Effects Of Pb-Stress On The Activity Of Photosynthetic Apparatus In Maize And Cucumber Seedling Leaves

By measurement of gas exchange, chlorophyll fluorescence, chlorophyll a fluorescence transient and light absorbance at 820 nm, etc., effects of Pb-stress on photosynthetic apparent in cucumber and maize seedling leaves were investigated in this paper. The main results are as follows:1. After treated with different Pb concentration for 20days, the growth of cucumber and maize seedlings was significantly inhibited; leaf area, chlorophyll content and biomass were decreased in Pb treated seedlings leaves and the damage accelerated with the increase of the Pb concentration.2. Chlorophyll content significant decreased in Pb treated cucumber and maize leaves. There was no significant decrease in the concentration of Mg, N, P, K, Fe and Mn irons Pb treated leaves, which showed that the decreased content of chlorophyll was not caused by the change in the content of the above irons.3. Pb treatment resulted in a big decrease in net photosynthetic rate, stomatal conductance, apparent quanta yield, as well as carboxylation efficiency, whereas a considerably increase in intercellular CO₂ concentration was observed in Pb treated leaves. The decrease of photosynthesis was not due to stomatal limiting under Pb stress. The decrease in carboxylation efficiency indicated that the inhibition of dark reaction was the limiting factor of photosynthesis decrease in Pb treated leaves.4. Pb treatment decreased the actual efficiency of PSII (ФPSII), photochemical quenching (qP) and the efficiency of light energy capture by open PSII reaction centers (Fv'/Fm') in leaves of cucumber and maize . However, non- photochemical quenching was increased in these leaves .5. The parameters obtained from the chlorophyll a transient such as PIABS,φEo andψO decreased and WK increased with the increase of the Pb concentration. The changes of these parameters showed that donor and acceptor sides of PSII were damaged in Pb treated leaves. Photons absorbed by per excited cross-section (ABS/CSm), photons used to move electron transport beyond QA-per excited cross-section (ETo/CSm) and the density of active reaction center per excited cross-section (RC/CSm) decreased and the dissipated flux per excited cross-section (DIo/CSm) increased in Pb treated leaves. It was observed that the fraction of photons used to move electron transport beyond QA-per reaction center (ETo/RC) and trapped photons per reaction center (TR/RCm) increased markedly in the Pb treated leaves, which was attributed to that more absorbed photons by leaves was transported to active reaction centers and resulted in charge separation.6. The activity of PSI decreased with the increase of the Pb concentration. Hower further studies are needed to clarify which system btween PSII and PSI was damaged first under the Pb treatement.7. The activity of active oxygen scavenging enzymes SOD and APX were declined in Pb treated leaves. Pb stress decreased the content of Cu in cucumber and maize leaves, which might affect the activity of SOD and APX. The decrease in the activity of active oxygen scavenging enzymes may result in more active oxygen species which would damage the large biomolecule and membrane.8. The content of MDA increased in Pb treated leaves, indicating the damage of membrane. The damage of membrane affected the activity of membrane bonding protein associated with photosynthesis, which might result in the decrease of photosynthesis. The activity of nitrate reductase (NR) was damaged in Pb treated leaves, which decreased NH₄⁺ application.In conclusion, Pb stress decreased the content of chlorophyll, the efficiency of carboxylation, the activities of PSII and PSI, oxygen scavenging enzyme and nitrate reductase, which decreased the Pn in cucumber and maize seedling leaves. The decrease of Pn resulted in the decrease of biomass.