Chlorophyll fluorescence as a biological feedback signal -for optimized plant growth conditions and stress diagnosis

Doctoral thesis, 2021

The use of light emitting diodes (LEDs) instead of traditional high pressure sodium lamps, in greenhouses and indoor growing facilities, enables tuning and optimization of both light intensity and light spectrum. This opens for an energy saving potential not possible otherwise. Furthermore, such controllable lamps can be used to generate low intensity light excitations, which cause changes in the plants' chlorophyll fluorescence (ChlF). Analysis of these changes can then be used for plant diagnosis. We have conducted such experiments on plants to evaluate if and how proximal sensed ChlF, measured on canopy level, can be used as a biological feedback signal for spectrum optimization, stress detection, and for light intensity optimization.

We found that steady-state ChlF have a strong correlation with short term photosynthesis and can be used for estimation of relative efficiency of different LED colors with respect to each other. We did not find significant changes in the relative efficiencies when light intensity or spectrum was changed, as was initially hypothesized. However, the method can still be applicable for spectrum calibration, as the efficiencies of different LED colors vary individually as the diodes degrade with time and they also vary to different degree depending on the operating temperature.

Experiments on abiotically stressed plants (drought, salt, and heat) showed that variations in the dynamics of the ChlF signal can be used to classify plants as healthy or unhealthy. Experiments with the root infection Pythium ultimum indicated that severe infection is detectable. This is promising as it by its nature is hard to detect without harvesting. More research is needed though, to statistically verify if this, and other biotic stress factors can be detected, and if so, how severe the infection must be.

Fast ChlF gain, defined as the amplitude of the ChlF signal caused by light pulses with a high frequency and a low intensity, was found to have a concave shape with respect to light intensity. Furthermore, the light intensity corresponding to the maximum of the fast ChlF gain coincide with the light level where the photosynthetic rate starts to saturate, which in some sense can be regarded as an optimal light level for efficient growth. Hence, we suggest the use of an extremum seeking controller to force the light intensity level to this point and demonstrates how this works in a simulation study.