# The Fitting of the OJ Phase of Chlorophyll Fluorescence Induction Based on an Analytical Solution and Its Application in Urban Heat Island Research

## TL;DR

This paper introduces a new analytical method to study chlorophyll fluorescence, revealing insights into photosystem II dynamics and urban heat island effects.

## Contribution

The paper presents an analytical solution for fitting the OJ phase of chlorophyll fluorescence induction, enabling precise parameter estimation.

## Key findings

- The functional absorption cross-section of PSII (σPSII) and other parameters dynamically change during the O to J transition.
- Higher-temperature-acclimated leaves show increased σPSII and decreased QA− oxidation rate (kox).
- Urban heat islands influence PSII characteristics, resembling shade-type adaptations.

## Abstract

Chlorophyll (Chl) fluorescence induction (FI) upon a dark–light transition has been widely analyzed to derive information on initial events of energy conversion and electron transfer in photosystem II (PSII). However, currently, there is no analytical solution to the differential equation of QA reduction kinetics, raising a doubt about the fitting of FI by numerical iteration solution. We derived an analytical solution to fit the OJ phase of FI, thereby yielding estimates of three parameters: the functional absorption cross-section of PSII (σPSII), a probability parameter that describes the connectivity among PSII complexes (p), and the rate coefficient for QA− oxidation (kox). We found that σPSII, p, and kox exhibited dynamic changes during the transition from O to J. We postulated that in high excitation light, some other energy dissipation pathways may vastly outcompete against excitation energy transfer from a closed PSII trap to an open PSII, thereby giving the impression that connectivity seemingly does not exist. We also conducted a case study on the urban heat island effect on the heat stability of PSII using our method and showed that higher-temperature-acclimated leaves had a greater σPSII, lower kox, and a tendency of lower p towards more shade-type characteristics.