Investigations on Photorespiration With a Sensitive ' 4 C-Assay Israel

A leaf disk assay for photorespiration has been developed based on the rate of release of recently fixed "'CO., in light in a rapid stream of CO,. free air at 300 to 350. In tobacco leaves (Havana Seed) photorespiration with this assay is 3 to 5 times greater than the ' "CO, output in the dark. In maize, photorespiration is only 2 % of that in tobacco. The importance of open leaf stomata, rapid flow rates of CO.,-free air, elevated temperatures, and oxygen in the atmosphere in order to obtain release into the air of a larger portion of the 14iCO, evolved within the tissue in the light was established in tobacco. Photorespiration. but not dark respiration, was inhibited by a-thydroxv.2-,pyridinemethanesulfonic acid, an inhibitor of glycolate oxidase, and by 3-(4-chlorophenyl)-1,l-dimethylurea (CMU), an inhibitor of photosynthetic electron transiport, under conditions whieh did not affect the stomata. These experiments show that the substrates of photorespiration and dark respiration differ and also provide additional support for the role of glycolate as a major substrate of photorespiration. It was also shown that at 350 the quantity of '4CO released in the assay may represent only 33 ', of the gross "CO., evolved in the light, the remainder being recycled within the tissue. It was concluded that maize does not evolve appreciable quantities of CO., in the light and that this largely accounts for the greater efficiency -of net photosynthesis exhibited by maize. Hence low rates of photorespiration may be expected to be correlated with a high rate of CO., uptake at the normal concentrations of CO., found in air and at higher light intensities. The stuggestioni that CO. evolution in the light in photosnlthietic ti-sues, kinowvin as photorespi ration. may be considerably greater than the easily measured dark respiration was clearlstated by Decker (2, 5) and was based upon observations of the rapid postilluminationi hturst of CO2. in several species. This view wvas recently supported 1)w the findinig that wihenl photorespiration in tobacco leaf disks va. ; inihibited witlh an a-hvdroxvsulfonate. whichi inhibits -lvcolate oxidase activity (22), net photosynthesis w%-as greatly enhanced (26). Under the sale conditions disks of maize leaves were uinaffected. Maize has a much greater net photosynthesis thani tobacco ait high light intenisities atnd normial concentrations of CO. lpresent in the air ( 19) and possesses an app)arelntly negligible photorespiration (71, 13'). These results suggested that if the rate of photorespiration wvere decreased by genetic or biochemical means, a greater net photosynithesis may be expected in those species where photorespiration occurs. The studies described here have beeni carried out in orider to learin more about the natnrle of this plhlotorespiration process and to develop ani assay helpful in selecting genetic material w-itlh a hiigher photosvntlhetic efficiency such as is ex.hibite(d bymaize. Several methods ha've already been tise(l by investigators in attempts to obtain some measure of the CO., released by photosynthetic tissues in the light. Since some photosynthetic CO. fixation must always. take lplace while measurenm,ent of the released CO., is being 1nid(le. it is apparent that no method will l)ermit a measure of the gross photoresl)iratioin even when the directioni of the CO., flux is reverse(d 1)v providing a CO.-free itmosl)here ('15). The qluantity of CO., released fronii a leaf in the light will depen(l on the magnitude of the photorespiration, which increases greatly at elevated temiiperatures anl concentrations of oxygen exceedilig 2 O/( to 33 % ill those slecies possessing this reaction (6, 18l). In aIdditioIl it will depenld oIn what proportion of the evolved CO., diffuses to the atmosphere in comparisoIn to the amiouint that is refixed in the chloroplasts. Thus the magniitude of the (liffusive resistance from the site of CO., evolutionl to the atmosphere (including stomiiatal and leaf to air resistance) as well as the gross photosynthesis by the chloroplast (photos;ynthetic efficiency) will affect the amilotunit of CO. released from the leaf (1). In order to minimize the turniover of CO. inside the leaf and tlhus permit a greater fraction to be released, it is therefore essential to have the stomata as widely openi as possible and to have ,i rapi(d airflow over the leaf tissue. Some of the earlier examinations of l)llotoreslirationl have been made by observing factors that influenice the l)ost-illumiination burst of COC. (2. 4, 18). by plotting the rate of net photosynthesis at atmospheric CO. concentrationis above the CO., compensatioin point and extrapolating to 'zero CO." concenitration (3, 6). 'by observing the 'CO., released in air from tissue supplied with glycolate-l-14C (26). -anld 1o measurin, the evolution of CO., in the light 1829 www.plantphysiol. org on April 30, 2019 Published by Downloaded from Copyright © 1968 American Society of Plant Biologists. All rights reserved.