Aitken, S. M. | Search bioRxiv

bioRxiv2026

Towards a standard approach to investigating the Thermal Load Sensitivity of photosystem II via chlorophyll fluorescence

Evaluating the drivers of variation in plant thermal tolerance limits requires a clearer understanding of how methodological matters can lead to different tolerance estimates. Chlorophyll fluorometry - to measure the temperature-dependent change in FV/FM - is a well-established approach to derive tolerance thresholds of photosystem II (PSII) in plants, but one-off, time-specific thermal exposures do not consider the fundamental dose-dependent effect of heat. The resurgent thermal death time (TDT) approach integrates both the temperature intensity and the exposure duration to derive time-based critical temperature thresholds and sensitivity parameters. We build upon this foundation to develop a protocol for evaluating thermal load sensitivity (TLS; non-lethal heat stress) of PSII in plants. Through five experiments across four diverse species, we tested the moderating effects of light, leaf sectioning, time since collection, and the temporal dynamics of FV/FM recovery. There were dramatic changes in tolerance threshold estimates based on thermal load (i.e. dose-dependent) effects on FV/FM, and strong effects of light intensity during heat and the presence of light post-heat. We offer recommendations pertaining to method implementation and discuss future empirical avenues. Appraising cumulative heat stress will enhance the utility of thermal tolerance estimates - the TLS approach outlined here moves us toward a new standard.

ecology↗

bioRxiv2025

Morphological and heat-tolerance traits are associated with progression and impact of, but not vulnerability to, tree decline

Warming and drying climate trends are driving tree dieback worldwide with broad-reaching impacts on ecosystem services. Studying decline is unavoidably a retrospective exercise in which researchers are challenged to determine whether trait values that are associated with dieback are drivers versus consquences of decline. In this study, we use the subalpine snow gum (Eucalyptus pauciflora, ssp niphophila) as a case study to illustrate how to identify whether plant traits may explain vulnerability of individual trees, assess how progression of dieback symptoms affect traits and physiological tolerance, and ask whether those changes could exacerbate decline. While the impact of drought on tree mortality has been well documented, we consider the potential role of heat which has received considerably less attention. We assessed changes in leaf and stem morphology, and stomatal anatomy across a dieback severity and an abiotic elevation gradient. We also assessed the relationship between these traits, photosystem heat tolerance and dieback progression, using results to model leaf viability under current and future climate scenarios. While severely symptomatic trees exibited trait values indicating water stress, trees with low or moderate dieback were not different from unaffected ones. Thus the differences in severely affected trees are likely responses to water stress caused by woodborer girdling and provide no evidence of underlying trait difference driving vulnerability. Severely symptomatic trees, however, had lower photosystem heat tolerance and models indicated leaves were likely to accumulate lethal damage to photosystems within a growing season, thus contributing to a feedback cycle of decline even under current thermal regimes. HighlightsO_LIAccounting for abiotic, spatial variation explains complex relationships in dieback research C_LIO_LIInsect mediated dieback induces trait variation consistent with water limitation C_LIO_LINo evidence of variation in vulnerability to dieback among snow gum individuals C_LIO_LIDieback-induced reduction in heat tolerance compounds heat load effects C_LIO_LIWarming induced heat loading will compromise carbon gain and exacerbate decline C_LI

ecology↗