I think most reported leaf burn from high light intensity is because of the heat buildup. Not of the photons intensity themselves. The leaves can heat up because the light itself is a heat source, because even LEDs are not 100% efficient. So the light warms up, and if the leaves get close, they warm up too. And the energy of the photons is observed by the leaves. Leaves try to convert that energy into chemical bonds. But photosynthesis is actually really inefficient, so heat is produced as a result. So as long as you stay below 400 PPFD, and the temperatures of the leaves stay low enough, long days should be fine leaf burn wise. The second question is if the plant can handle long days, or if it depends hormonally on a long dark period where it goes into the dark cycle/calvin cycle. If the plant has a circadian rhythm like us humans and going into the calvin cycle is hard-coded in, 24 hour days could stress it out. On top of that, it seems that there are non-photosynthetically related reasons for not having 24 hour cycles. One reason is that during the night, the plants provide sugars to the rhizosphere. And having the lights on 24/7 would starve the rhizosphere. And starving the rhizosphere would reduce its ability to take up minerals. You can also see in type lapses of plants that during the night, they whirl or dance around. And during the day, they follow the sun. And if you grow them inside, it takes them some time to adapt to the light cycle you have going. Only after a while, they will dance strongly during the dark period.
Some points of clarification:
There are three main modes of heat transfer. Conduction, convection and radiation. Conduction is the most efficient, then convection, then radiation.
In the example of heat transfer from light to plant...if the light housing heats up during use then:
Conduction can transfer heat if the plant is touching the housing. This one is pretty clear. Touch something hot, you get burned.
Convection can transfer heat by moving it from the light to the plant by the air molecules hitting the light, picking up energy from the warmer surface then carrying that energy to the leaf where the air molecules deposit that energy when they hit the leaf.
Radiation can come from two sources. The hot part of the light housing emits infrared photons. Those photons can radiate to the plant and deposit their energy into the leaf. IR photons aren't very efficient at driving photosynthesis and cannot otherwise be used by the leaf, so the leaf heats up because that energy has to go somewhere. ALSO, the photons from the light itself are a source of energy (heat) that radiates on the plant. Photosynthesis IS efficient at using red and blue photons so they get captured and turned into intermediate products. This reaction is endothermic. The energy from the photon gets bound up into the products of the reaction and do not heat up the leaf. However, chlorophyll is not 100% efficient. It's more like 25-30% efficient. 25% of the radiative heat goes into usable product. The rest...70% or so...is lost to heat buildup in the plant tissue. Actually, some fraction of that 70% is reflected or transmitted and leaves the leaf...but much of it does convert to heat inside the leaf itself.
Even if there were a perfect vacuum between the light source and the leaf...a perfect vaccuum is a perfect insulator to conduction and convection...the leaf would heat up because of radiation from the light itself. Hot air around the leaf prevents that heat from escaping out of the tissues. But even with cooler surrounding air, a bright enough light will burn plant tissues as the heat cannot leave fast enough.
As far as the Calvin cycle: it is NOT called the dark reaction because it happens at night. In fact, it can't run in the dark for very long at all! The Calvin cycle requires NADP to run. NADP is a short lived byproduct of photosynthesis. It cannot be stored until night. The Calvin cycle is called the "dark" reaction because, unlike photosynthesis, it does not require light to run. Photosynthesis is the "light" reaction because it requires light...the Calvin cycle does not require light making it the "dark" reaction. Scientists are too clever for their own good sometimes
en.wikipedia.org
What does occur during the dark is a lot of maintenance work. Chlorophyll that was burned up during the day needs to be regenerated. Converted sugars from the Calvin cycle need to be transported to useful places. The plant may need to regenerate other hormones and growth factors and may actually grow during the night. Basically, the light/dark reactions result in a warehouse full of goodies that need to be distributed and used and the factory itself needs some routine maintenance
The efficiencies of those processes help determine how much dark time the plant needs to catch up. A lot of these activities also happen during the day, but they are slower and sometimes the plant can't keep up. For example, if the light is too intense, the plant may burn up chlorophyll faster than it can regenerate it. The Calvin cycle can build up sugars much faster than they can cross membranes and diffuse to other parts of the plant. A few hours of darkness where nothing new is breaking can help the maintenance men catch up and clear off the shelves so new product can be produced.
. same color as new growth on my variegated serissas. caught in the act!
