Increasing soil salinity suppresses both productivity and fiber quality of cotton, thus, an appropriate management approach needs to be developed to lessen the detrimental effect of salinity stress. This study assessed two cotton genotypes with different salt sensitivities to investigate the possible role of nitrogen supplementation at the seedling stage. Salt stress induced by sodium chloride (NaCl, 200 mmol·L −1 ) decreased the growth traits and dry mass production of both genotypes. Nitrogen supplementation increased the plant water status, photosynthetic pigment synthesis, and gas exchange attributes. Addition of nitrogen to the saline media significantly decreased the generation of lethal oxidative stress biomarkers such as hydrogen peroxide, lipid peroxidation, and electrolyte leakage ratio. The activity of the antioxidant defense system was upregulated in both saline and non-saline growth media as a result of nitrogen application. Furthermore, nitrogen supplementation enhanced the accumulation of osmolytes, such as soluble sugars, soluble proteins, and free amino acids. This established the beneficial role of nitrogen by retaining additional osmolality to uphold the relative water content and protect the photosynthetic apparatus, particularly in the salt-sensitive genotype. In summary, nitrogen application may represent a potential strategy to overcome the salinity-mediated impairment of cotton to some extent.Plants 2020, 9, 450 2 of 20 most sensitivity to salinity during the seedling growth stage, and remained sensitive at the reproductive stage [9]. Excess salinity inhibits plant growth and development by inducing osmotic stress, particular ion toxicity (Na + and Cl − ), oxidative damage, and/or nutritional disorders in plant tissues, which subsequently lead to weakened plant growth and endurance [10][11][12]. Salt stress has been shown to significantly decrease the uptake and metabolism of numerous mineral nutrients (such as nitrogen, potassium, phosphorus, and calcium), and accelerate the damage of the photosynthetic machinery as well as the whole salt tolerance mechanisms of plants. Salt stress-induced plant metabolism alterations are the secondary consequence of carbon and nitrogen metabolism [13]. Moreover, a high salt concentration perturbs electron transport systems in both chloroplasts and mitochondria [14,15]. This accelerates electron leakage from electron transportation chains and induces the generation of reactive oxygen species (ROS), such as superoxide radicals, hydroxyl radicals, and hydrogen peroxide [16,17]. The over-accumulation of ROS not only damages cellular and biological activities [18][19][20], but also degrades chlorophyll pigments and obstructs the synthesis of proteins, amino acids, lipids, deoxyribonucleic acid, as well as enzymatic and non-enzymatic activities of plants [17]. To avoid ROS-induced oxidative damage, plant-assured endogenous tolerance approaches, such as the antioxidant defense system as well as the accumulation of organic solutes and secondary metabolites [21]....

Chemical defoliation is an important part of cotton mechanical harvesting, which can effectively reduce the impurity content. Thidiazuron (TDZ) is the most used chemical defoliant on cotton. To better clarify the mechanism of TDZ promoting cotton leaf abscission, a greenhouse experiment was conducted on two cotton cultivars (CRI 12 and CRI 49) by using 100 mg L −1 TDZ at the eight-true-leaf stage. Results showed that TDZ significantly promoted the formation of leaf abscission zone and leaf abscission. Although the antioxidant enzyme activities were improved, the reactive oxygen species and malondialdehyde (MDA) contents of TDZ increased significantly compared with CK (water). The photosynthesis system was destroyed as net photosynthesis (Pn), transpiration rate (Tr), and stomatal conductance (Gs) decreased dramatically by TDZ. Furthermore, comparative RNA-seq analysis of the leaves showed that all of the photosynthetic related genes were downregulated and the oxidation-reduction process participated in leaf shedding caused by TDZ. Consequently, a hypothesis involving possible cross-talk between ROS metabolism and photosynthesis jointly regulating cotton leaf abscission is proposed. Our findings not only provide important insights into leaf shedding-associated changes induced by TDZ in cotton, but also highlight the possibility that the ROS and photosynthesis may play a critical role in the organ shedding process in other crops.Previous studies have found that ethylene accelerates organ abscission, while auxin inhibits this process [5][6][7][8][9]. Meanwhile, researchers have paid attention to the fact that multiple cell wall-degrading enzymes are activated to dissolve the cell wall and middle lamella of abscission zone, including cellulases, pecticlyases, polygalactosidases [10][11][12]. Some research has found that hydrogen peroxide (H 2 O 2 ) acts downstream from ethylene and plays a role in the cell-wall degradation process in abscission signaling [13,14].Reactive oxygen species (ROS), including H 2 O 2 , superoxide, singlet oxygen, and the hydroxyl radical, are produced in response to environmental stress [13]. Environmental stresses (such as drought, cold stress, salt stress, and pathogen attack) are often accompanied by leaf abscission [15][16][17], but the relationship between stress and abscission is rarely studied. Excessive ROS can damage cellular components, including lipids, protein, and nucleic acids [18]. ROS produced at the abscission zone site play an important role in regulating leaf abscission under drought stress in cassava (Manihot esculenta Crantz) [19]. Recent studies have found that ROS do not work in the fruit abscission zone of olives (Olea europaea L.), but only alter oxidative stress in the abscission zone of leaves and then mediate abscission induced by ethephon [20]. Previous studies have shown that insufficient carbohydrate accumulation and distribution can lead to flower and fruit abscission. For example, carbohydrate stress can induce longan (Dimocarpus longan Lour.) fruit abscissi...

Background: Salt stress significantly inhibits the growth, development, and productivity of cotton because of osmotic, ionic, and oxidative stresses. Therefore, the screening and development of salt tolerant cotton cultivars is a key issue towards sustainable agriculture. This study subjected 11 upland cotton genotypes at the seedling growth stage to five different salt concentrations and evaluated their salt tolerance and reliable traits. Results: Several morpho-physiological traits were measured after 10 days of salinity treatment and the salt tolerance performance varied significantly among the tested cotton genotypes. The optimal NaCl concentration for the evaluation of salt tolerance was 200 mmol•L − 1. Membership function value and salt tolerance index were used to identify the most consistent salt tolerance traits. Leaf relative water content and photosynthesis were identified as reliable indicators for salt tolerance at the seedling stage. All considered traits related to salt tolerance indices were significantly and positively correlated with each other except for malondialdehyde. Cluster heat map analysis based on the morpho-physiological salt tolerance-indices clearly discriminated the 11 cotton genotypes into three different salt tolerance clusters. Cluster I represented the salt-tolerant genotypes (Z9807, Z0228, and Z7526) whereas clusters II (Z0710, Z7514, Z1910, and Z7516) and III (Z0102, Z7780, Z9648, and Z9612) represented moderately salttolerant and salt-sensitive genotypes, respectively. Conclusions: A hydroponic screening system was established. Leaf relative water content and photosynthesis were identified as two reliable traits that adequately represented the salt tolerance of cotton genotypes at the seedling growth stage. Furthermore, three salt-tolerant genotypes were identified, which might be used as genetic resources for the salt-tolerance breeding of cotton.