The function of carotenoids during chloroplast development. V. Correlation between carotenoid content, ultrastructure and chlorophyll b to chlorophyll a ratio

The ultrastructural effect of carotenoid deficiency in wheat (Triticum aestivum L.) was studied after adding the herbicide SAN‐9789 to the growth medium. The presence of SAN‐9789 (28 mg I‐1) resulted in an almost complete absence of carotenoids. For plants grown in darkness the lack of carotenoids was accompanied by a reduction in partitions between primary thylakoids as well as a change in appearance of the plastoglobuli from small and black (osmiophilic) to large and greyish white (less osmiophilic). When plants were grown in weak red light (16 mW m‐2), the presence of SAN‐9789 also resulted in an almost complete absence of grana, a decrease in the ratio of chlorophyll b/chlorophyll a from 0.25 to 0.1, and an almost complete absence of prolamellar bodies. The greatest differences in carotenoid content, in amount of grana, in chlorophyll b/chlorophyll a ratio, and in number of prolamellar bodies, all occurred between 0.28 and 2.8 mg I‐1 of SAN‐9789. The change in appearance of plastoglobuli occurred at a lower SAN‐9789 concentration, between 0.028 and 0.28 mg I‐1. The ultrastructural responses to the SAN‐9789 treatment could be related to the effect on the carotenoid synthesis in different ways. The disappearance of thylakoid partitions was connected to a structural role of carotenoids, while the disappearance of the prolamellar bodies was dependent on the photoprotective role of carotenoids. The change in shape and size of plastoglobuli was not correlated to the presence of carotenoids. However, a connection to the accumulation of the carotenoid precursors is still possible.