The Effects of Leaf Age and Senescence on the Distribution of Carbon inLolium temulentum

Abstract

Assimilate distribution in leaves of Lolium temulentum was established by root absorption of [¹⁴C]sucrose and after exposure to ¹⁴CO₂. Age determined the amount of carbon assimilated, with more label being incorporated during expansion than at maturity. During senescence ¹⁴C assimilation was much lower. Ethanol-soluble extracts from various tissues of root-labelled plants contained most of the radioactivity chiefly in basic and acidic compounds. The neutral fraction was composed predominantly of sucrose. Sucrose was comparably labelled in leaves from plants fed equal amounts of either [¹⁴C]sucrose, glucose, or fructose and only traces of labelled monosaccharides appeared in extracts. Radioactive sucrose was translocated rapidly from mature leaves whereas, in the expanding leaf, carbon incorporation was directed towards growth and the greater proportion of label present at ligule formation was in ethanol-insoluble material. Induced senescence, of a mature leaf fed during expansion, produced a rapid loss from the pool of insoluble ¹⁴C. This was accompanied by a reduction in the contents of chlorophyll and soluble protein and an accumulation of amino acids. The onset of senescence caused changes in leaf sugar levels which were correlated with increased rates of respiration.