Chlorophyll accumulation and breakdown in light‐grown barley transferred to darkness: effect of seedling age

Abstract

Diurnally grown barley (Hordeum vulgare L. cv. Clipper) seedlings of various ages (3–4, 5–6 and 10–11‐days‐old) were transferred to darkness for 17 h and changes in leaf fresh weight, chlorophyll a, chlorophyll b and protochlorophyllide measured. The results were consistent with previous evidence of a light‐independent chlorophyll biosynthetic pathway in light‐grown barley. There was a net gain in chlorophyll (μg leaf‐1) in 5–6‐ and 10–11‐day‐old plants after 17 h dark treatment. The amounts of chlorophyll that accumulated were similar (5.9 and 4.3 μg Chl leaf‐1), despite a twofold difference in leaf size at T0. The rate of leaf expansion in 5–6‐day‐old plants greatly exceeded the rate of chlorophyll accumulation and leaves were noticeably paler after dark treatment i.e. there was a reduction in chlorophyll concentration (μg g fresh weight‐1) in spite of an increase in chlorophyll content (μg leaf‐1). The ability of light‐grown barley to accumulate chlorophyll in darkness was a function of seedling age. Very young seedlings (3–4‐day‐old) generally lost chlorophyll in darkness. The decrease in chlorophyll per leaf resulted mainly from loss of chlorophyll b. Preferential loss of chlorophyll b resulted in dramatic increases in the chlorophyll a:b ratio. Since 3–4‐day‐old seedlings (1) accumulated 5‐aminolevulinic acid in the presence of levulinic acid at a rate comparable to older seedlings, and (2) converted exogenous 5‐aminolevulinic acid to chlorophyll in the absence of light, it is unlikely that failure of the youngest plants to accumulate chlorophyll in darkness was due to blocks at these steps in the pathway. Net loss of chlorophyll (μg leaf‐1) in 3–4‐day‐old seedlings in darkness was eliminated by the addition of chloramphenicol, which occasionally produced a small, but significant, gain in total chlorophyll. These results imply that chlorophyll degradation in young barley leaves is strongly influenced by the chloroplast genome, and is a major factor influencing changes in chlorophyll levels in darkness. The present findings are consistent with the suggestion that the failure of 3–4‐day‐old barley seedlings to accumulate chlorophyll in darkness may be due to chlorophyll turnover in which the rate of degradation exceeds the rate of synthesis.