The Effect Of Low Root-zone Temperature On Photosynthesis And Root Growth In Tomato Seedings

In winter and spring protected vegetable cultivation in northern China,it is very vulnerable to suffer low temperature stress.Solar greenhouse is limited by its own structure.In winter and spring,rainy and snowy days,the low temperature in solar greenhouse is caused by the lack of timely heat replenishment.Especially at night,the decrease of temperature in greenhouse and soil will lead to the decrease of temperature in greenhouse and soil.In daytime,atmospheric temperature returns quickly,while soil temperature returns slowly.Therefore,the soil temperature can not rise in time and affect the growth of crops.At this time,ground temperature becomes one of the key factors restricting the growth and development of protected vegetables in winter and spring.In this experiment,the seedlings of tomato(Lycopersicon esculentum Mill.)`Liaoyuan Duoli' cultivated in northern China were cultured in nutrient solution at different root zone temperatures in order to explore the effects on root growth and photosynthesis of tomato plants.The following conclusions are drawn from the experiment.1.The effects of different root zone temperatures on the growth of tomato plants were studied.Root growth,plant height and stem diameter were severely inhibited when the root zone temperature was 7 ?,moderately inhibited when the root zone temperature was 13 ?,slightly inhibited when the root zone temperature was 16 ?,and slightly inhibited when the root zone temperature was 25 ?.Plant growth was the best under the root zone environment of 19 ?.Therefore,it can be divided into 7 ~ 16 ? as low temperature,22 ~ 25 ? as high temperature and 19 ? as control.2.It was clear that low root zone temperature inhibited photosynthesis of tomato seedlings and led to photoinhibition of photosystem.The net photosynthetic rate(Pn),stomatal conductance(Gs)and transpiration rate(Tr)of leaves decreased first and then stabilized at low root temperature(7 and 13 ?),and intercellular CO2 concentration(Ci)decreased sharply after 3 d of treatment and then returned to normal level.The stomata were closed after 10 d of treatment.The decrease of Fv/Fm,Y(II),ETR and the sharp decrease of q P in a short period of 3d indicated that the structure of photosystem II was damaged,the electron transfer was blocked and photoinhibition was produced at low root zone temperature.However,the increase of Y(NPQ)and NPQ indicated that the photosynthetic system consumed excess energy through heat dissipation and others.And that played a protective role.3.It was clear that low root zone temperature inhibited root growth and mitochondrial activity of tomato.Root length,surface area,volume,number of root tips and number of branches decreased significantly after 6 d of low root zone temperature treatment at 7 ?,and that reached significant level after 12 d of low temperature treatment at 13 ?.Low root zone temperature also reduced root hairs and root activity,thus reducing root water and nutrient uptake.Roots produce reactive oxygen species(ROS)at low temperatures,and at the same time,the lipid peroxidation of mitochondrial membranes in roots is impaired.Activating the antioxidant system made the activity of SOD and CAT highest at 3 d,and then decreased with the increase of treatment time.This is related to the increase of mitochondrial membrane permeability,the decrease of membrane potential and cytochrome c/a in roots caused by reactive oxygen species.