Effects of Water and Salt Movement On Photosynthetic Production Characteristics and Yield Formation of Cotton On Coastal Saline Land

Background In order to study the effects of spatial distribution and yearly migration variations of soil water and salt in coastal saline land on photosynthetic production and yield formation of cotton, spatial distribution characteristics of water content, salinity, and pH in soil at 0–200 cm depths in 3 cotton elds that were similar in locality but differed markedly in degree of salinization were determined in April through October, and photosynthetic characteristics and photosynthate accumulation of cotton were also determined. Results The study shows that, the slightly salinized cotton eld had lower soil salinity and pH, where soil water content was lower in rainy season (July–August), and at the late reproductive stage (September–October), soil water content was markedly higher than that in the moderately salinized cotton eld, where cotton suffered smaller salt-alkali stress, photosynthetic production matched well with hydrothermal resource, the sink organ had a long photosynthate accumulation time and was at the active material accumulation stage for a long time; salt-alkali stress to the moderately salinized cotton eld was relieved in rainy season, but at the early reproductive stage (April–June) and the late reproductive stage of cotton, salt-alkali stress remained evident, and photosynthetic production t more poorly with the rich photothermal resource stage; the severely salinized cotton eld subjected to prolonged high salt-alkali stress resulted in low levels of photosynthetic production capacity and yield. cotton eld, salt inhibition agronomic actions taken with greater efforts; for the moderately salinized cotton eld, water and fertilizer management should be tightened at the early reproductive stage of cotton; for the slightly salinized cotton eld, high-quality ecient cotton production should be developed.


Introduction
Soil salinization has caused serious impact on global agricultural productivity and sustainable development, and to date, there is about 9.5×108 hm2 of salinized land worldwide, and over time, the area of the salinized land continues to increase and the level of salinization is continuously exacerbated (Qadir et al. 2008; Setia et al. 2012). China has a considerably large area of coastal saline land accounting for 40% of the total area of the saline land nationwide (Yang 2008;Dong 2010), the coastal areas have generally annual precipitations above 500 mm, indicating huge potential for agricultural production exploitation and utilization. Soil salinization of the coastal saline land affects soil physiochemical properties and plant growth (Zhang et al. 2010); rational exploitation and utilization of coastal saline land resource is one of important ways to address farmland resource plummeting and guarantee grain production security in China.
Numerous researchers have conducted in-depth studies on factors of spatial distribution of soil salinity in coastal saline land, effects of saline land amelioration actions on soil physiochemical properties, and mechanism of action of plant photosynthetic production (Fall 2017;Shan et al. 2011;Zhang et al. 2012; Wang et al. 2015; Wang et al. 2017), revealing a phenomenon during production practice that amelioration performance varied much after the same amelioration action was taken in the same area with the same groundwater salinity. In previous studies, variations in spatial distribution of soil water and salt in coastal saline land are mostly used to evaluate amelioration action performance, and more studies on soil water and salt movement focus on one period or part of the soil layer, resulting in discrepancy between soil amelioration performance evaluated based on the data during one period and crop production. Crop growth and development on the salinized land was seriously affected by soil salinity and pH level [4]; salinity altered crop photosynthetic system or structure-related physiological and biochemical processes, leading to decline in net photosynthetic rate (Kao et al. 2006). Fewer of the studies to date focus on the impact of variation in spatial distribution of soil water and salts during crop reproduction on crop photosynthetic production, while highly salt-resistant cotton is generally accepted as a pioneer crop for saline land exploitation (Dong 2010) and one important cash crop in China where its planting area accounts for about 15% of the cotton planting area in the world and its yield accounts for about 25% of the cotton yield worldwide (Yu 2013); planting cotton in coastal saline land has important social and economic bene ts. In this study, cotton elds in native state that differ much in degree of salinization within one coastal region were selected, variability of spatial water-salt distribution in soil layers of the salinized cotton elds at 0-200 cm depths was investigated in 3 time intervals, April-June, July-August, and September-October, depending on cotton production, climate, and precipitation in the coastal region, photosynthetic production characteristics of cotton varying with spatial distribution characteristics of soil water and salts were summarized, and their effects on photosynthetic production and yield of cotton were analyzed, so as to provide certain theoretical rationale for amelioration action establishment and adjudication objectivity and comprehensiveness.

