Temperature-Dependent maximum dark-adapted Fluorescence in Lantana camara: Implications for Accurate NPQ Measurements

Continuous active chlorophyll fluorescence measurements, e.g. using the MONI- or MICRO-PAM instruments by Walz, are widely used because they provide valuable, real-time information about the photosynthetic activity and stress status of plants by measuring chlorophyll fluorescence. Its ability to provide non-invasive, precise data on photosystem performance makes it an essential tool in both research and practical applications.

However, some derived parameters from these measurements depend on measurements taken from dark-adapted leaves. With continuous measurements, these values are collected during the night. Considering that, in addition to light availability, many other environmental parameters (such as temperature, VPD, etc.) differ significantly from daytime conditions, the question arises whether directly relating parameters measured at night to those measured during the day might lead to errors. One such parameter that is potentially affected is NPQ (non-photochemical quenching), which is calculated from the ratio of the maximum fluorescence of a dark-adapted leaf (Fm) to the maximum fluorescence of a light-adapted leaf (F’m).

We thus present here the results of a laboratory experiment in which we investigated the temperature dependence of Fm in Lantana camara under otherwise constant conditions. We were able to demonstrate that Fm shows a clear dependence on ambient temperature, with Fm increasing as the temperature rises. This implies that, under typical field conditions, where night-time temperatures are lower than daytime temperatures, Fm measured at night would underestimate the actual values of Fm observed under warmer daytime conditions. Ultimately, this leads to an underestimation of NPQ, when calculated from these underestimated Fm values.

In parallel, we investigated whether a typical dark adaptation period of 30 minutes is sufficient to reach a dark-adapted state during daytime conditions. Our results showed that 30 minutes was never enough to ensure adequate dark adaptation in the leaves.