One of my plants died, any ideas why? lotsa pics!

first things first this is my first post here, so hi all
i know the plants i have are more sativa based and most of you go for an indica strain, but i pefer sativa and these are for me.
i also know that the plants are big (5 foot high or so) and most of you go for SOG or SCROG methods, but for me this is a hobby and it amuses me to have big plants

anyway i have 3 flowering plants under a 600W HPS light with heat absorb glass in a garage storeroom.
im living in Aus and its bloody hot this time of year
not using an auto watering pump system, just one of those water itself pots that you can get anywhere with the resovoir in the bottom.
2 parts perlite 1 part vermiculite medium, AAA lush nutes
the light (hot) is 2 feet above the plants so shouldnt be burning them

one of the plants is dying the other 2 arent. Its the one of the far right. sorry the photos are shit, its a very red light that HPS puts out, and a red brick background.

i would love to know why, any suggestions so i dont do it again ?

EDIT : a day after taking the photos and ive stopped watering it cause its well dead now

more pics of how dead it is

i dunno, maybe you gave them too much water, they need less when they are flowering, especially towards the end.. i dunno how far along yours were though.

looks to me like they don't have enough water? They look dry and crisp to me, although the photo quality is limiting my thoughts.

have you tested the ph in the soil?
you can go to the hardware store and get a soil ph tester. HomeTownStores.com - Detail
maybe!!! the ph in the soil was to high/low.

the leaves are dry and crispy for sure
it gets really hot in there and i suspect that plays a part too
ill get a PH tester when i get a chance and give that a go : its something i didnt think of

Pics are too crappy to really tell. Prob burned them with too high temps from light!!