Materials And Methods
The experiment was performed in Hebei state-owned Haixing Farm (38°21′N, 117°31′E) in 2015-2016. Three cotton elds (each having an area greater than 3.0 hm 2 ) differing much in topography, elevation, and bulk density of soil were selected, the soil of each cotton eld was coastal saline soil, and according to previous coastal saline land classi cation method (Dong et al.2009), these 3 cotton elds were classi ed into 3 categories varying in degree of salinization based on salt content of the plow layer (0-20 cm deep) in the second half of April; soil salinity and nutrient composition of the soil plow layer in each cotton eld are shown in Table 1. topdressing fertilizer (urea: 150 kg·hm -2 ) was applied once in rainy season, while eld management actions such as simpli ed pruning, chemical regulation, and disease and pest control were the same as conventional ones in local agricultural production.
Throughout cotton reproduction stage (April through October), the soil of each cotton eld at 0-200 cm depths was aliquotted into 10 soil layers, the soil in each soil layer was monthly sampled for determination of water content and salinity, and soil sampling each time was done in one day in order to minimize the impact of variability of soil water-salt distribution with time on sampling. Each sample was weighed, dried to constant weight at 105°C (6-8 h), cooled down in a desiccator for 30 min, and then weighed again to determine water content of the soil sample by mass; next, the dried soil and water were mixed at a ratio of 1:5 to result in extraction solution; then, pH was determined by PHS-3 pH meter, electrical conductivity was determined by DDS2307 conductivity meter, and soil salt content was obtained using conversion formula y=2.8311x+0.2932, where y is the salt content (g·kg -1 ) and x is the electrical conductivity (ms·cm-1). Six points per cotton eld were selected for the assays.
At basal and critical development stages of cotton yield formation, the following photosynthetic indicators were determined: 1) Leaf area index: determined by LAI-2200 canopy analyzer (LI-Cor, USA) in early morning or at dusk without direct sunlight. Firstly, the operator placed the probe level above the canopy, pressed down the test button, and after the instrument buzzed twice, placed the probe into crop population, kept it level, pressed the test button, and after listening to two buzz sounds, moved the probe horizontally at a uniform rate, and measured leaf areas at different positions in horizontal direction inside the canopy.
2) Canopy apparent photosynthesis rate and canopy respiration rate: directly determined by GXH-305 infra-red CO 2 analyzer in the eld. The height of the assimilation chamber was set depending on plant height at each reproductive stage, the chamber was a closed-circuit system, inside which a fan was installed to agitate the gas while the frame was shrouded with transparent polyester lm. Canopy apparent photosynthesis rate was determined in sunny days while light intensity was stabilized at 1200-1400 µmol·m -2 ·s -1 (at 9:00-11:00 Beijing time). Duration of each assay was 60 s. While the light was masked by special lampshade, canopy respiration rate was determined by the same method as that for determination of canopy apparent photosynthesis rate. For cotton plants treated in different ways, recurrent measurement was conducted. Meanwhile, soil respiration rate of a plot with similar structure of cotton population was measured to correct the measured values of population photosynthesis rate and population respiration rate.
3) SPAD value of relative chlorophyll content: Leaf SPAD value was determined by SPAD-502 chlorophyll meter (Minolta, Japan). Top fourth leaf of the cotton's main stem prior to topping and top second leaf of the cotton's main stem after topping were selected for measurement, and 10 leaves were selected and delineated at each of 3 selected points per cotton eld.

4) Dry mass accumulation and allocation:
In each cotton eld, 6 cotton plants representative in growth and development were selected, degraded, and xed at 105°C for 30 min, completely dried at 80°C, and then weighed; logistic equation y=a/(1+be-cx) was used to simulate cotton plant photosynthate accumulation process.

