Heat Stress?

if the light meter is accurate, which a phone app is definitely not accurate, you might be giving too much light per day. But, definitely some nutrient-related issues here too. you'll be better off working first from teh specs of your light, calculate PPFD, then reference a DLI table and shoot for 35-40. This will be more useful than a phone app. Observe and react from there. You will need some slight adjustment but once dialed in, you can take a reading with that phone app and use that as your baseline in future. Skip the trial and error. The phone's camera lense is not a quantum meter. It cannot measure photons. It is measuring klux then converting using some factor that is not necessarily tailored to your light -- you have to cnvert the "lumens" curve because it weights Green wavelengths extra heavy. So, cooler CCT lights will read higher with you app than warmer CCT lights and many othe rreasons why the reading will be false by at least 10%. It is however a consistent reading under any 1 light (or a light with exact same properties). You will have to callibrate how you read it, like above finding the sweet spot, first, under each different light you own. 28C does not cause heat stress unless your RH is incredibly low... Maybe, 30% or lower? if not.. heat is not the problem. (vpd, really as both temp and RH need to be considered together and not separately) If RH is low you need a humidifier, not a foliar spray. There's no foliar spray that will fix this without potentially causing many problems for no reason. Simply start providng what the plant needs at the roots will fix the nutrient issue seed in leaves and a better climate will fix any 'heat' issues. Foliar sprays are a good tool to use for very specific contexts - you can provide immediate relief to a deficiency of some molecules, but only if it can traverse the stomata -- not all nutes have that ability. Constantly spraying your plant is not a good habit to form. It becomes more about an individuals OCD not related to actual cause and effect and simply not needed. They feel like they are adding their own 'special sauce' bur really more akin to onanism. it only adds risks when done without a good reason. leaf coat is just a horticultural oil or similar and it's going to plug up your stomata... LOL says so right in their own description in not so many words. Stomata are how the plant gets it's carbon from the atmosphere (transpiration) as well as respiration at night. It is essential. You should not apply things that will purposefully block up your stomata, LOL.. that is retarded. AN is a shit-ass company, lol. BWahaha coating a leaf with latex.. do you want smaller buds? use this product. BWAHAHA This company likes to exploit people's OCD and onanism, lol. It's not about reality.. it's about some skewed idea, correlation or relationship that they then bastardize and sell as a product that doesn't really do anything useful. They like to split up fertilizr that can be packaged together just so you have to buy more bottles of it. they instruct too high of doses so that you run out sooner. this company is trash-dick city. They are predators with limited understanding of fertilization. Bet they don't employ one chemist or plant biologist, lol. If they do, they have whored themselves out to business interests. you look to be in hydro/soilless... your rockwool cube seems to have black mold on it? This is a nutrienet imbalance and rootzone issue i bet. Usually burnt tips around outside relates to low potassium, but could also be too high of phosphorous, too. If you work out PPMs of your fertilizer (in total, there are online calculators to helo) and assuming no major ratio imbalances causing lockout, P should be around 45-70ppm and K 180-210ppm give or take. These ranges shouldn't cause too many issues unless something else is out of balance causing lockout or a stimulative effect ("mulder's chart").