Salinity stress is a major limitation to global crop production. Sugar beet, one of the world's leading sugar crops, has stronger salt tolerant characteristics than other crops. To investigate the response to different levels of salt stress, sugar beet was grown hydroponically under 3 (control), 70, 140, 210 and 280 mM NaCl conditions. We found no differences in dry weight of the aerial part and leaf area between 70 mM NaCl and control conditions, although dry weight of the root and whole plant treated with 70 mM NaCl was lower than control seedlings. As salt concentrations increased, degree of growth arrest became obvious In addition, under salt stress, the highest concentrations of Na and Cl were detected in the tissue of petioles and old leaves. N and K contents in the tissue of leave, petiole and root decreased rapidly with the increase of NaCl concentrations. P content showed an increasing pattern in these tissues. The activities of antioxidant enzymes such as superoxide dismutase, catalase, ascorbate peroxidase and glutathione peroxidase showed increasing patterns with increase in salt concentrations. Moreover, osmoprotectants such as free amino acids and betaine increased in concentration as the external salinity increased. Two organic acids (malate and citrate) involved in tricarboxylic acid (TCA)-cycle exhibited increasing contents under salt stress. Lastly, we found that Rubisco activity was inhibited under salt stress. The activity of NADP-malic enzyme, NADP-malate dehydrogenase and phosphoenolpyruvate carboxylase showed a trend that first increased and then decreased. Their activities were highest with salinity at 140 mM NaCl. Our study has contributed to the understanding of the sugar beet physiological and metabolic response mechanisms under different degrees of salt stress.

Soil-related obstacles resulting from continuous monoculture have limited the sustainable development of the tomato industry in China. An experiment on tomatoes with seven continuous monoculture treatments (the 1st, 3rd, 5th, 7th, 9th, 11th, and 13th crops, respectively) was conducted in a solar greenhouse, to investigate the influence of monoculture on soil quality. Most soil quality indicators first increased and then decreased with increasing continuous monoculture crops, and significant differences among crops were observed. Indicators at the 13th crop were significantly lower than those at the other crops in terms of average well color development (AWCD), substrate richness (S), the Shannon diversity index (H), and the McIntosh index (U) of the soil microbial community (SMC), soil urease (UR), and neutral phosphatase (N-PHO) activities, and available nitrogen (AN) and potassium (AK). However, fungal abundance (FUN) at the 13th crop was significantly higher than that at the other crops. As principal component analysis (PCA) revealed, SMC functional diversity at the 1st, 11th, and 13th crops were similar, and were obviously distinguished from those at the other crops. Moreover, the tomato yield was significantly and positively correlated with soil-available potassium and SMC functional diversity indexes. Our findings indicated that short-term continuous monoculture, e.g., for fewer than seven or nine crops, was beneficial for soil quality improvement. However, continuous monoculture for greater than 11 crops had adverse effects on soil enzyme activities, soil microbial abundances, soil chemical properties, soil SMC functional diversity, and the tomato yield, particularly at the 13th crop.

Soil salinization is a common environmental problem that seriously affects the yield and quality of crops. Sugar beet (Beta vulgaris L.), one of the main sugar crops in the world, shows a strong tolerance to salt stress. To decipher the molecular mechanism of sugar beet under salt stress, we conducted transcriptomic analyses of two contrasting sugar beet genotypes. To the best of our knowledge, this is the first comparison of salt-response transcriptomes in sugar beet with contrasting genotypes. Compared to the salt-sensitive cultivar (S710), the salt-tolerant one (T710MU) showed better growth and exhibited a higher chlorophyll content, higher antioxidant enzyme activity, and increased levels of osmotic adjustment molecules. Based on a high-throughput experimental system, 1714 differentially expressed genes were identified in the leaves of the salt-sensitive genotype, and 2912 in the salt-tolerant one. Many of the differentially expressed genes were involved in stress and defense responses, metabolic processes, signal transduction, transport processes, and cell wall synthesis. Moreover, expression patterns of several genes differed between the two cultivars in response to salt stress, and several key pathways involved in determining the salt tolerance of sugar beet, were identified. Our results revealed the mechanism of salt tolerance in sugar beet and provided potential metabolic pathways and gene markers for growing salt-tolerant cultivars.