Photosynthetic Performance Index (PPI): what does it mean?

01

What is the photosynthetic performance index (PPI)?

The PPI is a daily index, expressed as a percentage, that shows how efficiently your crop used the light on that day.

The index looks at electron transport in the leaf. That is the first step of photosynthesis: the leaf converts captured light into energy. The plant then uses that energy to fix CO₂. So the PPI does not measure how much CO₂ your crop took up, nor how much it grew.

What does the number mean?

• 100%: your crop performed optimally for the available light.
• Below 100%: your crop could have performed better.
• Above 100%: your crop did better than the model expected.

02

Why compare only days with roughly the same amount of light?

On a day with little light, your crop can by definition convert less light than on a sunny day. The goal is to understand why your crop had a high PPI on one day and a low PPI on another.

That is why the PPI compares each day only with days that had a similar amount of daily light. That amount is the daily light integral (DLI): all the light your crop receives in a day, in mol/m²/day. MyLedgnd puts days with roughly the same amount of light into the same group. Such a group is called a light class.

Your own best performance as the reference

Think of it like running. You do not compare your 5K pace with your marathon; you compare it with your previous 5Ks. In the same way, your crop has a highest and a lowest PPI per light class. These are the days when things went particularly well or badly, and therefore the days you want to zoom in on.

03

How is the PPI calculated?

You do not need to know the calculation to use the index. In short, it works in three steps.

Measure: what did your crop do?

A fluorescence measurement determines how well the leaf converts the light it captures. That measurement is combined with the amount of light (PAR). This tells you how much electron transport actually took place.

Calculate: what was possible?

The model calculates how much electron transport would be optimal under that same light. The model is parameterised, which makes it specific to each crop.

Add up and divide

Both are added up over the whole day. The ratio between the measured and the optimal total is your PPI.

Researchers use the same approach to monitor the photosynthesis of a greenhouse crop. The PPI makes this visible at a daily level.

Do you often score above 100%? Then the model needs to be calibrated for your crop and specific variety. Let us know.

04

A high index does not automatically mean more growth

This is where it often goes wrong. In the example, the crop scored 95% yesterday. On the best day it scored 100%. Almost the same, then. But look at the figures below.

Yesterday the crop used the light almost as well. But there was much less light. As a result, there was about 43% less electron transport.

The lesson: a high PPI on a dark day means the little light available was used well. It does not mean your crop did a lot. So always look at the PPI and the daily light integral together.

05

How do you use the PPI in your cultivation?

The PPI only becomes really useful when you put a day next to your best day. Here is how:

• Choose a period of at least four weeks. That gives you enough days per light class.
• Check how many days have good measurement data. If that is below 90%, be careful with your conclusions.
• Find your best and weakest days in the PPI table. Only compare days in the same row.
• In the climate table, put a day next to your best day. Click the date. You will then see the course of the day, with the index for each moment.

Found something? Treat it as a lead, not as proof. Irrigation, fertilisation, the growth phase of the crop and the condition of the leaf can also affect the index. So use the PPI as a starting point, and check the crop yourself as well. Want to dig deeper, moment by moment? Then light deviation helps: how much of the light your crop is not using at a given moment.

You often cannot see this kind of stress from the outside, or only when it is too late. Read which signals your plant gives earlier in the whitepaper How to spot invisible plant stress.

06

What does the PPI not tell you?

An index only works if you know what it is meant for. Keep these four things in mind.

How much your crop produced

The PPI measures how well the light was used. Not how much CO₂ your crop fixed, and not how much it grew.

How days with a lot and a little light compare

Only compare days in the same row of the PPI table. Want to know how much more a sunny day delivered? Then look at the daily light integral (DLI) and the electron transport over the day in the climate table.

How you are doing compared with other growers

The model differs per crop and sometimes per variety. The index is designed to compare your crop with itself, not with another grower's crop.

Whether the measurement was right

The index is only as good as the measurement. Is more than 10% of the measurement data missing on a day? Then the index gets a warning. Is more than 20% missing? Then no index is shown. Seeing 0% under normal light? Then the measurement has probably failed. Report the date to support.

07

Your own best day as the benchmark

Data does not replace your craftsmanship. But it does give your experience a fixed reference point. The PPI does not compare your crop with a textbook standard, but with itself, on days with roughly the same amount of light. That way you know which day you want to repeat, and which climate went with it.

Start simple: put yesterday next to your best day with roughly the same amount of light. Then look at the trend over a few weeks, and at how it relates to VPD and other early stress signals.