Limitation of Photosynthesis by Carbon Metabolism

Abstract

It has been investigated how far electron transport or carbon metabolism limit the maximal rates of photosynthesis achieved by spinach leaves in saturating light and CO2. Leaf discs were illuminated with high light until a steady state rate of 02 evolution was attained, and then subjected to a 30 second interruption in low light, to generate an increased demand for the products of electron transport. Upon returning to high light there is a temporary enhancement of photosynthesis which lasts 15 to 30 seconds, and can be up to 50% above the steady state rate of 02 evolution. This temporary enhancement is only found when saturating light intensities are used for the steady state illumination, is increased when low light rather than darkness is used during the interruption, and is maximal following a 30 to 60 seconds interruption in low light. Decreasing the temperature over the 10 to 30°C range led to the transient enhancement becoming larger. The temporary enhancement is associated with an increased ATP/ADP ratio, a decreased level of 3-phosphoglycerate, and increased levels of triose phosphate and ribulose 1,5-bisphosphate. Since electron transport can occur at higher rates than in steady state conditions , and generate a higher energy status, it is concluded that leaves have a surplus electron transport capacity in saturating light and CO. From the alterations of metabolites, it can be calculated that the enhanced 02 evolution must be accompanied by an increased rate of ribulose 1,5-bisphosphate regeneration and carboxylation. It is suggested that the capacity for sucrose synthesis ultimately limits the maximal rates of photosynthesis, by restricting the rate at which inorganic phosphate can be recycled to support electron transport and carbon fixation in the chloroplast. When a plant leaf is photosynthesizing in low light or low C02, the rate of photosynthesis can be increased by raising the light intensity or the CO2 concentration. However, a point is reached where further increases of light or of CO2 do not lead to any further increase in the rate of photosynthesis. At this point, it would appear that an internal ceiling has been reached where at least one of the components of the photosynthetic apparatus cannot operate any faster. In principle, this ceiling could be imposed by the number or turnover rate of the photochemical centers, by the capacity for photosynthetic electron transport, or by the rate at which ATP and NADPH can be consumed in the reactions of carbon metabolism which convert CO2 and H20, ultimately, to carbohydrate end products. We do not have simple methods to show to what extent these components are imposing a ceiling on the maximal rates of photosynthesis. The aim of this and the following (17) paper is to describe simple procedures ' Supported by the Deutsche Forschungsgemeinschaft. which reveal when carbon metabolism is restricting the rate at which the available light and CO2 can be utilized, to investigate in what conditions this may happen, and to ask which components of carbon metabolism may be responsible for this restriction. The approach described in this paper depends upon electron transport and the various reactions of carbon metabolism responding in a different way to a sudden decrease in the light intensity. After darkening, electron transport is immediately stopped, while stromal enzymes are only gradually inactivated (3, 8, 9), and the synthesis of sucrose in the cytosol continues for 15 to 60 s (22). It will be shown here that this slower inhibition of carbon metabolism can be exploited to generate an increased 'demand' for the products of electron transport. This should allow a transient enhancement of the rate of 02 evolution immediately after returning a leaf to high light in conditions where the leaf possesses excess capacity for electron transport which is not being utilized during steady state photosynthesis. MATERIALS AND METHODS Spinach (Spinacia oleracea var Mazurka) was grown in hydro-ponic culture (13) under a 9 h light/15 h dark cycle with a light intensity of 340 ,E-m2 * s-' and a CO2 concentration in the light of 380 ,l/L. The temperature was 22°C in the light and 1 6C in the dark. Leaf discs were taken from fully expanded leaves of 5-to 7-week-old plants. Xanthium was grown under a 12 h light/ 12 h dark cycle, with a light intensity of 300gE m 2-s-' and a temperature of 24°C in the light and 14C in the dark. The shade leaf grew at a light intensity of about 100 AE m-2 s'. Ivy was obtained from a plant growing in a shaded site in the botanical garden. 02 evolution was measured in a Hansatech leaf disc 02 electrode (4). Temperature was controlled by water flow through a water jacket. Light was provided by projectors, the intensity being varied with neutral filters. CO2 (about 5%) was supplied from 200 ,ul of a 2 M KHCO3/K2CO3 mixture (pH 9.3) on felt in the base of the electrode. Increasing the pH to 10.0 or decreasing the concentration to 0.3 M did not decrease the rate of photosyn-thesis. Usually, photosynthesis of 3 to 5 leafdiscs (1 cm diameter) was measured. The leaf discs were placed on a fine sheet of aluminum, which was punctured repeatedly with holes to allow gas exchange under the leaf material, but not punctured in the area without plant material. The aluminum prevented light penetrating into the electrode chamber below the leaf, where thermal expansion of the electrode walls and felt matting could produce artefacts with the light sources used in these experiments. After inclusion of the aluminum, the highest light intensities used did not produce significant alterations in 02 concentration due to thermal expansion when control experiments were carried out using leaf material which had been killed by freezing in liquid N2 and rethawed. 1115 www.plantphysiol.org on August 11, 2020-Published by Downloaded from