Coping Strategies Of Paulownia Fortunei Seedlings Under Different Drought Stresses And Between Day And Night

Plants respond to environmental changes by changing their morphological characteristics,physiological activities,and recruiting different root microorganisms.Drought is a natural disaster with the widest distribution,longest duration,and greatest harm.Research on drought resistance of plants has been continuously deepening.Paulownia is a fast-growing tree species with strong adaptability,great economic value,and ecological benefits.During its popularization and planting,it also encountered drought stress problems.Previous studies on the drought resistance of paulownia lack attention to underground roots,especially the carbon-based root respiration,root secretion,NSC storage and utilization,and mycorrhizal infection,and the understanding of paulownia’s response to drought stress is not comprehensive.At the same time,our earlier research found that paulownia has the same diurnal reversal of photosynthetic pathway as CAM,and the diurnal rhythmic changes of plants are also an important form of plant adaptation to the environment.The diurnal rhythmic changes of paulownia under drought environments have also aroused our interest.Therefore,this study used Paulownia fortunei as the experimental material,and set up non drought(over 80% of the field water capacity),moderate drought(50% to 60% of the field water capacity),and severe drought(30% to 40% of the field water capacity)experimental treatments,and explored their drought adaptation strategies from morphological,physiological,and microbial levels in daytime and nighttime,respectively.The main experimental results are as follows:(1)Morphological changes: During drought,the root biomass utilization efficiency in Paulownia fortunei seedlings increased,the root diameter decreased,the specific root length and tissue density increased;Leaf area and thickness decreased,specific leaf area and tissue density increased,and carbon turnover during litter and regrowth accelerated;Biomass allocation is more transferred from above ground to underground.Under moderate drought conditions,there was no significant difference in root biomass allocation;During severe drought,biomass is more allocated to coarse roots.There was no significant difference in the apparent morphological characteristics in Paulownia fortunei seedlings under different drought stress treatments between day and night.(2)Physiological changes: During moderate and severe drought,the net photosynthetic rate of leaves in Paulownia fortunei seedlings decreased by 23.02% and 48.74%,respectively.The light compensation point and light saturation point increased,and the total amount of carbon transported to the ground decreased.During daytime and moderate drought,the respiration rate per unit root weight and the secretion rate per unit root weight in Paulownia fortunei seedlings significantly increased by 75.89% and 86.02%,respectively;During severe drought,compared to moderate drought,the root exudation rate decreased by66.98%,and the respiratory rate continued to increase by 51.77%;Physiological activities such as root respiration and secretion at night were lower than those during daytime;Drought stress increased the nitrogen content in the chemical components of root exudates.During nighttime and moderate drought,the starch content in the root system significantly decreased by 29.02%,maintaining a high level of intracellular soluble sugar content;During severe drought,the soluble sugar content in roots significantly decreased by 49.05%(P<0.01).Under long-term drought adaptation,the total NSC content and SOD activity in the root and leaf tissues in young Paulownia fortunei seedlings maintained similar levels to those of seedlings grown without drought.Under moderate and severe drought,the activity of acid phosphatase in the roots in Paulownia fortunei seedlings significantly decreased by33.84% and 58.25% during the day,respectively.There was no significant difference in enzyme activity at night,and the difference in enzyme activity between day and night increased with the increase of drought severity.Under severe drought,the activity of acid phosphatase in the roots in Paulownia fortunei seedlings at night was 120.25% higher than that during the day(P<0.01).(3)Root microbial changes: The biological diversity,relative species abundance,and functional classification of fungal communities with significant differences in the roots in Paulownia fortunei seedlings have small differences between day and night,and are more affected by drought stress.As the degree of drought increases,the biodiversity of the fungal community in the root system in Paulownia fortunei seedlings decreases,the relative abundance of the Saccharomyces fungal community decreases,the relative abundance of the Ascomycete fungal community increases,and the relative abundance of the Basidiomycete fungal community fluctuates slightly.The relative abundance of saprophytic bacterial communities at night is lower than during the day,and the more severe the drought,the more significant the diurnal difference.With the increase of drought,the relative abundance of arbuscular mycorrhizal fungi in the roots in Paulownia fortunei seedlings decreased,while the relative abundance of saprophytes and pathogens increased.