5)
Yield: Prior to harvesting, 6 sampling points, 30 m2 each, in each cotton eld were randomly selected, total number of plants and total number of bolls at each sampling point were investigated and converted into number of bolls per plant and number of bolls per unit area, and the yield was calculated based on actually harvested seed cotton; 2 neighboring narrow rows per point were randomly selected, each plant per row was investigated, a total of 10 plants were harvested, air dried to determine average boll weight, and ginned to determine lint percentage, then lint yield was calculated based on the average boll weight and the lint percentage.
Experimental data were statistically analyzed by SPSS11.0 and plotted by SigmaPlot 10.0.

Results
3.1. Characteristics of soil water and salt movement in cotton elds varying in degree of salinization 3.1.1. Spatiotemporal variation characteristics of soil water content Water content determination of soil layers at 0-200 cm depths in different types of cotton elds ( Figure 1) shows that, soil water content increased gradually in ascending order of depth, the lowest water content was noted in April through June, while the highest water content was noted in July through August. Among variably salinized cotton elds, the severely salinized cotton eld had the highest soil water content. In April through June, soil water contents of soil layers at 0-80 cm depths in the slightly salinized cotton eld were 12.6%-19.8%, not evidently differing from those in the moderately salinized cotton eld and 11.4%-30.5% lower than those in the severely salinized land; in July through August, soil water contents of soil layers at 0-140 cm depths were 14.1%-24.5%, which were 4.6%-15.5% lower than those in the moderately salinized cotton eld and 9.6%-33.7% lower than those in the severely salinized cotton eld; in September through October, soil water contents of soil layers at 0-140 cm depths were 13.9%-24.4%, which were 4.7%-12.8% higher than those in the moderately salinized cotton eld and 4.2%-29.8% lower than those in the severely salinized cotton eld.

Spatiotemporal variation characteristics of salinity
Soil salinity is an important indicator characterizing degree of salinization. Salt content determination of soil layers at 0-200 cm depths in different types of cotton elds ( Figure 2) shows that, soil salt content increased gradually in ascending order of depth. In terms of variability between months, the highest salinity was noted in April through June while the lowest salinity was noted in July through August. In terms of variability between cotton elds varying in degree of salinization, the highest salt content was observed in the severely salinized cotton eld, and the salt contents of each soil layer was not lower than 5.0 g·kg -1 ; in the slightly salinized cotton eld, salt contents of soil layers at 0-140 cm depths were not higher than 3.0 g·kg -1 ; in the moderately salinized cotton eld, salt contents of soil layers at 0-100 cm depths were 3.0-5.0 g·kg -1 in April through June, not higher than 3.0 g·kg -1 in July through August, and not more than 4.0 g·kg -1 in September through October; in the slightly and moderately salinized cotton elds, smaller differences in salt contents of soil layers at 0-140 cm depths in July through August were observed. Salt contents of soil layers at 0-140 cm depths in the slightly salinized cotton eld in April through June were 39.2%-65.1% lower than those in the severely salinized cotton eld, and 0.6%-42.9% lower than those in the moderately salinized cotton eld; in July through August, the salt contents were 40.0%-77.0% lower than those in the severely salinized cotton eld and 3.5%-43.1% lower than those in the moderately salinized cotton eld; in September through October, the salt contents were 37.6%-73.4% lower than those in the severely salinized cotton eld and 3.0%-51.2% lower than those in the moderately salinized cotton eld.

