Responses of leaf stomatal traits and gas exchange process of cherry tomato to NaCl salinity stress

Abstract: Salinity stress due to the increased use of non-potable water resources for irrigation imposes major limitations on plant growth in salt-affected soils, and meanwhile excessive salinity in the soil may cause osmotic and ionic stresses leading to various physiological and morphological damages to plants. In order to examine the responses of stomatal traits and gas exchange of cherry tomato to salinity stress, we grew cherry tomato seedlings in growth chambers for 21 days with different NaCl concentrations of nutrition medium including 0 (CK), 0.05, 0.10, 0.15, 0.20, and 0.25 mol/L. The parameters related to stomata of leaf were measured. The photosynthesis characteristics were analyzed. The results showed that NaCl stress significantly increased the stomatal density of cherry tomato leaves (P<0.05), and obvious differences on the stomatal density were found among different NaCl stresses. By contrast, the stomatal length, stomatal width, stomatal perimeter, and stomatal shape index of cherry tomato seedlings were substantially decreased by NaCl stress (P<0.05) with the minimal values of stomatal length, stomatal width, stomatal perimeter, stomatal area, and stomatal shape index under the NaCl concentration of 0.15 mol/L. Moreover, we also found that the spatial distribution pattern of stomata was scale dependent with regular patterns at small scales and random patterns at larger scales on cherry tomato leaves. NaCl stress significantly changed the spatial distribution pattern of stomata with a substantial increase in the average nearest neighbor distance between stomata under the NaCl concentrations of 0.05, 0.20, and 0.25 mol/L, whereas the nearest neighbor distance between stomata was obviously decreased with salinity treatments of the NaCl concentrations of 0.10 and 0.15 mol/L. The net photosynthetic rate (Pn) of cherry tomato seedling was gradually decreased with the increase of NaCl concentrations, and the Pn under severe NaCl stress (0.25 mol/L) was significantly different with that of the other NaCl concentrations (P<0.05). Compared to CK, the leaf Pn of cherry tomato seedling were marginally decreased by 47%, 71%, 77%,78%和89% (P<0.05) with NaCl concentrations of 0.05, 0.10, 0.15, 0.2, 0.25 mol/L, respectively. Meanwhile, the transpiration rates (Tr), stomatal conductance (Gs), and intercellular CO2 concentration (Ci) were also substantially decreased with the increase of NaCl concentrations, and significantly lower than the CK (P<0.05). As a result, salinity stress resulted in decline of leaf water use efficiency (water use efficiency, WUE), especially for the WUE under severe NaCl stress (0.25 mol/L) and under the severe stress the WUE was significantly decreased by 48% than that of CK (P<0.05). In addition, the chlorophyll contents (chlorophyll a, chlorophyll b, chlorophyll a+b, and chlorophyll a/b) featured bell-shaped curves with the increases of NaCl stress and all the maximal values of chlorophyll a, chlorophyll b and chlorophyll a+b were at the NaCl concentration of 0.05 mol/L. The NaCl stress higher than 0.10 mol/L significantly decreased the contents of chlorophyll a, chlorophyll b, chlorophyll a+b (P<0.05), especially for the cherry tomato under severe NaCl stress of 0.25 mol/L. These results suggested that cherry tomato may maximize the leaf gas exchange efficiency through changing both the aperture size and shape of single stoma and the stomatal distribution (stomatal density and spatial distribution pattern of stomata) under NaCl stress, although the ability for maximizing leaf gas exchange efficiency was limited under higher salinity stress. Our results are not only important for understanding the relationships among stomatal traits, leaf gas exchange, and chlorophyll content under salinity stress, but also provide data and theoretical support for planting cherry tomato on saline alkali soil and selection salinity tolerance cherry tomato cultivars.