The quantitative determination of photosystem I density on intact leaves using single turnover flashes

Abstract

The in vivo measurement of photosynthesis has beenlargely limited to fluorescence. which is hard to interpretbeing a very indirect measurement and to gas exchange incontinuous light. which infers only about the overallefficiency of the leaf and does not give any quantitativedata about the concentrations and absolute activities ofphotostynthetic complexes.In this study. a method. proposed for in vivodetermination of the concentration and redox state ofPhotosystem I. one of the photoactive complexes ofphotosynthesis. has been corrected. Most of the complexesin the photosynthetic unit can be measured with exactly thesame principles and similar equipment. This would makepossible a diagnostic test system for nondestructive testingof photosynthetic stress The necessity of P700measurement for such a system and the opportunities itpromises have been briefly discussed. It has been concludedthat the magnitude and exact site of damage onphotosynthesis by environmental factors can be determinedwith te help of PS I measurement. For the quantitativejustification of the measurements. the influence of thenonideal optical properties of the leaf on the measurementshave been investigated by experimental means and it has beenobserved that the refraction and scattering phenomena in theleaf magnify the apparent absorption changes by a factor ofca. six. A method for the exact determination of active PS Icenters, through measurement of the oxygen evolved by singleturnover flash excitation has been developed.The elongation of the pathlenght of the light in theleaf up to about six times the thickness of the leaf hasbeen found to be due to mostly the refractions in the lipidair interface at the spongy parenchima. This organisation ofthe leaf structure appears to be an intentional event, withthe purpose of increasing the light harvesting efficiency ofthe leaf at the spectral region between the peaks ofchlorophyll absorption.