Spatiotemporal variation characteristics of pH
Determination of pH for soil layers at 0-200 cm depths in different types of cotton elds ( Figure 3) shows that, the severely salinized cotton eld had higher pH values than other types of cotton elds; pH values of soil layers at 0-140 cm depths in the slightly salinized cotton eld were lower than those in the moderately salinized cotton eld, whereas pH values of other soil layers in various types of cotton elds did not differ signi cantly. In terms of variability of pH between months, the highest pH occurred in April through June while the lowest pH was observed in July through August. All the pH values of various soil layers in the severely salinized cotton eld during the assay were greater than 8.0. In April through June, pH values of soil layers at 0-140 cm depths in the slightly salinized cotton eld were 7.4-8.0, 5.8%-11.1% lower than those in the severely salinized cotton eld and 3.5%-7.6% lower than those in the moderately salinized cotton eld; in July through August, pH values remained at 7.0-7.5, 12.2%-17.2% lower than those in the severely salinized cotton eld and 1.4%-3.7% lower than those in the moderately salinized cotton eld; in September through October, pH values in the slightly salinized cotton eld remained at 7.5-7.8, 5.0%-9.9% lower than those in the severely salinized cotton eld and 5.0%-7.2% lower than those in the moderately salinized cotton eld.
3.2. Photosynthetic characteristics of the cotton in cotton elds varying in degree of salinization Page 6/18

Leaf area index and chlorophyll content
The study shows that (Figure 4), for each type of cotton eld, both leaf area index and chlorophyll content of the cotton canopy increased until peaking in the second half of August, and then started to decrease; the differences among cotton elds varying in degree of salinization were evident. In the severely salinized cotton eld, values and ranges of variation of both indicators were smaller, remaining within narrower ranges throughout the reproductive stage; the values of both indicators in the moderately salinized cotton eld in June and July were obviously lower than those in the slightly salinized cotton eld, and when they peaked in the second half of August, their differences from those in the slightly salinized cotton eld became smaller, then they decreased quickly; the values of both indicators in the slightly salinized cotton eld were higher, and decreased more slowly at the late stage. In the slightly salinized cotton eld, values of leaf area index were 0.33-4.38, 105.3%-290.0% higher than those in the severely salinized cotton eld and 0.1%-73.2% higher than those in the moderately salinized cotton eld; chlorophyll SPAD values were 34.8-49.6, 23.5%-54.3% higher than those in the severely salinized cotton eld and 0.4%-38.0% higher than those in the moderately salinized cotton eld.

Canopy apparent photosynthesis rate and canopy respiration rate
The study shows that (Figure 3), canopy apparent photosynthesis rate and canopy respiration rate of cotton in each type of cotton eld increased until peaking in the second half of August, and then started to decrease; the differences in both indicators of the cotton between cotton elds varying in degree of salinization were signi cant. In the severely salinized cotton eld, values and ranges of variation of both indicators were smaller; the values of both indicators in the moderately salinized cotton eld in June and July were markedly less than those in the slightly salinized cotton eld, and when they peaked in the second half of August, their differences from those in the slightly salinized cotton eld became smaller, and then the values decreases quickly; the values in the slightly salinized cotton eld were higher, and decreased more slowly at the late stage. In the slightly salinized cotton eld, canopy apparent photosynthesis rates were 21.8-38.6 μmol/(m 2 s), 30.9%-339.5% higher than those in the severely salinized cotton eld and 7.4%-183.6% higher than those in the moderately salinized cotton eld; the canopy respiration rates were 3.65-9.51 μmol/(m 2 s), 46.0%-162.8% higher than those in the severely salinized cotton eld, 3.6%-22.2% higher than those in the moderately salinized cotton eld at the early reproductive stage, and 16.2%-25.3% lower than those in the moderately salinized cotton eld at other reproductive stages.

