Measurements of mesophyll conductance, photosynthetic electron transport and alternative electron sinks of field grown wheat leaves

Abstract

Photosynthetic electron transport drives the carbon reduction cycle, the carbon oxidation cycle, and any alternative electron sinks such as nitrogen reduction. A chlorophyll fluorescence- based method allows estimation of the total electron transport rate while a gas-exchange-based method can provide estimates of the electron transport needed for the carbon reduction cycle and, if the CO2 partial pressure inside the chloroplast is accurately known, for the carbon oxidation cycle. The gas-exchange method cannot provide estimates of alternative electron sinks. Photosynthetic electron transport in flag leaves of wheat was estimated by the fluorescence method and gasexchange method to determine the possible magnitude of alternative electron sinks. Under non-photorespiratory conditions the two measures of electron transport were the same, ruling out substantial alternative electron sinks. Under photorespiratory conditions the fluorescence-based electron transport rate could be accounted for by the carbon reduction and carbon oxidation cycle only if we assumed the CO2 partial pressure inside the chloroplasts to be lower than that in the intercellular spaces of the leaves. To further test for the presence of alternative electron sinks, carbon metabolism was inhibited by feeding glyceraldehyde. As carbon metabolism was inhibited, the electron transport was inhibited to the same degree. A small residual rate of electron transport was measured when carbon metabolism was completely inhibited which we take to be the maximum capacity of alternative electron sinks. Since the alternative sinks were small enough to ignore, the comparison of fluorescence and gas-exchange based methods for measuring the rate of electron transport could be used to estimate the mesophyll conductance to CO2 diffusion. The mesophyll conductance estimated this way fell as wheat flag leaves senesced. The age-related decline in photosynthesis may be attributed in part to the reduction of mesophyll conductance to CO2 diffusion and in part to the estimated decline of ribulose 1,5-bisphosphate carboxylase amount.

Photosynthetic parameters of birch (Betula pendula Roth) leaves growing in normal and in CO2‐ and O3‐ enriched atmospheres

“…In 21 kPa O 2 , where carboxylation resistance increased, the relative role of r md was smaller. These r md estimates generally agree with those for other species (Loreto et al 1994;Laisk & Loreto 1996;Evans & Loreto 2000), but are rather approximate due to methodological difficulties. The higher K C value used by Medlyn et al (1999) compensates for the neglected r md ; as a result of this their V m values are expected to be more or less correct.…”

Carbon Dioxide Diffusion inside Leaves

“…4). The correlation has been confirmed in several subsequent papers (Loreto et al, , 1994Epron et al, 1995). This has firmly established that pc is about 30% lower than pi for many species when leaves are actively photosynthesizing in high irradiance.…”

“…This has firmly established that pc is about 30% lower than pi for many species when leaves are actively photosynthesizing in high irradiance. The correlation holds not only for young leaves, but for older ones as well; as wheat leaves aged, both photosynthetic capacity and gw declined in parallel (Loreto et al, 1994). For amphistomatous leaves, where intercellular air space resistance is probably the minor component (see below), g, should be proportional to the surface area of chloroplasts exposed to intercellular air space per unit of leaf area, S, (Laisk et al, 1970).…”

In Situ Investigation of Leaf Water Status by Portable Unilateral Nuclear Magnetic Resonance

“…Leaves were maintained under a 1 L min 21 flow of clean synthetic air (80% N 2 , 20% O 2 , and 370 mmol mol 21 CO 2 ) while thermostatted at 25°C and illuminated with 800 mmol m 22 s 21 white light with the system described by Loreto et al (1994) for CO 2 -and water-exchange measurements. However, part of the air leaving the cuvette was redirected through a Teflon valve to a proton transfer reaction mass spectrometer (Ionicon) for measurements of released volatile organic compounds (Lindinger et al, 1998).…”