Addressing the long-standing limitations of double exponential and non-rectangular hyperbolic models in quantifying light-response of electron transport rates in different photosynthetic organisms under various conditions

Abstract

The models used to describe the light response of electron transport rate in photosynthesis play a crucial role in determining two key parameters i.e., the maximum electron transport rate (J _max) and the saturation light intensity (I _sat). However, not all models accurately fit J-I curves, and determine the values of J _max and I _sat. Here, three models, namely the double exponential (DE) model, the non-rectangular hyperbolic (NRH) model, and a mechanistic model developed by one of the coauthors (Z-P Ye) and his coworkers (referred to as the mechanistic model), were compared in terms of their ability to fit J-I curves and estimate J _max and I _sat. Here, we apply these three models to a series of previously collected Chl a fluorescence data from seven photosynthetic organisms, grown under different conditions. Our results show that the mechanistic model performed well in describing the J-I curves, regardless of whether photoinhibition/dynamic down-regulation of photosystem II (PSII) occurs. Moreover, both J _max and I _sat estimated by this model are in very good agreement with the measured data. On the contrary, although the DE model simulates quite well the J-I curve for the species studied, it significantly overestimates both the J _max of Amaranthus hypochondriacus and the I _sat of Microcystis aeruginosa grown under NH₄ ⁺-N supply. More importantly, the light intensity required to achieve the potential maximum of J (J _s) estimated by this model exceeds the unexpected high value of 10⁵ μmol photons m^-2 s^-1 for Triticum aestivum and A. hypochondriacus. The NRH model fails to characterize the J-I curves with dynamic down-regulation/photoinhibition for Abies alba, Oryza sativa and M. aeruginosa. In addition, this model also significantly overestimates the values of J _max for T. aestivum at 21% O₂ and A. hypochondriacus grown under normal condition, and significantly underestimates the values of J _max for M. aeruginosa grown under NO₃ ^-N supply. Our study provides evidence that the 'mechanistic model' is much more suitable than both the DE and NRH models in fitting the J-I curves and in estimating the photosynthetic parameters. This is a powerful tool for studying light harvesting properties and the dynamic down-regulation of PSII/photoinhibition.

Keywords: double exponential model; dynamic down-regulation; electron transport rate; mechanistic model; non-rectangular hyperbolic model; photoinhibition.