Chlorophyll turnover in etiolated greening barley transferred to darkness: Isotopic (1‐14C glutamic acid) evidence of dark chlorophyll synthesis in the absence of chlorophyll accumulation

Abstract

Synthesis of chlorophyll was initiated in 5‐ to 6‐day‐old dark‐grown barley (Hordeum vulgare L. cv. Clipper)seedlings by exposing them to light in the presence of 1‐14 C glutamic acid supplied via the roots.The plants were then returned to darkness. At the end of light treatment (T) and after 7 or 18 h dark treatment chlorophylls a and b were extracted, quantified (μgleaf1). purified by HPLC to their magnesium‐free derivatives (pheophytin a and b) and their molar radioactivities determined. After 2 h exposure to light followed by 6 h illumination in the presence of 1‐14 C glutamic acid, seedlings had accumulated 4‐7 nmol chlorophyll leaf1 and had incorporated between 900‐1 350 Bq (g fresh weight)1 of radioactive label into the chlorophyll pool. When seedlings were transferred to darkness, label continued to be incorporated and after 18 h the radioactivity of the chlorophyll pool had increased by 300‐700 Bq (g fresh weight)1. Net chlorophyll content, however, remained constant during dark treatment. The increase in radioactivity of the chlorophyll pool in darkness represented the difference between a net increase of label incorporated into chlorophyll a and a small loss of label from chlorophyll b. The absence of measurable radioactivity in the phytol moiety of labelled chlorophyll a, extracted at the endof dark treatment, demonstrated thatincorporation of label was into the tetrapyrrole moiely of chlorophyll and not into the phytol chain. Light‐independent incorporation of 1‐14 C glutamic acid into chlorophyll of greening barley seedlings transferred to darkness indicates that chlorophyll synthesis continues when light is withheld. We interpret the net gain in radioactivity of chlorophyll in darkness, in the absence of a net gain in chlorophyll content, to chlorophyll turnover i.e. to simultaneous synthesis and breakdown of chlorophyll when etiolated greening barley seedlings are transferred to darkness.