Studies On Adaptation Mechanism And Root Morphogenesis Of Cotton Seedling In Floating Nursing In Nutrient Water-bed

At present,a lot of cotton seedling cultivation technologies such as nutritive bowls nursing seedling(NBNS),nursing cotton seedling with reed-tube,soilless seedling,plug seedling and so on,have been developed in China cotton production. Cotton seed was directly sown into soil in Yellow River Basin and Northwestern region in China,and the seedling transplanting technique is adopted in Yangtze River Basin where hybrid cotton is mainly planted.The seedling transplanting technique is mainly applied with some shortcomings such as low seedling emergence and stand rate and labor-consuming.Based on the present seedling nursery methods of cotton, vegetable,and tobacco in domestic and abroad,a new cotton seedling nursery technology,the floating nursing seedling in nutrient water-bed(FNSNWB) is developed by Cotton Research Institute of Hunan Agricultural University.Cotton seeds are sown in patented mix media carried in a porous polyethylene plate which is floated on nourishing water according to the technology of FNSNWB.The seedling emergence of cotton is above 95%and stand rate above 90%by the new method. Compared with other seedling nursing technologies,the FNSNWB has many advantages with low cost,high efficiency,labor-saving and yield increasing.The FNSNWB has achieved greatly and was applied so far in Hunan,Hubei,Jiangxi and Anhui provinces since 2005.In order to explore the morphological characteristics and the physiological-ecological adaptation mechanisms of roots of cotton seedling nursed with the method of FNSNWB,Nongza 66 and Xiangzamian 8 were used as the materials to study the growth and development of roots and shoots and their adaptation to water environment at morphological,cytological and physiological-biochemical levels in the present paper. 1 The morphology and physiological-biochemical characteristics of cotton seedling roots of FNSNWB(1) It is found that the best sowing pattern was to lay cotton seed radicles slope with underneath,which the root through porous polyethylene hole rate reach 97.0% and the rate of seedling with seed coat was 3.0%.The sowing depth from 1.0 to 2.0 cm,where the moisture of medium around the seedlings was about 68-85%,was beneficial not only to the root system's formation and growth but also to the lateral roots occurrence.(2) According to the results of research on the morphological characteristics,the roots of FNSNWB can be divided into two different types.The roots growing in the medium were called drought roots with spring shape,and the roots growing in the nutrient water were called hydrophilic ones.The hydrophilic roots are white and tender with fat,soft and flexible tips.From one leaf to five leaves stages,the fresh and dry weight of root,ratio of root to shoot,root volume,root length and root density of seedlings with FNSNWB were significantly more than that of the seedlings with NBNS.Meanwhile,the lateral roots of FNSNWB grew more,which enhance the total root surface area and active absorption area of root.(3) The drought roots have a lot of characteristics with closely arranged and neatly compacted epidermis cell,thickened cell wall and the cortical cell forming some parenchyma.The hydrophilic roots also have a lot of remarkable characteristics with arranged loosely and disorderly epidermis cell,argumented cortical cell.The cortical cells merged into a lot of parenchyma for the transportation of oxygen. Therefore,the formation of parenchyma in root system was the main trait of cotton seedlings that adapted to aquatic environment.The root cortex thickness of cotton seedlings of FNSNWB became thinner,and the root stele radius and root diameter vessel was increased comparing with NBNS,which improved the ability of water trasportion of root system.(4) Compared with the roots of seedlings with NBNS,the TTC reductive intensity in the ones of seedlings with FNSNWB increased by 12.4%,11.4%,14.2%, 22.9%and 10.5%from one leaf to five leaves stages using Nongza 66 as experimental material.The TTC reductive intensity increased by 10.3%,12.4%,14.8%,21.0%and 8.8%from one leaf to five leaves stages using the Xiangzamian 8 as experimental material.During the stage of seedling,the activities of superoxide dismutase(SOD), peroxides(POD) and catalase(CAT) of cotton seedlings roots fostered by FNSNWB were higher than those of the NBNS.Meanwhile,the accumulation of O₂^- and MDA content of cotton seedlings roots decreased.Our results showed that the FNSNWB can increase the activities of SOD,POD and CAT,and decrease the function of membrane lipid peroxidation of cotton roots.Therefore,FNSNWB can protect the intactness of membrane structure and increase the stress resistance of cotton seedlings.(5) From one leaf to five leaves stages the cellulose activities of cotton seedlings roots of FNSNMB were stronger than that of NBNS.The cellulose activities reach the peak at two leaves stage,when the cellulose activities of the drought roots of the two varieties(Nongza 66 and Xiangzamian 8) reach 0.548μg/(g·min) and 0.668μg/(g·min), respectively,and the cellulose activities of hydrophilic roots reach 0.551μg/(g·min) and 0.702μg/(g·min),respectively.Under the water cultural condition,the cellulose activities of cotton seedlings roots were stimulated and led to accelerate cell fusion. Moreover,compared with NBNS,the ethylene content and its synthesize rate of cotton seedlings roots of FNSNWB from one leaf to five leaves stages were increased, which accelerated the death of the cortical cells and the aerenchyma formation in cortical cell.(6) From one leaf to five leaves stages,the activities of LDH and ADH in roots fostered by FNSNWB were stronger that that of NBNS.For the two different types of roots,the ADH activities of the drought roots were higher than that of the hydrophilic roots.The LDH activities of the hydrophilic roots were higher than that of the drought roots.Therefore,FNSNMB can increase LDH and ADH activities in a wide range in roots and enhance the respiration enzyme activities,which avoid the damage of massive accumulations of lactate and acetaldehyde and thypoxia resistance.Thus,the hypoxia resistance of cotton seedlings fostered by FNSNWB was enhanced. 2 The physiological and biochemical characteristics of cotton seedling shoots of FNSNWB(1) The leaf chlorophyll,soluble protein and sugar contents in cotton seedling of FNSNWB from one leaf to five leaves stages were significantly different from that of seedling of NBNS.(2) From one leaf to five leaves stages,the activities of SOD,POD and CAT of cotton seedlings leaves fostered by FNSNWB were higher than those of the NBNS. Meanwhile,the accumulation of O₂^- content and MDA content of cotton seedlings leaves decreased.Our results showed that the FNSNWB can increase the activities of SOD,POD and CAT and decrease function of membrane lipid peroxidation of cotton leaves.Therefore,FNSNWB may protect the intactness of membrane structure and increase the stress resistance of cotton seedlings.