Photosynthate accumulation and allocation characteristics of organs
Photosynthate accumulation and allocation determination for source and sink organs of cotton plants in different types of cotton elds (Table 2) shows that, for photosynthate accumulation of source and sink organs in the severely salinized cotton eld, linear growth stage started and ended earlier, duration and active mass accumulation stage were shorter, and the maximum growth rate was smaller; source organs in the moderately and slightly salinized cotton elds differed not much in linear growth stage ending time and maximum growth rate onset time, but varied much in linear growth stage starting time; photosynthate accumulation of the sink organ increased with higher degree of salinization, linear growth stage ending time and maximum growth rate onset time were expedited, while the growth stage, maximum growth rate, and active mass accumulation stage decreased. In terms of photosynthate accumulation of the source organ, linear growth in the severely salinized cotton eld, respectively, and 14-18 d, 25-35 d, and 20-26 d earlier than those in the moderately salinized cotton eld, respectively; the linear growth stage, maximum growth rate, and active mass accumulation stage of photosynthate accumulation were 44.3%-69.6%, 163.4%-182.1%, and 44.3%-69.6% higher than those in the severely salinized cotton eld, respectively, and 28.7%-43.0%, 49.7%-50.6%, and 28.7%-43.0% higher than those in the moderately salinized cotton eld, respectively.

Yields of cotton and yield components
Based on statistics of yield measurements and actual yields (Table 3), different types of cotton elds differed signi cantly in yield. In the slightly salinized cotton eld, seed cotton yield was more than 3000 kg·hm −2 , average lint yield reached 1200 kg·hm −2 , and the lint yields were 91.0%-130.8% higher than those in the severely salinized cotton eld and 36.0%-40.7% higher than those in the moderately salinized cotton eld. The lint yields in the moderately salinized cotton eld were above 900 kg·hm −2 , 40.4%-64.0% higher than those in the severely salinized cotton eld. It's found in further analysis of yield components that, the cotton elds varying in degree of salinization differed signi cantly in number of harvested plants and number of bolls per unit area, the lower the degree of salinization, the more the number of harvested plants and number of bolls per unit area; differences in boll weight and lint percentage between the severely and the moderately salinized cotton elds were insigni cant, and both indicators were signi cantly lower than those in the slightly salinized cotton eld. No evident difference was noted in cotton yield and yield components in two severely salinized cotton elds. Therefore, number of bolls per unit area was the main contributor to lower cotton yield in the severely salinized cotton eld, and boll weight and lint percentage limited further increase in the yield of cotton in the moderately salinized cotton eld.  1992). This study shows that, regardless of type of salinized cotton eld, soil salinity and pH peaked in April through June (arid and rainless spring and summer), decreased signi cantly in July through August (rainy summer and autumn), and increased somewhat in September through October (post-rainy autumn). In July through August, soil salt washing of the slightly salinized cotton eld caused further decline in soil salinity and pH. In the moderately salinized cotton eld, soil salinity and pH decreased markedly due to precipitation washing, too. In September through October, pH values of the soil at 0-140 cm depths in the slightly salinized cotton eld increased insigni cantly, whereas soil salinity and pH in the moderately salinized cotton eld increased markedly. Soil salinity and pH in the severely salinized cotton eld were signi cantly higher than those in other types of cotton elds. Meanwhile, low salinity and low pH are conducive to increase in leaf chlorophyll content of cotton in the slightly salinized cotton eld, laying a foundation for improvement in photosynthetic production and yield of cotton at the late reproductive stage (Sun and Hong 2011); in the cases of moderately and severely salinized cotton elds, even with higher soil water contents, salt-alkali stresses such as high salinity and high pH not only result in more intense ion injury to cotton growth but will lead to physiological drought, therefore, not only leaf growth is suppressed but leaf chlorophyll synthesis is also hindered (Yang et al. 2011). In July through August (rainy season), in slightly and moderately salinized cotton elds, salt-alkali stress is signi cantly relieved, leaves grow quickly, and chlorophyll content also increases signi cantly, resulting in signi cant increase in photosynthetic rate (Dong et al. 2012), indicating stronger photosynthetic production capacity. However, when early growth of cotton in the moderately salinized cotton eld was evidently suppressed, soil water content was relatively high and vegetative growth became particularly pronounced; in September through October (post-rainy autumn), salinity and pH increased quickly, leaves turned yellow quickly and fell, resulting in evident decline in photosynthetic rate. In the case of severely salinized cotton eld, salt-alkali stress remained at a higher level throughout the reproductive stage, resulting in signi cantly insu cient photosynthetic capacity.
Water content transport in coastal saline land features strong season and salt-alkali synergy. Based on this study, in different types of salinized cotton elds, soil water content peaked in April through June (arid and rainless spring and summer), decreased signi cantly in July through August (rainy summer and autumn), and increased somewhat in September through October (post-rainy autumn). Soil water contents in the severely salinized cotton eld signi cantly were higher than those in other types of cotton elds; in September through October, soil water contents in moderately salinized cotton eld decreased markedly, and water contents of the soil at 0-140 cm depths in the slightly salinized cotton eld were markedly higher than those in the moderately salinized cotton eld. As indicated in previous study, soil salinity accumulation will enable permeation stress to plants and preclude water content-related permeation regulating material in the plant from exerting its role, so that the moisture fails to be effectively transported to leaves (Singh et al. 2015). This study shows that, in the slightly salinized cotton eld, the cotton at seedling stage was subjected to minimum salt-alkali stress, and in rainy season, the salt-alkali stress was relieved, soil water content was modest, and photosynthetic production was more robust; furthermore, in the post-rainy autumn, milder salt-alkali stress and higher soil water content were noted, leaves remained at a longer physiological stage, and photosynthetic rate was kept at a higher level for a long time. In the case of the moderately salinized cotton eld, as early growth was evidently inhibited, soil water content was relatively high and vegetative growth was particularly pronounced in July through August (rainy summer and autumn), resulting in signi cant increases in number of owers and number of bolls; in September through October (post-rainy autumn), soil water content dropped quickly and extent of salt-alkali stress increased markedly, thus salt-alkali drought led to evident reduction in photosynthetic rate. In the case of the severely salinized cotton eld, although soil water content was high throughout reproductive stage, salt stress will undermine the water absorption capacity of plant root system by reducing soil water potential, and will further weaken the effectiveness of water conduction via the xylem so that each leaf stoma will be partially closed to reduce transpiration and chloroplast structure will be disrupted, leading to intense inhibition of photosynthesis (Rahnama et al. 2010).

