Temperature treatments of dark-grown pea seedlings cause an accelerated greening in the light at different levels of gene expression.

We have previously shown that heat-shock in the dark evokes photomorphogenesis-like effects and circadian rhythmicity at the level of mRNAs when applied to emerging pea plantlets during several consecutive days [15]. Here we extend these findings by showing that a temperature shift to 10 degrees C above average and a single heat-shock are sufficient for induction of circadian rhythmicity and changes in morphogenesis. The maximum response to a single heat-shock occurs at days 2 to 3 after sowing indicating additional developmental control of the response. An increasing number of heat-shock treatments intensifies the morphogenetic effect. The heat-shocked plantlets have an elevated level of the xanthophyll lutein in the dark. Upon illumination of heat-shocked plantlets accumulation of chloroplast pigments as well as that of individual thylakoid membrane proteins and their corresponding mRNAs occur much faster than in the etiolated controls. This is reflected in an accelerated formation of grana stacks. Therefore, heat-shock seems to evoke a responsiveness of plantlets similar to that obtained earlier by other authors using pre-illumination. The working hypothesis is put forward that induction or synchronization of circadian rhythmicity by either light or heat-shock might be sufficient to explain the observed morphogenetic changes.