Leaf Development in Lolium temulentum: Formation of the Photosynthetic Apparatus in the Presence of the Porphyrin Synthesis Inhibitor Gabaculine

Gabaculine, a potent inhibitor of the biosynthesis of aminolaevulinic acid in plants, was supplied via the roots to seedlings of Lolium temulentum and chloroplast development along the age gradient of the emergent leaf was followed. Exposure to the inhibitor prevented the development of an organized photosynthetic membrane system and a noticeable accompanying feature was the appearance of aggregates of osmiophilic globules even in young plastids. In untreated leaves these were only seen to any extent in chloroplasts undergoing senescence. Though chlorophyll formation was inhibited by gabaculine, levels of carotenoid in treated tissue were somewhat elevated. However, there was a selective depletion ofβ-carotene which, in situ, is closely associated with the photosynthetic reaction centre. Amounts of two plastid-localized tetrapyrrole-binding proteins, light-harvesting chlorophyll protein of photosystem-2 (LHCP-2) and cytochrome f, were determined. There was a close correspondence in inhibition between LHCP-2 and its prosthetic group with the greatest decrease, compared to leaves of untreated plants, being at the leaf base. In contrast, formation of the apoprotein and haem components of cytochrome f was apparently not so tightly coupled. Amounts of apoprotein were similar to those of untreated controls, but haem was concentrated in the region proximal to the base and declined along the age gradient, being absent from the leaf tip. Appearance of novel haem-peroxidase activities in the basal region of treated leaves was observed.