Effects of variations in photosynthetic characteristics of cotton in coastal saline cotton eld on photosynthetic production and yield of cotton
Under abiotic stresses, a plant maintains growth by altering morphological characteristics (Huang et al. 2012). The effect of salt-alkali stress on cotton in coastal saline land is also represented in photosynthate accumulation characteristic (Wang et al. 2014). Immediate impact of salt-alkali stress on photosynthetic production of cotton is one of the main causes of variation in yield level of cotton in salinized land (Dong et al. 2012). This study shows that, in coastal cotton elds varying in degree of salinization, cotton growth was maintained and cotton yield was formed, thanks to different photosynthate accumulation characteristics of the source and sink organs. In the severely salinized cotton eld, photosynthate accumulation of the source and sink organs had a linear growth stage starting and ending earlier, shorter duration, shorter active mass accumulation stage, and smaller maximum growth rate; heavier salt-alkali stress and longer duration lead to low match between photosynthetic production and photothermal resource, lower photosynthetic accumulation rate (Stefanov et al. 2018), and shorter duration of fast accumulation. Therefore, photosynthate accumulation decreased signi cantly probably because soil salt concentration reaching a threshold caused permeation stress to the crop and disturbed nutrient ion balance, resulting in dynamic imbalance of ions in the plant, metabolic disorder of intracellular reactive oxygen, reduced photosynthesis and retarded in vivo energy metabolism, inhibited crop growth and development, and even massive cotton shriveling and yield reduction (Chen and Murata 2011); fewer harvested plants and bolls as well as lower boll weight resulted in low yield of cotton in the severely salinized cotton eld. Cotton in the moderately salinized cotton eld was subjected to milder salt-alkali stress, which became relieved for a while but exacerbated again at the late reproductive stage, thus, photosynthetic production matched with photothermal resource to some extent, but late salt-alkali stress enabled premature ending of photosynthate accumulation, which is very detrimental to reproductive organ growth and development, and resulted in insu cient boll development and yield reduction. Compared with those in the moderately salinized cotton eld, the linear growth stage starting time, linear growth stage ending time, and maximum growth rate onset time of source-organ photosynthate accumulation in the slightly salinized cotton eld were -4 d, (-1)-3 d, and (-1)-(-3) d earlier, respectively, the linear growth stage, maximum growth rate, and active mass accumulation stage of photosynthate accumulation were 6.0%-16.9%, 8.0%-18.1%, and 5.9%-17.0% higher, respectively, the linear growth stage starting time, linear growth stage ending time, and maximum growth rate onset time of sink-organ photosynthate accumulation were 14-18 d, 25-35 d, and 20-26 d earlier, respectively, and the linear growth stage, maximum growth rate, and active mass accumulation stage of photosynthate accumulation were 28.7%-43.0%, 49.7%-50.6%, and 28.7%-43.0% higher, respectively. Therefore, in the slightly salinized cotton eld, photosynthate accumulation matched with photothermal resource well, the linear growth stage, maximum growth rate, and active mass accumulation stage were markedly higher, vigorous growth stage of the source organ started later while vigorous growth stage of the sink organ started earlier, in case over-allocation of photosynthate and vegetative growth would result in yield reduction at the vigorous growth stage; moreover, yield formation time was signi cantly extended, boll development was more su cient, and individual boll weight and lint percentage were higher, facilitating allocation of more accumulated photosynthates to reproductive growth so as to improve yield effectively.
Increasing photosynthetic capacity of a crop is an effective condition of yield improvement (Feng et al. 2014;Yao et al. 2016). Therefore, routine cultivation management measures should be speci cally adjusted. For severely salinized cotton elds, salt inhibition and other related agronomical measures should be strengthened to minimize the impact of salt-alkali stress on normal growth and development of cotton; for moderately salinized cotton elds, water and fertilizer management of cotton at early reproductive stage should be tightened to promote vegetative growth of cotton in case source-sink development mismatch problem arises at boll setting stage; in contrast, photosynthetic performance and photosynthetic production of cotton in the slightly salinized cotton eld are less affected, where high quality e cient production of cotton can be achieved.

