Study On The Leaf Coloration And The Mechanisms Of Physiological And Ecological Response Of Buxus Microphylla During Overwintering

Leaf reddening of winter evergreens is a widespread phenomenon.The study of this phenomenon is important not only for revealing the physiological and ecological mechanisms in winter evergreens,but also providing the practical guidance in winter landscape construction for human settlements.In this study,17 cultivars of Buxus genus plants which were introduced from aboard by Beijing Botanical Garden were selected for the testing shrubs.The relationship between the leaf coloration and cold resistance of the overwintering shrubs were studied.Based on the results,three cultivars of Buxus microphylla:'Greenbeauty','Sunnyside' and 'Wintergreen' which experienced different winter discolorations were selected for further study,including the pigment basis and the quantitative decisive temperature and light conditions for the winter leaf coloration as well as its adaptive photosynthetic physiological and ecological regulation mechanisms.The main results were as follows:1)The degree of leaf reddening of cultivars in Buxus plants did not show consistency with the cold resistance performance of the plant in winter.The cold resistance performance of the three different cultivars of Buxus microphylla('Greenbeauty,:almost kept green in winter;'Sunnyside':medium in the degree of leaf reddening but return green latest;'Greenbeauty':reddest in winter but return green earliest)were as follows:'Wintergreen'>'Greenbeauty'>'Sunnyside'.2)The leaf reddening of the three cultivars were inseparable from the interaction between low temperature and relatively high light during the overwintering process,and the three cultivars showed differences in reddening degrees,reflecting their tolerance and sensitivity to low temperature and high light.The critical temperature of the leaves in 'Greenbeauty' was 0.67-1.55 C for the adaxial side and 5.25-7.68? for the abaxial side.The critical temperature of the leaves in'Wintergreen' was 4.16-4.98? for the adaxial side and 9.41-10.23? for the abaxial side.The critical temperature of the leaves in 'Sunnyside' was 4.33-6.42? for the adaxial side and 8.43-9.41?for the abaxial side.The daily solar radiation during the critical reddening period was about 10 MJ·m-2.In the coldest month of January,almost no red pigments accumated in the sun leaves of'Greenbeauty' and 'Sunnyside' when the light intensity was droped to 50-60%and 30-40%,respectively,while a few red pigments still accumated in sun leaves of 'Wintergreen' when the light intensity was droped to 10-20%of the full sunlight.3)The leaf reddening in Buxus microphylla was not associated with the synthesis of anthocyanins in leaves.The content of carotenoids in the three cultivars of Buxus microphylla increased during overwintering while the content of chlorophylls(Chls)decreased,and this trend was opposite to that in the regreening process when spring came.The red pigments in the leaves of Buxus microphylla was mainly distributed in the upper layer of the leaves towards the sunlight side.4)The leaves of Buxus microphylla under high light and weak light took different strategies during winter acclimation.Overall,the sun leaves of Buxus microphylla tended to up-regulated Chl alb with increased reductions in Chl b and accumulated red pigments in upper leaf layers,thus to reduce the blue and red light absorption,and simultaneously,enhanced the sustained(DD(excitation energy dissipated as heat in the PSII antenna)to dissipate most of the light energy,whereas shade leaves maintained relatively high Chl a and Chl b contents and high red light absorption,showed regulated proportions of ?D and ?E(excitation energy dissipated as heat in PSII reaction centers)to maintain relatively high photochemical activity in winter.5)The difference in leaf coloration of the three cultivars of Buxus microphylla was related to their different strategies in utilization and dissipation of light energy during overwintering.Among them,the sun leaves of'Greenbeauty' could maintain a certain degree of photochemical efficiency in winter,the 'Wintergreen' greatly increased the capacity of heat dissipation in antenna system,and the'Sunnyside' showed intermediate capacities in photochemical efficiency and light energy dissipation in antenna system between 'Greenbeauty' and 'Wintergreen'.6)The activity of carbon reaction in the downstream of PSII-PSI-carbon assimilation chain in the sun and shade leaves of Buxus micorphylla 'Wintergreen' decreased with the decreasing of the temperature in the overwintering process.However,the two phenotypes under different light conditions took different strategies.In sun leaves,the heat-dissipate function of the sustained,non-relaxed PSII antenna system was activated to dissipate most of the absorbed light energy into heat,which greatly reduced the excitation pressure on the upstream PSII receptor side of the electron transport chain,thus to avoid the damage in photosynthetic apparatus suffered from excessive light.However,shade leaves under low light would give priority to activate the reversible inactivation in PSII reaction center to alleviate the electron transfer blocking caused by the decline of PSI photochemical activity in the early winter,when subjected to extreme low temperature in deep winter,they would simultaneously increase the heat dissipation process of the PSII antenna system,adjusting the balance of the reversible inactivation of the PSII reaction center and the heat dissipation of the antenna system to maintain a certain level of winter photochemical activity,thus providing the required energy limit for plant metabolism.7)The synthesis of red pigments in the leaves of 'Wintergreen' in winter played a down-regulation role in the Fv/Fm value by affecting the content and distribution of Chls.The usual measurement of Fv/Fm on red adaxial surface possibly overestimated the actual degree of photoinhibition compared with that of the whole leaf in the winter.The main contributions of this study were as follows:the relationship between the leaf reddening and the cold resistance in evergreens(took the Buxus cultivars as examples)in winter was investigated;the leaf redness of evergreen plants was proved to be closely related to the coupling factors of light and temperature,and the critical temperature and light conditions that determined the leaf reddening of three cultivars of Buxus micorphylla in winter were quantified;the temperature and light responding strategies of the three cultivars of Buxus micorphylla with different leaf coloration during the overwintering process were revealed;the technical method of chlorophyll fluorescence multi-stage induction curve was explored to study the temperature and light responding mechanism of sun-red and shade-green leaves of Buxus micorphylla ' Wintergreen' during overwintering from the perspective of PS?-PS?-carbon assimilation.