Study On The Photosynthetic Capacity And Photoprotection Mechanisms In Beta Vulgaris L. Under Salt Stress

To explore the photosynthetic capacity and photoprotection mechanisms inBeta vulgaris L. under salt stress, a combined measurement of basic physiologicalindex, gas exchange, chlorophyll fluorescence as well as chlorophyll fluorescencetransient were conducted. The main results are as follows:1. The effect of NaCl on the growth of Beta vulgaris L.With the increase of NaCl concentration, the growth indexes such as dry weightand fresh weight and leaf area of Beta vulgaris L. increased first and then decreased.The maximum value was obtained with100mM treatment. The growth of Betavulgaris L. was promoted when the NaCl concentration was low. However, highconcentration NaCl inhibited its growth.2. The effect of NaCl on the photosynthetic characters of Beta vulgaris L.Under the treatment of100mM NaCl, both Pn and Gs increased remarkably,but a dramaticl decrease was also observed with the NaCl concentration increased.The change of Ci was consistent with Pn. In contrast with Ci, Ls showed a greatincrease under the salt concentration of200mM, but decreased slightly when theNaCl concentration up to300mM. It can be concluded that under high saltconcentration, non-stomatal control contribute more to the decrease of Pn.The LCP of Beta vulgaris L. rose remarkably under salt stress, while the LSPreached the maximum at100mM NaCl concentration. The photosynthetic capacitywas partly facilitated by low NaCl concentration; however, high salt stress inhibitedit remarkably.As the NaCl concentration increased, the φPSII and qP showed the sametendency that rose at first and then fell. The maximum value appeared at100mM,just as Pn. As the NaCl concentration increased further more, it began to decrease.The photoinhibition showed up and the condition gradually with the increase of NaCl.3. The effect of NaCl on NPQ and DPS.NPQ gradually increased under salt stress. NPQ increased remarkablycompared to the control when it came to300mM NaCl concentration. Thephotochemical conversion efficiency was inhibited by salt stress, and more excitedenergy dissipated through NPQ. As a result, the PSII reaction centre wouldn’t bedamaged badly.qE, the major composition of NPQ, comprises63~69%of NPQ in Betavulgaris L.. In our experiment, qE and NPQ exhibited the similar trend, whichincreased with the increase of salt concentration. And a remarkable increase at300mM was observed compared to the control. This indicates that under salt stress, qEwhich is dependent on the energization of the thylakoid membrane, may be anefficient means to dissipate the heat.qT, known as state transition, is less than10%in NPQ. qT didn’t show visiblechange under salt stress, it can be indicated that qT plays little role in heatdissipation in Beta vulgaris L..qI accounted for only20%of NPQ. qI increased with the increase of NaClconcentration. When up to300mM, the NPQ showed a remarkable increase. Thephotoinhibition was greatly aggravated by salt stress. DPS show a similar trend withthe qE, which indicated that under salt stress, DPS dissipated more heat.4. Effect of salt stress on photosynthetic pigments.The content of Chl a, Chl b as well as the total content of Chl (a+b) decreasedunder salt stress, while the content of Anth and Car increased. Both of themincreased remarkably at300mM NaCl concentration. The two pigments play apositive role in the photoprotective mechanism under salt stress.5. The role of reactive oxygen species in photoprotective mechanisms undersalt stress of Beta vulgaris L.Although the activity of SOD decreased under high salt concentration, it wasstill higher than the control. The activity of APX increased under high saltconcentration. SOD was the major antioxidase under low salt concentration, as the salt concentration increased, PAX took its place.6. Effect of NaCl on the photochemical characters of PSII in Beta vulgarisL.The ratio of variable fluorescence Fk to the amplitude FJ-Fo(known asWk)didn’t change much under salt stress, which indicated salt stress didn’t dodamage to the donor side of PSII in Beta vulgaris L..Sm represents the size of PQ pool, which greatly decreased under high saltconcentration; The turn over number of QAin the time span from J-step to P-step (N)showed its maximum value at100mM, and then decreased as the salt concentrationincreased. The relative variable fluorescence at J-step(VJ) showed a remarkablyincreased at300mM. Which indicated the capacity of electron transport wasinhibited under high salt stress.RC/CSo decreased remarkably under300mM NaCl treatment. ABS/RC、TRo/RC、ETo/RC and DIo/RC shared the similar tendency, and is contrary toRC/CSo. With some light energy trapped in the reaction centre of PSII under saltstress, the energy trapped in inactived-PSII reaction centre developed as a efficientway to protect the reaction centre of PSII in Beta vulgaris L..