| Light is necessary for plant growth. But excess light energy will be generated when lightenergy absorbed by the plants is over the blade carbon assimilation, resulting inphotoinhibition. Under natural conditions, the occurrence of photoinhibition is inevitable.So in the long-term evolution, plants have evolved a series of photoprotection mechanisms toease the photoinhibition, such as cyclic electron transfer, photorespiration, heat dissipation,water-water cycle, as well as active oxygen scavenging system. Although suchintra-chloroplastic defense mechanisms have been researched extensively, few studies havebeen done about extra-chloroplastic defense mechanisms.With further research, it has been found that excess reducing equivalents would betransferred from the chloroplast to other organelles through multiple pathways under highlight intensity to be oxidized there. The latest research showed that plant leaves in bright lightconditions greatly increases the activity of AOX pathway. It is speculated that the excessreducing equivalents in mitochondria from chloroplast activates AOX pathways, acceleratingthe consumption of excess reducing equivalents in mitochondria, which increases the outputof excess reducing equivalents form chloroplast, and thus indirectly alleviates thephotoinhibition in plant leaves.Under natural conditions, the plants will inevitably suffer stress, such as hightemperature or low temperature. Chloroplast under low or high temperature is very fragile andvulnerable. Intra-chloroplastic defense mechanisms are easily affected by the stress to loss thefunction of photoprotection. So the extra-chloroplastic defence mechanisms at this time mayplay a more important role in photoprotection. However, so far, no direct data prove that AOXpathway plays an important role in photoprotection under low or high temperatures. In thisstudy, the physiological function of the AOX pathway in photoprotection of cucumber leaves was studied under different temperatures with different light intensities. In addition, theeffects of temperatures on the activities of AOX were also discussed. The main resultsobtained are as follows:(1) The PSII actual photochemical efficiency (ΦPSII) and electron transport rate (ETR)significantly decreased and the non-QBreducing reaction centers significantly increased incucumber leaves treated with SHAM under low light intensity (200μmol photons m-2s-1),though the Fv/Fm did not significantly change in cucumber leaves after treated with SHAM,which indicates that, even under low light, the inhibition of AOX pathway would enhancephotoinhibition. And the Fv/Fm, ΦPSIIand ETR in cucumber leaves treated with SHAM underhigh light intensity (1000μmol photons m-2s-1) were also significantly decreased, and thedecline extent in high light was greater than that in low light, suggesting that inhibition ofAOX pathway would cause severer photoinhibition in high light than in low light. In order tofurther investigate injury sites after inhibition of the AOX pathway, OJIP curve weremeasured after treated with low light or high light. The results showed that the J points weresignificantly increased in both low light and high light intensity, indicating that inhibition ofAOX pathway would block the transfer of electron from QAto QBno matter in low light or inhigh light. JIP-test analysis showed that, although the inhibition of AOX pathway did notaffect the absorption flux per unit leaf area, the absorption flux per reaction center and thetrapped energy flux per reaction center were both significantly increased, which indicates thatAOX pathway significantly affected the number of the reactive center but did not affect theability of absorption of light energy in cucumber leaves. After inhibition of AOX pathwayunder the low light, the scavenging enzymes of reactive oxygen species (ROS) as well asNPQ both increased to avoid over accumulation of ROS, which alleviated the photoinhibitionof cucumber leaves. Under high light intensities, the inhibition of AOX caused severerphotoinhibition in cucumber leaves, and the increased activities of the ROS scavengingenzymes and NPQ were unable to prevent the over accumulation of ROS due to the inhibitionof AOX pathway. The above results demonstrate that AOX pathway played an important rolein photoprotection in cucumber leaves under high and low light intensities. In addition, therole of AOX pathway in photoprotection could not be replaced by other photoprotectionmechanisms under high light. (2) The activity of AOX pathway increased significantly at high temperatures (40℃or45℃), while decreased significantly at low temperature (4℃or10℃). Under hightemperature, the Fv/Fm significantly decreased in cucumber leaves treated with SHAM andthe decrease extent of the Fv/Fm gradually increased as the temperature rises; but under lowtemperatures, the Fv/Fm did not change significantly in cucumber leaves treated withSHAM, which indicates that the inhibition of AOX pathway enhanced the photoinhibition incucumber leaves under high temperatures, while under low temperatures the inhibition ofAOX pathway did not effect the photoinhibition in cucumber leaves. The P point of the OJIPcurves significantly decreased, and the J point significantly increased in cucumber leavestreated with SHAM under high temperatures. However, under low temperatures the OJIPcurves did no significantly change in cucumber leaves treated with SHAM. JIP-test analysisshowed that the1-VJdecreased significantly in cucumber leaves treated with SHAM underhigh temperatures, and under low temperatures1-VJdid not change significantly in cucumberleaves treated with SHAM. The ETR significantly decreased and1-qP increased significantlyin cucumber leaves treated with SHAM under high temperature, while, under lowtemperatures, there were no significant differences in ETR between SHAM treatment andcontrol. The above results indicate that, under high temperature, the inhibition of AOXpathway increased the excitation pressure and decreased the number of active reaction center,and thus blocked the electron transfer from QAto QBand decreased the photosynthetic linerelectron transport rate. However, due to the low activity of AOX pathway in low temperature,the role of photoprotection was limited. In summary, the AOX pathway played a moreimportant role in photoprotection under high temperature than under low temperature.(3) The inhibition of AOX pathway was accompanied by decreases in activities ofNADP-MDH and NAD-MDH no matter under low light or high light at room temperature,which indicates that the photoprotection of AOX pathway under room temperature, at least,partly depends on the activity of malate-oxaloacetate shuttle. However, the change extents ofthese two enzyme activities were not correlated with the change extent of photoinhibition.Therefore it is reasonable to speculate that the photoprotection role of AOX pathway alsodepends on other process besides the malate-oxaloacetate shuttle. The observation that theactivities of NAD-MDH and NADP-MDH did not change significantly when the cucumber leaves was transferred from room temperature to high temperature also indicates that thephotoprotection of AOX pathway depends less on malate-oxaloacetate shuttle. Part of themetabolic processes of photorespiration may be involved in mitochondrial metabolism. Theactivity of photorespiration increased significantly under high temperature and decreasedsignificantly under low temperature.. The inhibition of photorespiration enhanced thephotoinhibition under high temperatures, and simultaneously inhibition of photorespirationand AOX pathway did not further aggravate photoinhibition. Based on the above facts, it isspeculated that the photoprotection of AOX pathway partly depends on photorespiration. |
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