Response Differences Of Different Root Types Of Rice To Drought Stress And Its Physiological Basis

Drought disasters occur frequently due to global climate change,and the area of rice subjected to drought stress is also increasing year by year.As the main water absorbing organ of plants,root system is also the first part to feel drought stress.The morphology and physiological characteristics of root system are closely related to drought resistance.In order to further clarify the relationship between root characteristics and drought resistance of rice,two root system types(large root system and small root system)screened out in the early stage of the experiment were used as materials.By setting the flood control(CK)and drought treatment(DS,soil water potential of-30 k Pa)during the whole growth period.The morphological characteristics,physiological and biochemical changes,dry matter accumulation and distribution,yield and quality of two rice lines and their responses to drought stress in the key growth stages were systematically studied.The results show that:(1)Compared with rice lines with small roots,rice lines with large roots had higher plant height,leaf area index,light interception rate,total root length,root surface area,root volume,vessel number in root xylem and thickness of thick-walled cells in outer cortex.Under drought stress,plant height,tiller number,total root length,leaf area per plant,leaf area index,root surface area,mean root diameter and root volume of rice were significantly decreased.Meanwhile,under drought stress,the growth period of rice was prolonged,the number of root xylem vessels increased,and the thickness of root exocarp thickened.Compared with the two types of rice,the plant height,total root length and root surface area of rice with large root system decreased significantly after drought stress,while the leaf area per plant of rice with small root system decreased significantly.Among them,the total root length of rice with large root system decreased most significantly at heading and flowering stage in 2020,from 48909 cm to 7670 cm,a decrease of 85.0%.The results showed that the rice with large root system had larger biomass,and its root morphological index decreased greatly after drought stress,while the rice with small root system had smaller biomass and decreased less after drought stress.(2)Under CK,large root lines had higher leaf water potential and root bleeding rate,while small root lines had higher root activity,leaf sucrose synthase(SS)activity and jasmonic acid content.Under drought treatment,leaf water potential,root bleeding rate and root starch content decreased significantly,while root activity,leaf and root soluble sugar content,leaf starch content,abscisic acid and auxin content increased significantly.Compared with the two types of rice,the root bleeding rate of the large-root type decreased significantly after drought stress,and the decrease was most significant at heading and flowering stage in 2020.The large-root type decreased from 1.48 g/h to 0.07 g/h,with an average decrease of 95.2%.The soluble sugar content of the root and starch content of the leaf increased significantly.3.4 times and 2.0 times of flood control,respectively.Under drought stress,leaf water potential decreased and root activity increased significantly.The results showed that under drought stress,the physiological and biochemical activities of the underground part of rice with large root system,such as root bleeding rate,decreased greatly,while the physiological and biochemical activities of the overground part,such as leaf water potential,decreased relatively little,while that of the small root system was opposite.(3)After drought,the total dry matter accumulation of rice plants decreased significantly,and the dry matter accumulation of stem,leaf,sheath,panicle and root organs decreased significantly.But the proportion of dry matter distributed among different organs varies.In most cases,after drought stress,the proportion of root and sheath increased in small-root lines,while the proportion of stem and ear decreased,while the proportion of leaf and sheath increased in large-root lines,while the proportion of root and ear decreased.Especially in the reproductive growth stage,more dry matter was allocated to the root system under drought stress,which resulted in a significant increase in root-shoot ratio.However,rice with large root system left more dry matter in the aboves under drought conditions,which resulted in the reduction of root-shoot ratio.(4)Under flood irrigation control,the yield of large-root lines was significantly higher than that of small-root lines,and the yield of large-root lines also decreased significantly under drought stress.The average yield of rice line with large root system decreased from4.31 t·hm-2 to 3.08 t·hm-2,decreasing by 28.5%.The average yield of rice lines with small roots decreased from 2.48 t·hm-2 to 2.04 t·hm-2,a decrease of 17.8%.There was no significant difference in processing quality between the two kinds of rice,but in cooking quality,the large root system was better than the small root system.In conclusion,under drought stress,the morphological structure and physiological and biochemical activities of the roots of rice with large roots decreased more than those of the two root types,while the leaf morphology and physiological and biochemical activities of the aboveground decreased less,and the distribution ratio of dry matter in the roots and aboveground changed and the root-shoot ratio decreased.Under drought stress,rice with large root system could maintain higher yield and better rice quality.The results can provide a theoretical basis for drought-resistant rice cultivation and selection of drought-resistant varieties.