Conclusions
In the slightly salinized cotton eld, soil water content in rainy season was lower than that at boll opening stage; the cotton was subjected to smaller salt-alkali and waterlogging stresses, and with higher photosynthetic performance and better match between photosynthetic production and hydrothermal resource, the cotton remained at active mass accumulation stage for a long time; in the moderately salinized cotton eld, evident salt-alkali stress was present at early and later reproductive stages, and yield formation stage was poorly matched with rich photothermal resource stage; in the severely salinized cotton eld, the cotton was subjected to prolonged high salt-alkali stress and its photosynthetic performance became heavily suppressed, resulting in inferior photosynthetic production capacity and yield level.

Declarations
Ethics approval and consent to participate Not applicable.

Consent for publication
Not applicable.
Availability of data and materials The datasets used and analysed during the current study are available from the corresponding author on reasonable request.

Competing interests
The authors declare that they have no competing interests.  Vertical distribution and variation of soil moisture in the cotton eld during the cotton growth period relative to soil salinealkali level Note: A: Cotton eld with heavy saline-alkali degree; B: Cotton eld with middle saline-alkali degree; C: Cotton eld with mild saline-alkali degree.

Figure 2
Vertical distribution and variation of soil salt content in the cotton eld during the cotton growth period relative to soil saline-alkali level Note: A: Cotton eld with heavy saline-alkali degree; B: Cotton eld with middle saline-alkali degree; C: Cotton eld with mild saline-alkali degree.
Page 16/18 Change of leaf area and SPAD value in the cotton eld during the growth period relative to soil saline-alkali level Note:A: Cotton eld with heavy saline-alkali degree; B: Cotton eld with middle saline-alkali degree; C: Cotton eld with mild salinealkali degree.