NITROGEN AVAILABILITY ALTERS PATTERNS OF ACCUMULATION OF HEAT STRESS-INDUCED PROTEINS IN PLANTS

Mounting evidence suggests that heat-shock proteins (HSPs) play a vita l role in enhancing survival at high temperature. There is, however, c onsiderable variation in patterns of HSP production among species, and even among and within individuals of a species. It is not known why t his variation exists and to what extent variation in HSPs among organi sms might be related to differences in thermotolerance. One possibilit y is that production of HSPs confers costs and natural selection has w orked towards optimizing the cost-to-benefits of HSP synthesis and acc umulation. However, the costs of this production have not been determi ned. If HSP production confers significant nitrogen (N) costs, then we reasoned that plants grown under low-N conditions might accumulate le ss HSP than high-N plants. Furthermore, if HSPs are related to thermot olerance, then variation in HSPs induced by N (or other factors) might correlate with variation in thermotolerance, here measured as short-t erm effects of heat stress on net CO2 assimilation and photosystem LI (PSII) function. To test these predictions, we grew individuals of a s ingle variety of corn (Zea mays L.) under different N levels and then exposed the plants to acute heat stress. We found that: (1) high-N pla nts produced greater amounts of mitochondrial Hsp60 and chloroplastic Hsp24 per unit protein than their low-N counterparts; and (2) patterns of HSP production were related to PSII efficiency, as measured by F-v /F-m. Thus, our results indicate that N availability influences HSP pr oduction in higher plants suggesting that HSP production might be reso urce-limited, and that among other benefits, chloroplast HSPs (e.g., H sp24) may in some way limit damage to PSII function during heat stress .