Effects of saline-fresh water rotation irrigation on photosynthetic characteristics and leaf ultrastructure of tomato plants in a greenhouse

To reveal the mechanisms of saline-fresh water rotation irrigation that affect the leaf ultrastructure and photosynthetic characteristics of tomato plants and to optimize the strategy of saline water irrigation of greenhouse tomatoes, a two-season tomato experiment with four treatments of saline-fresh water rotation irrigation was conducted in a greenhouse in southern Xinjiang. The four treatments consisted of rotation irrigation with four times saline-fresh water (W1), rotation irrigation with two times saline water and two times fresh water (W2), rotation irrigation with two times fresh water, four times saline water, and two times fresh water (W3), and freshwater irrigation as a control (CK). The three rotation patterns had the same amount of saline water and fresh water, but the rotational interval was different. The results indicated that the saline-alkali stress introduced by saline water significantly reduced the gas exchange parameters of tomato leaves and water use efficiency at the leaf scale, and both stomatal and non-stomatal factors played a key role in limiting leaf gas exchange. The chloroplast granular lamellae structure was disrupted in tomato leaves treated with W1 and W2. Compared with CK, W1 and W2 decreased leaf chlorophyll content by 4.59% and 10.89%, net photosynthetic rate by 26.82% and 40.11%, and yield by 60.62% and 67.63%, respectively. In contrast, W3 presented a relatively intact mesophyll cell structure and relatively high chlorophyll content and photosynthetic efficiency. In W3 treatment, no significant differences were found in the number of fruits per plant (only increased by 8.16% and the yield by 4.03%), while with better quality compared with CK. The results suggested that tomato growth and yield were neither poor nor detrimental when rotated with saline water during the flowering and fruiting stage-fruit expansion stage and freshwater irrigation during other growth stages. W3 can be used as a saline-fresh water rotation pattern for tomato production in greenhouses in arid and saline areas.