Photosynthetic thermotolerance is quantitatively and positively correlatedwith production of specific heat-shock proteins among nine genotypes of Lycopersicon (tomato)

We recently showed that the chloroplast small heat-shock protein therein re ferred to as chip Hsp24) protects photosystem 2 (PS2) during heat stress, a nd phenotypic variation in production of chip Hsp24 is positively related t o PS2 thermotolerance. However, the importance of chip Hsp24 or other Hsps to other aspects of photosynthesis and overall photosynthetic thermotoleran ce is unknown. To begin investigating this and the importance of genetic va riation in Hsp production to photosynthetic thermotolerance, the production of several prominent Hsps and photosynthetic thermotolerance were quantifi ed in nine genotypes of Lycopersicon, and then the relationships between th ermotolerance of net photosynthetic rate (P-N) and production of each Hsp w ere examined. The nine genotypes exhibited wide variation in P-N thermotole rance and production of each of the Hsps examined (chlp Hsp70, Hsp60, and H sp24, and cytosol Hsp70). No statistically significant relationship was obs erved between production of chip Hsp70 and P-N thermotolerance, and only a weak positive relationship between cytosolic Hsp70 and P-N was detected. Ho wever, significant positive relationships were observed between production of chip Hsp24 and Hsp60 and P-N thermotolerance. Hence natural variation in production of chip Hsp24 and Hsp60 is important in determining variation i n photosynthetic thermotolerance. This is perhaps the first evidence that c hip Hsp60 is involved in photosynthetic thermotolerance, and these in vivo results are consistent with previous in vitro results showing that chip Hsp 24 protects PS2 during heat stress.