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.].