Occurrence of a high temperature sensitivity of chloroplast ribosome formation in several higher plants.

A specific high temperature-induced deficiency of chloroplast ribosome formation, as indicated by the absence of chloroplast rRNA, has been observed in the leaves of light- or dark-grown seedlings of Avena sativa L., Hordeum vulgare L., and Triticum aestivum L. at certain temperatures between 28 and 34 C. While the growth of the leaves (size, morphology, total amino nitrogen content) was little affected by the elevated temperature, chlorophyll accumulation was strongly inhibited, amounting to only 2 to 20% of its content in 22 C-grown leaves which were used as a reference for normal development. The carotenoid contents were also lower but still reached at least 15 to 20% of the corresponding measurements at 22 C. Ribulose-1,5-bisphosphate carboxylase was absent at the higher temperature while NADP-glyceralde-hydephosphate dehydrogenase reached high activities. For the peroxisomal marker enzyme hydroxypyruvate reductase, 30 to 70% of the activity present in 22 C-grown leaves was found in extracts from high temperature-grown leaves. Fumarase reached 1.5- to 4-fold higher activities at the elevated growth temperature than at 22 C. Leaves of Pisum sativum L. were completely chlorotic and deficient of 70S ribosomes at 33 C but simultaneously suffered from a severe general inhibition of their growth. In Zea mays L., a formation of chlorotic leaves was not observed at elevated temperatures.