| Ilex×attenuata ’Sunny Foster’ is a unique ornamental plant.It is characterized by spineless,golden yellow leaves,red fruit,fast growth,and strong adaptability.The seedlings germinated from the seeds of Ilex×attenuata’Sunny Foster’were green,and the new buds and leaves grown in the sun turned yellow.The stronger the light,the deeper the yellow,and the yellow leaves could return to green in the shade environment.However,the light response patterns and the mechanism of the yellow leaves turning green are not clear.In order to verify the process of yellow leaves turning green,the annual seedlings wiht yellow leaves were artificially shaded.Before treatment,the light intensity was1500μmol·m-2·s-1,after shading,the light was 550μmol·m-2·s-1,and the treatments lasted for 32days.In the process of shade treatment,the sample periods were divided into six times,with the whole yellow leaf color（T0,0 day）as the control,the sampling interval was 6-7 days:T1,day 6;T2,day 15;T3,day 22;T4,day 29;and T5,day 32,respectively.The leaf structure,physio-biochemical parameters,photosynthetic parameters,and chlorophyll biosynthesis pathway were systematically analyzed during the period of shade,and the physiological response of the Ilex×attenuata’Sunny Foster’to low light was studied,so as to lay a foundation for further studies on the mechanism of yellow leaf re-greening.The main results are as follows:（1）Compared with before shading（T0）,the thickness of leaves,palisade tissue and spongy tissue decreased significantly（P<0.05）for the whole green leaf at 32 days（T5）,by28.3%,39.9%,and 29.0%,respectively.This may be a mechanism for plants to prevent photosynthetic organs from injury,and also a response to light stress environment.This response was especially significant in the early stage of shading（from T0 to T1）,and the difference was greater.Therefore,leaf characteristics showed a more rapid and direct response to shading.The ultrastructure of chloroplast was the most mature in T5,but immature in T0.（2）In the process of shading,the content of chlorophyll a,chlorophyll b,and total chlorophyll increased significantly（P<0.05）,and reached the maximum value in T5,making the leaves green again.The content of chlorophyll a changed significantly in T2,while the inflexion of chlorophyll b and total chlorophyll content appeared in T3,increased 554.6%,165.3%,and358.3%,respectively compared with T0.The content of carotenoid decreased,while the ratio of chlorophyll to carotenoid increased significantly（P<0.05）.The content of anthocyanin increased by 281.4%.The change of pigment content leads to the change of leaf color,and the change of chlorophyll content plays a leading role in the process of leaf color changing from yellow to green.（3）From T0 to T5,the maximum conversion efficiency of PSⅡFv/Fm and non photochemical quenching coefficient NPQ showed a significant upward trend（P<0.05）,by19.6%and 470.0%separately.Fq’/Fm’and qP showed a significant downward trend（P<0.05）,27.3%and 18.3%,separately.Fv’/Fm’showed a first downward and then upward trend.The soluble sugar increased by 72.2%.There was no difference in starch content,which was about12～16 mg·g-1.The results showed that the Ilex×attenuata’Sunny Foster’grew well under low light with an increasing tendency in chlorophyll content,photosynthetic capacity,and accumulation of carbon and water chemicals,and the leaves turned green.These physiological indexes showed significant difference only at T2 and T3.Thus,the coordination between the photosynthetic physiological functions between T2 and T3 was achieved..（4）The leaves were all yellow without shading,and the diurnal change of net photosynthesis was bimodal with"noon break".After shading,e.g.T1,T2,T3,T4,and T5,the diurnal changes of photosynthesis were unimodal without"noon break".Compared with T0,the net photosynthetic rate Pn,stomatal conductance Cond and intercellular CO2 concentration Ci all increased significantly（P<0.05）,reaching the peak value at T5,with an increase of 216.4%,45.2%,and 19.1%,respectively.Trmmol decreased significantly（P<0.05）,with a decrease of54.7%.During the shading period,the quantum efficiencyα,the maximum net photosynthetic rate Pmax and the dark respiration rate Rd all increased significantly,reaching the maximum value in T5,with an increase of 360.0%,204.9%,and 31.8%,respectively.The light saturation point（LSP）and the light compensation point（LCP）both increased and then decreased significantly,with a decrease of 25.5%and 71.9%compared with T0.（5）The activity of Rubisco showed a down-up-down trend,it increased by 10.4%in T4compared with T0,decreased in T5,but higher than T0.The PEPC decreased first and then increased.It reached the peak value at T5,which was 161.0%higher than T0.Sucrose synthetase showed a up-down-up trend.It reached the peak at T1 and the second peak at T5.The activity of sucrose phosphate synthetase first increased and then decreased,and from T0 to T2 it increased significantly（P<0.05）,and T2 reached the maximum and T0 was the lowest.In general,shading significantly affected the activity of photosynthetic enzymes,which were increased.（6）During shading,contents of ALA and PBG decreased significantly（P<0.05）,with46.2%and 38.8%,separately.The contents of UrogenⅢincreased significantly（P<0.05）,with T5 reaching the maximum.the contents of protoporphyriⅢ,protoporphyrin IX,Mg-protoporphyrin IX,and protochlorophyllin acid also increased significantly（P<0.05）,with100.2%,10.4%,29.4%,and 13.1%,respectively.Compared with T0,the activities of glutamyl tRNA reductase and ALAD decreased significantly（P<0.05）,with a decrease of 18.3%and32.82%,separately.The activity of UrogenⅢsynthetase increased significantly（P<0.05）,with an increase rate of 21.8%,starting from T2.The activity of chlorophyll enzyme decreased significantly at T5（P<0.05）,with a decrease rate of 5.13%.There was no significant difference in other treatments.The activity of demerged chelatase decreased significantly（P<0.05）,with a decrease of 11.97%,starting from T2.To sum up,the author preliminarily thinks that in the shade environment,the leaf color of Ilex×attenuata’Sunny Foster’appears to be greenish.Shade treatment has a significant impact on its physiological characteristics.Under low light,chloroplast developed best,the thickness of palisade tissue and sponge tissue reduced significantly,the content of chlorophyll increased significantly,the photosynthetic capacity increased,the accumulation of chlorophyll started from the increase of UrogenⅢ,mainly due to the increase of UrogenⅢsynthetase activity.The tired start time was between T1 and T2,and finally hte leaves returned to green.This experiment provided a certain theoretical support for the application of Ilex×attenuata’Sunny Foster’in landscape practice,and also laid a solid foundation for further studies on the regulation of leaf color formation. |
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