Journal of Experimental Botany, Vol. 54, No. 383, pp. 851-860,
February 1, 2003
© 2003 Oxford University Press
Salinity treatment shows no effects on photosystem II photochemistry, but increases the resistance of photosystem II to heat stress in halophyte Suaeda salsa
Received 6 August 2002; Accepted 11 October 2002
1 Photosynthesis Research Centre, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, P.R. China
2 Key Lab of Photosynthesis and Environmental Molecular Physiology, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, P.R. China
3 Department of Biology, Shandong Normal University, Jinan 250014, P.R. China
4 Department of Biology, Hong Kong Baptist University, Hong Kong, P.R. China
5 To whom correspondence should be addressed. E-mail: congminglu{at}yahoo.com; jzhang{at}hkbu.edu.hk
Abbreviations: Fo, minimal fluorescence level in dark-adapted leaves; Fo', minimal fluorescence level in light-adapted leaves; Fm, maximal fluorescence level in dark-adapted leaves; Fm', maximal fluorescence level in light-adapted leaves; Fv, maximum variable fluorescence level in dark-adapted leaves; Fv', maximum variable fluorescence level in light-adapted leaves; Fs, steady-state fluorescence level; Fv/Fm, maximal efficiency of PSII photochemistry; 
PSII, actual PSII efficiency; Fv'/Fm', efficiency of excitation energy capture by open PSII reaction centres; qP, photochemical quenching coefficient; qN, non-photochemical quenching coefficient.
Photosynthetic gas exchange, modulated chlorophyll fluorescence, rapid fluorescence induction kinetics, and the polyphasic fluorescence transients were used to evaluate PSII photochemsitry in the halophyte Suaeda salsa exposed to a combination of high salinity (100–400 mM NaCl) and heat stress (35–47.5 °C, air temperature). CO2 assimilation rate increased slightly with increasing salt concentration up to 300 mM NaCl and showed no decrease even at 400 mM NaCl. Salinity treatment showed neither effects on the maximal efficiency of PSII photochemistry (Fv/Fm), the rapid fluorescence induction kinetics, and the polyphasic fluorescence transients in dark-adapted leaves, nor effects on the efficiency of excitation energy capture by open PSII reaction centres (Fv'/Fm') and the actual PSII effciency (PSII), photochemical quenching (qP), and non-photochemical quenching (qN) in light-adapted leaves. The results indicate that high salinity had no effects on PSII photochemistry either in a dark-adapted state or in a light-adapted state. With increasing temperature, CO2 assimilation rate decreased significantly and no net CO2 assimilation was observed at 47.5 °C. Salinity treatment had no effect on the response of CO2 assimilation to high temperature when temperature was below 40 °C. At 45 °C, CO2 assimilation rate in control plants decreased to zero, but the salt-adapted plants still maintained some CO2 assimilation capacity. On the other hand, the responses of PSII photochemistry to heat stress was modified by salinity treatment. When temperature was above 35 °C, the declines in Fv/Fm, PSII, Fv'/Fm', and qP were smaller in salt-adapted leaves compared to control leaves. This increased thermostability was independent of the degree of salinity, since no significant changes in the above-described fluorescence parameters were observed among the plants treated with different concentrations of NaCl. During heat stress, a very clear K step as a specific indicator of damage to the O2-evolving complex in the polyphasic fluorescence transients appeared in control plants, but did not get pronounced in salt-adapted plants. In addition, a greater increase in the ratio (Fi–Fo)/(Fp–Fo) which is an expression of the proportion of the QB-non-reducing PSII centres was observed in control plants rather than in salt-adapted plants. The results suggest that the increased thermostability of PSII seems to be associated with the increased resistance of the O2-evolving complex and the reaction centres of PSII to high temperature.
Key words: Chlorophyll fluorescence, gas exchange, heat stress, photosystem II photochemistry, salinity treatment, Suaeda salsa L.
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