Effects Of Light Quality On Photosynthetic Efficiency And The Resistance To Sphaerotheca Fuliginea In Cucumber Plants

Greenhouse horticulture is an important integrative part of modern agriculture,however,vegetable development are severely affected by low light and pathogeninfection.Hence,there is a growing emphasis on understanding the mechanism oflight quality on vegetable growth and pathogen infection,and selecting alternativesartificial lights in modem vegetable industry.In this study,we investigated changes ofCO₂ assimilation,chlorophyll biosynthesis,senescence and supplemental lighting,aswell as incidence of powdery milder and associated metabolism in cucumber plants inresponse to different wavelength lights.The results are as follows:1.We examined how light quality influences plant photosynthesis by analyzingchanges in photosynthetic parameters and expression levels of some photosynthesisrelated genes of cucumber(Cucumis sativus L.cv.Jinyou No.1)plants.The resultsshowed that all plants grown under monochromatic light had reduced growth,CO₂assimilation rate(Pn)and quantum yield of PSII electron transport(φ_psâ…¡)ascompared with plants grown under W,and these reductions were more significant inthe plants under G,Y and R.Interestingly,P-and B-grown plants had higher stomatalconductance(Gs),total and initial Rubisco activities and higher transcriptional levelsof 10 genes which encode key enzymes in the Calvin cycle together with higher totalsoluble sugars,sucrose and starch contents as compared with W-grown plants,whereas in G-,Y-,and R-grown plants these parameters declined.Therefore,thereduction in Pn under P and B is likely the result of inactivation of photosystems,while under Y,G and R,the reduction is attributed to both photosystem inactivationand the closure of stomata and the transcriptional down-regulation of genes for theCalvin cycle enzymes.In conclusion,light quality alters plant photosynthesis by theeffects on the activity of photosynthetic apparatus in leaves and the effects on thegene expression and/or activity of the Calvin cycle enzymes.2.We investigated the effects of different light qualities on chlorophyll biosynthesis in five-day-old etiolated cucumber seedlings.Exposure to R and Bresulted in an increased content of chlorophyll,and P and B enhanced the content ofcarotenoid,whilst G and Y decreased both contents of chlorophyll and carotenoid.Furthermore,P,B and R up-regulated the transcriptional levels of GluTR,MgCH andFeCH,and down-regulated the expression of POR,whereas G and Y showed theopposite results.3.We investigated the efficiency of supplemental lights with different lightsources on the physiological and photosynthetic characteristics and its molecularmechanisms in cucumber seedling.The results showed that all plants grown under B,R and W increased the CO₂ assimilation rate(Pn)and transcriptional levels of 10genes which encode photosynthesis,whereas in G-and Y-grown plants theseparameters declined.In multiwavelength experiment,G decreased the effects of B andR on increasing the photosynthesis.Furthermore,the combination of B and R hadsignificantly increased Pn and up-regulated the expressions of genes encodingphotosynthesis,and its effect was more significant than that of W.Interestingly,wefound that the changes of transcriptional levels of photosynthesis related genes causedby different light qualities were companied with the changes of redox status of GSHand AsA.Therefore,we proposed that redox may be involved in the processes whichlight qualities act on4.The role of different light qualities in leaf senescence was investigated incucumber seedlings.P and B increased the transcript levels and activities ofantioxidant enzymes and delayed the decreases in the contents of chlorophyll andsoluble proteins,therefore,maintaining the lower content of MDA and preventingsenescence.However,G,Y and R accelerated plant senescence by inhibiting theactivities of antioxidant enzymes and leading to the decreases in the contents ofchlorophyll and soluble proteins and the increase in the content of MDA.Theseresults indicate that P and B prevented senescence by maintaining high levels ofantioxidant enzymes.6.To determine whether light quality affects the incidence of disease,cucumberplants were exposed to purple(P),blue(B),green(G),yellow(Y),red(R)and white (W)light with or without the inoculation of Sphaerotheca fuliginea for 72 h.Exposure to R resulted in an increased level of free salicylic acid(SA)together withincreased expression of defense genes such as PR-1 and H₂O₂ accumulation ascompared with plants exposure to W or other monochromatic lights.In comparison,plants grown under P and B exhibited higher activities of phenylalanineammonia-lyase and polyphenoloxidase together with increased level of flavonoids.Notably,red light increased whilst other monochromatic lights decreased theresistance to Sphaerotheca fuliginea as compared to W.Our findings highlight theimportance of red light in the light-induced resistance and the importance ofSA-dependent signalling pathway in the red-induced tolerance.