Effects Of Light And Temperature On Photosystemâ…¡Activity In Leaves Of Forsythia Suspensa

In natural conditions, the terrible environment problems(e.g., continues low or high light intensity and temperature) are always rapid and momentary changes in light intensity of variable duration(seconds to minutes) due to clouds or internal shading. The physiological stress and destroy are observed in field plants. Some researches showed that the midday depression of photosynthesis weren’t found in leaves of Forsythia suspensa under monsoon climate. However, diurnal variations of maximal photosystem efficiency of PSⅡin the dark(Fv/Fm) present the midday depression characteristic, in which high irradiance and temperature could play a great role in reducing carbon assimilation. Therefore, Forsythia suspensa was taken as test material and its leaves were dealt with heat stress, light stress and compound stress of high temperature and light, respectively. The chlorophyll fluorescence parameters measured by MINI-IMAGINGPAM and the photochemical reflectance index determined by QE65 spectrometer were used to study photosystemⅡactivity and de-epoxidation level of xanthophyll cycle. The behavioral characteristic of photosynthetic apparatus was revealed to enrich the research domain for efficiency of light capture and utilization. The main research results are as follows:(1)With increase of heat stress temperature(26℃、31℃、34℃、37℃、40℃、43℃、46℃、49℃、52℃) and heat stress time(5min、25min、45min), the heat inhibition of PSⅡactivity present accompanied with decrease of Fv/Fm. Meanwhile, multiple sites of PSⅡprotein complex are inactivated or destroyed due to reduce of actual photochemical efficiency(Y(Ⅱ)) and efficiency of excitation energy capture by open PSⅡreaction centers(Fv′/Fm′), increase of quantum yield of regulated energy dissipation(Y(NPQ)) and non-regulated energy dissipation(Y(NO)), firstly decrease and then increase of minmal fluorescence(Fo) and photochemical quenching coefficient(q P). Especially, the decrease of Fv/Fm is directly related to Fv′/Fm′. When the leaves of Forsythia suspensa are stressed by heat for 25 min or 45 min, Y(Ⅱ) continuously decreases, but Fv/Fm significantly decreases only from 37℃. Therefore, the effect of heat stress on the whole photosynthetic apparatus would emerge, and its action mechanism is also transformed from enzyme activity attenuation to PSⅡactivity attenuation.At the same time, coordination mechanisms of heat protection begin to act. On one hand, with increase of heat stress temperature at different stress time, the dissipation of surplus excitation energy mainly pass through Y(NO). However, photochemical reflectance index(PRI) presents an opposite trendency compared with Y(NO), which indicates that the xanthophylls cycle plays an important role in protecting photosynthetic apparatus of Forsythia suspensa leaves stressed with stronger heat. On the other hand, heat stress obviously decreases electronic transmit activity of PSII(Fm/Fo), which shows that the state transition of antenna can be allocated by adjusting excitation energy between PSⅡand PSⅠ.(2) With the illumination of light intensity(0μmol·m-2·s-1, 500μmol·m-2·s-1, 750μmol·m-2·s-1, 1000μmol·m-2·s-1, 1250μmol·m-2·s-1, 1500μmol·m-2·s-1, 1750 μmol·m-2·s-1, 2000μmol·m-2·s-1, 2250μmol·m-2·s-1) on leaves of Forsythia suspensa for 45 min, Fv/Fm gradually decreases and photoinhibition phenomenon appears. Moreover, the structure and function of PSⅡprotein complex are also changed and injured at multiple sites accompanied with decrease of Y(Ⅱ) and Fv’/Fm’, increase of Fo, and sum of Y(NPQ) and Y(NO). The decrease of Fv/Fm is also directly related to Fv’/Fm’.With increase of light intensity the ability of PSⅡenergy dissipation is obviously enhanced in Forsythia suspensa leaves, and the dissipated energy through Y(NO) is gradually greater than that through Y(NPQ). The illumination from 0μmol·m-2·s-1 to 1750μmol·m-2·s-1 decreases PRI, accompanied with increase of ability of PSⅡenergy dissipation. However, PRI also increases when light intensity is stronger than 1750μmol·m-2·s-1. The thermal energy dissipation depending on de-epoxidation level of xanthophyll cycle is observed in leaves of Forsythia suspensa illuminated below 1750μmol·m-2·s-1 light intensity, and the other thermal energy dissipation ways could be enhanced when light intensity is greater than 1750μmol·m-2·s-1. Meanwhile, the overflowing of excited stress from PSⅡto PSⅠleads to present of state 2 alleviate the degree of photoinhibition and light induced injury.(3)When Forsythia suspensa leaves are respectively treated with high temperature stress(37℃ and 0μmol·m-2·s-1), high light stress(26℃ and 1500μmol·m-2·s-1) and strong light coupled with high temperature(37℃ and 1500μmol·m-2·s-1) for 45 min, the Fv/Fm of leaves obviously decreases, and severe photoinhibition subsequently occurred. Compared with control, light stress and light coupled with temperature stress decrease Fo and cause reversible inactivation or injury to reaction center of PS Ⅱ. However, temperature stress increases Fo and damages antenna pigment of Light-harvesting complex. The change tendency of Fm、β/α-1、Fm/Fo under stronger light coupled with high temperature stress for 45 min is similar to the correspondingly change under 1500μmol·m-2·s-1 light intensity and photoinhibition appears. Furthermore, the change of Y(Ⅱ) and Y(NPQ) under strong light coupled with high temperature is similar to the correspondingly change under high temperature(37℃) and heat inhibition appears. q P is related to the each other cancelling between highlight inhibition and high temperature inhibition, and complex inhibition effect of high temperature and strong light recombination appears. In addition, there is no obvious change for PRI when Forsythia suspensa leaves are treated with high light and strong light coupled with high temperature for 45 min, but PRI decreases obviously when Forsythia suspensa leaves are treated with high temperature stress. Consequently, the xanthophyll cycle of Forsythia suspensa leaves isn′t main thermal energy dissipation way under strong light coupled with high temperature stress.