Why look at heat stress? Heat waves are becoming more common, and keeping crops healthy and productive during one is a big challenge. That is why breeders are looking for cultivars that are more tolerant to higher temperatures, without losing yield. But selecting the right cultivars takes a lot of time. Often, it is scored by eye, or increasingly with colour or multispectral imaging. However, in our experience, it takes two to three weeks before the plants start to show symptoms. Photosynthesis is much faster, so chlorophyll fluorescence should pick up the stress sooner. That is what Arjen Tegels tested during his MSc internship at PhenoVation.

Many of our standalone camera systems are used in plant research to non-destructively provide detailed insights into plant performance. The most popular measurements are those of photosynthetic efficiency. In combination with other measurements like chlorophyll content and morphological traits (see our website or related blogs), these parameters can help answer difficult research questions.

In part 1, I explained why the fluorescence of a leaf rises in steps (O, J, I and P) when you switch on a strong light. That curve is nice, but a curve is hard to compare. With 200 plants you cannot observe 200 curves next to each other and see which plants are stressed. In this part I show how we turn the curve into numbers. This is called the JIP test. The numbers describe what happens with the light energy that a leaf absorbs, so they tell you where in the photosynthetic machinery things go well, and where they do not. In short The JIP test reads the fluorescence at a few fixed moments of the curve (O, J, I and P, and some points in between) and puts them in formulas. Every number has a physiological meaning. The numbers follow the energy through the leaf: how much light is absorbed, how much of it reaches each step, and how much is lost as heat. The performance index (PI) combines several of these steps in one number. It is one of the most sensitive OJIP parameters. Under stress many parameters change, but whether they go up or down depends on the species, the type of stress and how severe it is. So look at several parameters together. Spider plots and leaf models help with that. What happens to light energy in a leaf? The JIP parameters follow the path of light energy, from the moment a photon is absorbed. The energy can be used to drive photosynthesis, or it is lost as heat or as fluorescence. At every step along the way, some energy is used and some is lost. We group the parameters in three types: Energy fluxes : how much energy flows through each step. Quantum yields : what fraction of all the absorbed energy reaches a certain step. Efficiencies : what fraction of the energy that reached one step continues to the next. Energy fluxes: how much goes where? A flux is an amount of energy flowing per second. The JIP test describes five of them: Absorption (ABS). Photons are caught by the antenna pigments. This creates excited chlorophyll. Trapping (TR). The energy of the excited chlorophyll is passed on to the reaction centre. There it starts the electron transport: QA takes up an electron. Electron transport (ET). Electrons that move on beyond QA, into the rest of the chain. Reduction of the end acceptors (RE). Electrons that arrive at the end of the chain, on the photosystem I side. (Scientists call taking up electrons "reduction".) Dissipation (DI). Energy that is lost as heat or as fluorescence. Most of it is heat; the fluorescence we measure is only a small part.

In short In an OJIP measurement, a dark-adapted leaf gets one second of extremely bright light, and we follow how its fluorescence climbs. The curve rises in steps, called O, J, I and P. Each step shows the flow of electrons in photosynthesis. From the shape of the curve we calculate how well the photosynthetic machinery works. Part 2 explains those parameters. 

In June 2025, we launched the CF2GO (Chlorophyll Fluorescence 2 Go) at the Greentech in Amsterdam, where the jury nominated it for the innovation award out of 47 products. The CF2GO is a system that accurately measures how efficiently a crop uses light for photosynthesis, in real time and from up to a meter away from the crop. No leaf clips, and not touching the plants. It brings the same scientific quality as our lab systems into the greenhouse. The CF2GO can hang in a fixed spot or move along a rail or boom, and it is already running at several commercial growers. Johan Langelaan, cultivation manager at Maarel Orchids, explains why they use it: "We can now see in real-time when the plants require more or less light for photosynthesis, allowing us to adjust screening and lighting immediately." And: "You get hard facts and measured values from the system, and you can immediately see what they mean." ( https://www.onderglas.nl/gewas-krijgt-exact-de-hoeveelheid-licht-die-het-nodig-heeft/ )

What is chlorophyll fluorescence?