Determining intensity and duration of supplemental growth light

For growers who are using or intend to use supplemental growth light, it is very important to understand the influence of light on their crop and the period of time the lamps should be used. The significant factors are:

In principle, the irradiation level determines the photosynthesis rate (CO2 uptake).At low light levels, photosynthesis is low and increases linearly with increasing irradiation.At a certain point, the photosynthesis rate becomes limited by other factors and reaches its saturation level.

The result is a Photosynthesis (Pn)-light response curve .The relation between light and photosynthesis is very much dependent on species, developmental stage and growth conditions of the leaves.The efficiency of photosynthesis is highest at low light levels.

This means that extra growth light is most beneficial when natural daylight (global radiation) is low and is clearly limiting growth. Depending on species and the objective of the grower, extra growth light is switched on when global radiation falls below 200-300 W/m2 (= 300-450 µmol.m-2 .s-1 inside the greenhouse).

Although growth light irradiance is very important for the photosynthesis rate, the SUM of growth light is the most important factor for plant growth. Based on his own experience, the Norwegian Professor Moe has created a plant classification, depending on their light sum needs for optimal growth.

Shade-plants

For example Saintpaulia and Lorraine-begonia and some green pot plants prefer low light conditions (5 -10 mol.m-2 per day). In fact these plants are easily damaged by full sunlight.

Medium tolerance plants

Most flowering pot plants have a medium tolerance in light sum (10 - 20 mol.m-2 per day). Examples are Kalanchoë, Poinsettia, pot-Chrysanthemum and Elatior-begonia.

Tolerant plants

Plants with a very high need for light (> 20 mol.m-2) are for example Roses,Tomato and Cucumber.

The sum of growth light is calculated as: irradiance (µmol.m-2.s-1) x duration (sec.)

For example: 100 µmol.m-2.s-1 during 16 hours gives a light sum of: 100 * (16 * 3600) / 1000 000 = 5.76 mol.m2