fitting), respectively. In relatively humid year of 2014, soil displayed normal soil-relative water content with low salt in the optimal groundwater depths of 1.6 m and 2.0 m (1.73 m for fitting). (2) Net photosynthesis, cotton yield and fiber quality all approached to the highest values in the optimal treatments. The reduction in net photosynthetic rate was mainly due to non-stomatal restriction in the treatment of 0.6 m in 2013 and 0.4 m in 2014. Meanwhile, in other treatments stomatal restriction was the main factor for photosynthesis limitations. As compared to 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 of 0.4 m, 2.4 m in 2014, respectively. Reduced boll number played a critical role to decrease seed cotton yield. In summary, the favorable soil-relative water contents for 0-20 and 20-40 cm soil depth were 54.68%-65.14% and 69.14%-79.13% in dry year of 2013 and 67.18%-69.39% and 73.00%-77.92% in humid year of 2014, respectively; similarly, electrical conductivity of a 1:5 distilled water for 0-20 and 20-40 cm soil depth was recorded as 0.92-1.20 dS m−1 and 0.70-0.95 dS m−1 in 2013, while 0.28-0.32 dS m−1 and 0.45-0.51 dS m−1 in 2014, respectively.