Costs of protein turnover and carbohydrate export in leaves of sun and shade species

Abstract

In mature leaves, adenosine triphosphate (ATP) provided by respiration is used for maintenance of leaves and carbohydrate export (starch mobilisation and phloem loading). The main maintenance processes of mature leaves include turnover of existing structures (e.g. proteins and membranes) and the trans-membrane transport of solutes. In order to mechanistically estimate these costs in mature leaves and compare them between sun and shade species, we measured the costs of protein turnover and carbohydrate export in mature leaves of Alocasia odora (Lodd.) Spach., a shade-tolerant species, and Phaseolus vulgaris L., a sun species, in the night. We estimated the rate of ATP production from the rate of O ₂ uptake, taking account of the contribution of the alternative respiratory pathway, assessed by the O ₂ isotope fractionation technique. The energy consumption of protein turnover was estimated from the decrease in the rate of ATP production in the presence of an inhibitor of cytosolic protein synthesis. Examination of the anatomy of the minor veins in the leaves revealed that A. odora employs symplastic phloem loading, while P. vulgaris was reported to employ apoplastic loading. Based on these phloem loading types and the difference between the rate of carbohydrate decrease and the CO ₂ efflux rate in the leaves, we calculated the ATP cost for carbohydrate export. The costs estimated for two processes amounted to about 40% of the ATP production rate in A. odora and 80% in P. vulgaris. The absolute costs for the two processes in the leaves of A. odora were much lower than those of P. vulgaris. Both the cost of protein turnover per unit leaf nitrogen and that of carbohydrate export per exported carbon were lower in the leaves of A. odora. Low ATP consumption rates by these cellular processes would explain the low respiratory rate in A. odora.