Physiological Resistance Analysis Of Citrus Plants Under Deficit Irrigation Conditions

Citrus is one of the most economically important evergreen fruit trees around the world,which is mainly distributed in over 135 tropical and subtropical countries.In China,because most of the citrus plants were cultivated in the hilly land,the deficit irrigation obviously plays an important role in saving water and improving the social,economic and ecological benefits.So it is of great theoretical and practical significance to study the mechanism of drought resistance in citrus plants. In this expriment,the influence of different soil moisture gradients at various stages on photosynthesis,endogenous hormones,oxidative stress-resistant enzyme system and non-enzyme system was investigated using W·Murcott(Citrus reticulata Blanco×Citrus sinensis Osb.Cv. Murcott) as the tested materials.The relationship among the drought resistance,photosynthesis, endogenous hormones,oxidative stress-resistant enzyme system and non-enzyme system were also discussed in our work.This study can provide a theoretical support for deficit irrigation in citrus W. Murcott,and the mainly results are as follows:1.Under deficit irrigation conditions,the growth and development of W·Murcott were significantly affected.Our results showed,the height and trunk diameter at different growth stages of tested plants under deficit irrigation condition were limited significantly compared to the control. when in mild stress(SRWC = 65%),compared to the control,there was no distinctive downward trend.However in severe stress(SRWC = 25%and SRWC =10%),the downward trend was very significant.Both the water content and water potential of shoots and leaves treated were lower than the control.The results also indicated that the water content and water potential of shoots and leaves positively correlated with the relative soil water content.2.Under deficit irrigation condition,the leaf proline and MDA significantly increased at different stages following the reduction in the relative soil water content.The content of leaf proline and MDA was negatively correlated to the relative soil water content.The results showed that water stress also caused the reduction of osmotic potential of the leaf cell protoplasts,Which indicated that the tested plants had a capacity of osmotic adjustment.Under the mild water stress(SRWC = 65%), minimal increase in MDA and a certain degree of strengthening of enzyme active in membrane protection system suggested that the stress-induced reactive oxygen species in plant can be eliminated in order to avoid lipid membrane peroxidation,which showed the tesed plants had an adaptability to water stress.However,under the severe stress(SRWC = 25%and SRWC =10%), both the reduced stability of membrane and membrane injury were the two main deleterious factors for citrus,caused by the rapid decrease of CAT,SOD,POD activity that resulted in imbalance between the generation and removal in reactive oxygen species,lipid membrane peroxidation strengthened and MDA content increased.3.Under deficit irrigation condition,a downward trend occurred in citrus leaf Pn,net photosynthetic rate and transpiration rate Tr,while increased in the stomatal resistance Gs following the reduction of the soil relative water content at different stages.Each different stress can cause net photosynthetic rate of citrus leaf,stomatal conductance,transpiration rate decline.This downward trend from high to low is:severe stress drop(SRWC = 25%and SRWC =10%),＞moderate stress (SRWC = 45%)＞mild stress(SRWC = 65%).However,under mild(SRWC = 65%) or moderate stress(SRWC = 45%) condition,compared to controls,the downward trend of various indicators of photosynthetic were not significant,while significant under severe stress conditions(SRWC = 25% and SRWC =10%).Our results also showed that under the different treatments,the net photosynthetic rate patterns of citrus leaf were different.The diurnal variation pattern of net photosynthetic rate of the control(SRWC = 85%),the mild stress(SRWC = 65%) and the moderate threat(SRWC = 45%) is a bimodal curve.When treated with severe stress(SRWC = 25%and SRWC =10%),the net photosynthetic rate patterns did not change significantly,showing a single peak curve.4.Deficit irrigation(water stress) caused the incease of ABA content in citrus leaves at different stages,and the extent of soil water stress had a significant positive correlation with ABA content. The IAA content of leaves decreased with the decline of relative water content in the soil,appearing increased at first and declined later.In the mild stress(SRWC = 65%) and moderate stress(SRWC = 45%) treatments,the IAA content of citrus leaves at different stages increased,while in severe stress treatment(SRWC = 25%and SRWC =10%),IAA content decreased,reaching a very significant level.5.We also studied the quality of citrus under deficit irrigation condition in the fields(different irrigation).The results showed that:Soil relative water content remained at 65%was the optimal condition to improve the quality of the fruit.Deficit irrigation(SRWC = 65%) increased the content of fruit sugars,organic acids,Vc,as well as soluble dry matter content,but caused reduction of yield to some extent.However,under serious stress(drought),it would have a serious influence on the yield and quality.This showed that moderate water deficit in citrus(SRWC = 65%) was not only conducive to improve the plant water use efficiency,but also can significantly improve the quality of fruit withoutr serious output loss.