Sunflowers were treated with mixing proportions of NaCl, Na2SO4, NaHCO3, and Na2CO3. Effects of salt and saltalkaline mixed stress on growth, photosynthesis, chlorophyll fluorescence, and contents of inorganic ions and organic acids of sunflower were compared. The growth of sunflower decreased with increasing salinity. The contents of photosynthetic pigments did not decrease under salt stress, but their contents decreased sharply under
salt-alkaline mixed stress. Net photosynthetic rates, stomatal conductance and intercellular CO2 concentration decreased obviously, with greater reductions under salt-alkaline mixed stress than under salt one. Fluorescence parameters showed no significant differences under salt stress. However, maximal efficiency of PSII photochemistry, photochemical quenching coefficient, electron transport rate, and actual PSII efficiency significantly decreased but non-photochemical quenching increased substantially under salt-alkaline mixed stress. Under salt-alkaline mixed stress, sunflower leaves maintained a low Na+- and high K+ status; this may be an important feature of sunflower tolerance to salinity. Analysis of the mechanism of ion balance showed that K+ but not Na+ was the main inorganic cation in sunflower leaves. Our results indicated that the change in organic acid content was opposite to the change of Cl-, and the contribution of organic acid to total charge in sunflower leaves under both stresses decreased with increasing salinity. This may be a special adaptive response to stresses for sunflower. Sunflower under stress conditions mainly accumulated inorganic ions instead of synthesizing organic compounds to decrease cell water potential in order to save energy consumption. and J. Liu, D.-C. Shi.
We recently showed that the chloroplast small heat-shock protein (herein referred to as chlp Hsp24) protects photosystem 2 (PS2) during heat stress, and phenotypic variation in production of chlp Hsp24 is positively related to PS2 thermotolerance. However, the importance of chlp Hsp24 or other Hsps to other aspects of photosynthesis and overall photosynthetic thermotolerance is unknown. To begin investigating this and the importance of genetic variation in Hsp production to photosynthetic thermotolerance, the production of several prominent Hsps and photosynthetic thermotolerance were quantified in nine genotypes of Lycopersicon, and then the relationships between thermotolerance of net photosynthetic rate (PN) and production of each Hsp were examined. The nine genotypes exhibited wide variation in PN thermotolerance and production of each of the Hsps examined (chlp Hsp70, Hsp60, and Hsp24, and cytosol Hsp70). No statistically significant relationship was observed between production of chlp Hsp70 and PN thermotolerance, and only a weak positive relationship between cytosolic Hsp70 and P N was detected. However, significant positive relationships were observed between production of chlp Hsp24 and Hsp60 and PN thermotolerance. Hence natural variation in production of chlp Hsp24 and Hsp60 is important in determining variation in photosynthetic thermotolerance. This is perhaps the first evidence that chlp Hsp60 is involved in photosynthetic thermotolerance, and these in vivo results are consistent with previous in vitro results showing that chlp Hsp24 protects PS2 during heat stress. and P. J. Preczewski ... [et al.].