Hi,
As mentioned, this arrangement will ensure even current sharing between series strings of LEDs.
Tom... 
Thank you.
Maybe we have a misunderstanding. What I want is "Growth speed vs. light intensity". LED current, PWM or others are just means of achieving the desired light intensity.
Thanks again for your suggestion. But I don't quite understand,
With average light output fixed what happens when you change the PWM duty?
"change the PWM duty"? You are talking about PWM frequency? If it is, someone else has already done it, and there is a difference between the frequencies. 
Yeah, just go to Mouser or RS-Components website (or whatever supplier you like) and search for "led driver IC". Hundreds of components will pop up; look for ones that support analog dimming. It comes in different names and approaches; some IC's take 1 ~ 10V, others 0.3-2.5, yet others 0-1.25V, etc. etc.
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I connected the following circuit to see if it would work as requested.
Gives good DC brightness control.
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Thank you so much.
Hi
You can use this tutorial
On scanning the reference document about the effect of pulsed light on vegetable growth, I began to wonder whether the authors really controlled their experiments appropriately to justify their interpretations. I did not see anything that suggested they did a spectral analysis of the led output at each pulse frequency to ensure that no wavelength shifts where occurring. A quick (maybe too quick) search suggested that there can be circuit temperature effects on the led wavelengths. Whether that could lead to spectral shifts large enough to affect growth, I do not know.
I wonder if there are good references describing intensity-pulsing-wavelength shifts in leds that might be appropriate, and might influence the direction of your study, as another degree of freedom.
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