The studies are going on and on. It is now known that different plant species need different wavelengths for optimal growth. So it was recently found that the wavelength of 730 nanometers promotes the growth of certain plants such as roses or peppers. That's why not only light intensity is important, but also spectrum.
You are in Australia, I assume that you have enough sunlight. So you should not look for the maximum, but minimum light intensity at which the plants are well colored and look good. To achieve good color saturation, the natural light is best suited because it has complete spectrum and does not cost anything.
Photosynthesis rate can not be increased indefinitely by irradiation of higher light intensities. A chlorophyll molecule absorbs about 45 photons per second. Once this light saturation is reached, you could add dozens of growth lamps, but the result would be the same except for a high energy bill.
The small seedlings do not need sunlight, it is even deadly for the little ones. Reducing light intensity should not be a problem, I have another problem: too little light. That's why I'm forced to help with artificial light to get good-looking plants.
The light intensity requirements are therefore different for adult Lithops and seedlings. In addition, there are Lithops species that are colored worse with much sunshine. These are mainly green acf, which then turn yellow. In addition, there are species that grow in nature in the grass and the others in the open areas. So it's best to find out for yourself which amount of light is best for which species.
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