Activities of the cyanide-resistant respiratory pathway in leaves of sun and shade species

Abstract

Higher plant mitochondria have a cyanide-resistant alternative respiratory pathway of electron transport (AP) that is not coupled to proton translocation. To characterise the ecophysiology of this apparently ‘wasteful’ pathway, we constructed a system consisting of a gas-phase oxygen electrode and an air sampling line for measurement of stable oxygen isotope ratios. With this system, we were able to measure respiratory rates of a small amount of leaf segments of ca 0.6 g fresh weight, and collect about 100 L of the air from the oxygen electrode chamber several times. The ¹⁸ O/¹⁶ O ratio in the air samples was measured by mass spectrometry. The activity of AP was estimated based on the isotopic discrimination of ¹⁸ O. We used the leaves of Alocasia odora (Lodd.) Spach., a shade species, and Spinacia oleracea L. and Phaseolus vulgaris L., sun species. These plants were grown at two levels of photosynthetically active photon flux density (PPFD). Three main findings were: (1) in the leaves of A. odora, the contribution of AP was less than 10% of the total respiratory rate, irrespective of growth PPFD; (2) for the sun species grown at high PPFD, the contribution of AP in the leaves was about 40% early in the night, but decreased dramatically late in the night; and (3) when S. oleracea was grown at low PPFD, the contribution of AP in the leaves declined. The low activity of AP in the leaves of A. odora suggests that the efficiency of adenosine triphosphate (ATP) production (ATP/O ₂ ) of this species is high. This may be especially important in shaded environments where input of light energy is low. We also suggest that, in the leaves of sun species, ATP/O ₂ changes depending on the conditions.