Effect of soil flooding stress on morphological and physiological characteristics of young plants of 'Newhall Navel Orange'/Poncirus trifoliata(L.) Raf.

Abstract: 【Objective】Soil flooding constitutes a seasonal factor that negatively affects plant performance and crop yield.‘Newhall Navel Orange'is the leading citrus cultivar in southern Jiangxi province. It often encounters the waterlogging stress caused by seasonal rainfall during its growth and development period. The present study aims to clarify the changes in morphological characteristics of navel orange plants after flooding treatment, and to reveal the physiological mechanism of navel orange/Poncirus trifoliata(L.) Raf. in response to flooding stress. It is of great significance for timely taking corresponding measures to alleviate the adverse effects of soil flooding stress on the growth, development,fruit yield and quality of navel orange.【Methods】Young‘Newhall Navel Orange'/Poncirus trifoliata(L.) Raf. plants with only 1 shoot were transplanted into plastic pots(24 cm diameter × 26 cm height). These pots contained 9.0 kg of soil obtained from a commercial citrus orchard. Waterlogging stress treatments were carried out 3 months after transplanting. Soil flooding was imposed by placing the treated pot into a larger plastic bucket(35 cm diameter × 33 cm height) filled with tap water to 3 cm above the bottoms of the pots. The non-waterlogged pots were well-watered(60% to 80% of field moisture capacity) throughout the experiment. Navel orange plants were sampled after 5, 10, 15, 20 and 45 days of the treatments, respectively. The plants were then separated into upper leaf, lower leaf and fine roots. The potted plants were used to compare the effects of waterlogging on the changes of morphology and anatomical structure, reactive oxygen metabolism, antioxidant enzyme activities superoxide dismutase(EC 1.15.1.1), peroxidase(EC 1.11.1.7), catalase(EC 1.11.1.6), oxidative damage in terms of malonaldehyde(MDA) concentration, soluble protein and soluble sugar under full flooding condition.【Results】The control plants did not show any symptoms throughout the experimental period. Within 20 days of soil flooding treatment, there were no obvious visible symptoms in above-ground plant parts,and the leaf microstructure did not change significantly. The roots showed a schizog-enous air cavity.After long-term(45 days) flooding, the main veins, lateral veins and fine veins of the upper leaves in navel orange plants had obvious yellowing symptoms, while the inter-veins remained relatively green;the lower leaf veinsd, the main veins turned yellow, the lateral veins turned slightly yellow, and the interveins remained green; root growth was inhibited and accompanied with blackening. Compared with the non-waterlogging treatment, waterlogging treatment did not affect the relative chlorophyll content in the lower leaves within 20 d treatment, but significantly increased the relative chlorophyll content in the upper leaves on the 5 th day of flooding. Long-term flooding significantly decreased the relative chlorophyll content both in the upper and lower leaves. There was no significant difference in MDA content of upper leaves between the flooded plants and the control within 20 days of treatment. On the 5 th day of flooding treatment, the MDA content in the lower leaves was even lower than that of the control. With the prolongation of flooding time, the MDA content increased but remained not significantly different from the control. The content of active oxygen species in leaves and roots was increased by waterlogging stress, and then the antioxidant enzyme system started. The rootO 2-·concentration increased significantly accompanying with the root MDA content on the 10 th day of flooding treatment, indicating that the root cells were damaged and the membrane lipid peroxidation increased. The activity of POD and CAT was significantly higher than that of the control on the 15 th day of flooding. After long-term flooding stress, the upper-and lower-leaf antioxidant enzymes maintained high activity and the root enzyme activity decreased significantly. In addition, the POD activity in the roots of navel orange plants was much higher than that in the leaves, and the CAT activity was much lower than that in the leaves. The upper leaves and roots of navel orange plants accumulated more soluble sugar in the early flooding period(5 days), while the lower leaves were less affected by flooding stress. Within 20 days of flooding stress, there were significant organ differences in the response of soluble sugar and soluble protein to waterlogging stress in upper leaves, lower leaves and roots. On the 5 th day of flooding treatment, the soluble protein content of the upper leaves increased significantly, while the soluble protein content of roots decreased significantly. On the 10 th and 15 th days following, the soluble protein content of the lower leaves was significantly lower than that of the control and the roots were significantly higher than that of the control in the soluble protein content. After long-term(45 days) flooding, the soluble sugar content of leaves and roots was significantly higher than that of the control, while the soluble protein content was significantly lower than that of the control.【Conclusion】With a period of flooding stress to‘Newhall Navel Orange'/Poncirus trifoliata(L.) Raf navel orange plants, the sensitivity and regulation mechanism of different organs in response to flooding stress were significantly different. Each organ would be co-regulated by changes in morphological structure, antioxidant enzyme activities and changes in osmotic adjustment substance content. However, long-term flooding stress would affected the metabolic process within the cell, and would induce obvious visible symptoms in the leaves and the roots, and inhibit the growth and development of the‘Newhall Navel Orange'/Poncirus trifoliata(L.) Raf plants.