Nothing serious -
Trichs
The sole function of trichomes is still ambiguous, looking at many species from all habitats only a few reasons stand out as sole functions. Theorised and tested some though heat or cold protection, some though increasing the boundary layer or creating turbulence, others simply protection from the sun. None really gained much acceptance except in a few species where their role was easily seen. From lab studies the boundary layer and turbulence didn’t add much or enough to warrant the resources needed to produce them over evolutionary scales. Some credence was given to heat and light protection but this had the effect of warming the tissue as heat conductance rose and heat stress.
Marijuana forms it’s densest trichomes at two stages, the hypocotyl on the seedling which layer gets shed once the stem turns woody and lenticels become more pronounced. The second mid to late flowering where buds, sugar leaves and the underside veins are coated in them.
Back to all the other species with trichomes we find one reason that fits and that is species that like to increase the temperature of the seed production areas. The reason for the hypocotyl trichomes would also be the same increasing that tissue temperature for better growth and as it wasn’t much of a solar collector in size like a leaf. As soon as it turns more woody it sheds that layer of trichomes as the woody stem is more resilient to climate and internal temperature a lot more stable.
Coincidentally some light is blocked especially in certain spectrums, so on the opposite side it negates some heat stress but conversely keeps the tissue hotter as heat conductance dominated the issue. This helps the seeds grow which prefer a higher temperature to the rest of the plant . I guess the underside of leaf veins on the sugar leaves simply kept the vein a little warmer but that’s really past the point of most research I could find so I can’t really speculate that precise function.
VPD
Trichs did not raise the boundary layer by much. Looking at leaf morphologiy it comes down to resources mainly, did the evolution of that plant or this favour heat or cold protection, did it favour water/drought protection and a few other things. Where did it put it’s resources to, note a forest with trees with many different shapes, they rarely favoured what the next species did and hence all the different tree leaf shapes all next to each other.
One thing we know is that with less reliable resources leaves grow longer and thinner i.e. sativa. Great at drought prevention, less fertile ground, very hot weather. Indica bigger wider where you find more resources and slightly less demanding climate and more water stability/availability.
So the plant grew leaf shapes that focused on these resources rather than others for better survival. They are pinnate which also gives smaller boundary layer and quicker feedback I e. It’s stomata change apperature much quicker tracking real time climate unlike others that don’t on broader palmate types This favours survival in challenging situations better than a palmate type large singular round leaf which although has greater potential for photosynthesis and transpiration but dosent offer much protection to stresses like drought and heat for instance like some in the tropics where resources are abundant all year.
VPD
My issue is that simple VPD charts don’t give credence for the hundred times VPDs power that wind does. Add in air movement and boundary layers drop enough that VPD charts are now impaired i.e. humidity dosent matter that air is sucking water faster than high humidity can stop it.
Against the grain the leaf and plant functions well in windless environments, take a hot stagnant day in a hemp field, shit grows fast no problems and canopy down there is little perceptible air movement some grow tent growers would doubt the vpd readings of the whole situation.
The leaf morphologiy is great at this kind of situation, I’ve done grows with no perceptible air movement just heat exchange for good air temps and always some of my best. Just some wind can screw up seedlings and most grows past a very low level and direct wind just causes burn as the leaf can’t keep up it’s water potential against the wind sucking it out of the stomata.
Just some wind is the difference between green brown stemmed seedlings and purple ones from anthocyanins. You can try that easily repeatable and the actual correct colour for seedling hypocotyls or stems is green brown and certainly not purple which indicates stress.
The pinnate leaf serrated at edge is further better equipped to shed heat from it’s edges and induce updrafts but technically stagnant air is hard to produce, even on a no wind day in the middle of a tightly packed hemp field air is still moving everywhere. Heat was the only issue in my grows wind wasn’t ever needed in fact just dialling down my exhaust in my early grows was a big step up to growing stress free plants and think most tents indoors have gar too much air flowing through almost akin to wind tunnels not a field on a hot day growing at its max. At that point water availability was the limiting factor not wind in the field. Wind also cools leaves fat too much, few plants in the world even like wind and plant recommend indoor lab levels barely get above 0.3m/s to 0.5m/s with diminished gains thereafter.
To support that idea of a no wind grow (move some air for heat but no perceptible movement to us) Google kratky growers with lettuce tomato and so on. They just string some lights no fans and produce some wonderful indoor grows. Yer low tech whatever, them plants grow well illustrating how out of whack out views on ventilation can get.
So there’s definite limits which we easily surpass and then probably blame oh or calcium or even the breeder why it all went to pot.
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