Torn about root zone temperatures.

So I bought a chiller for my RDWC the other day. Everything looking great got temp at 68F. Only thing is, despite what everyone says about lower root temps being better (definitely for pathogen/ oxygen), every research topic I could find on various types of plants including tomato suggest that anything less then 72F is quite detrimental to growth and carbon assimilation. All the studies I could find point to reduced yield/ shoot growth with anything less then 24 C (about 75F). Thing is, there are variables like light intensity. It does seem like with higher light / closer to plants, lower temperatures are more ideal since in order to utilize all the light the plants must take in CO2 faster which is easier done with lower temperatures. Another thing I was wondering is if temperature is strain specific. I mean a Columbian sativa probably evolved with higher root temperatures then a high altitude mountain ruderalis. I guess what I really want to know is what temperature has everyone generally found to work best for a high light, HID setup? Has there ever been a situation in which you felt root zone temperature would have been better warmer?

Some studies:
"For the short-term (48 h) evaluation of different soiltemperatures, temperatures within a 5 °C range of ambient
soil temperature (25 °C) resulted in optimal CO assimilation
rates at either CO concentration (Fig. 1). Temperatures
which varied by 10° above or below a 25 °C soil temperature
resulted in significant reductions in CO assimilation. For
all soil temperatures tested, a temperature of 30 °C resulted
in maximum values of assimilation.
http://aob.oxfordjournals.org/content/81/6/717.full.pdf


"Tomato seedlings (Lycopersicon esculentum Mill. cv Vendor) were grown hydroponically with their root systems maintained at a constant temperature for a 2-week period commencing with the appearance of the first true leaf. Based on fresh and dry weight and leaf area, the optimal root-zone temperature for seedling growth was 30°C."
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1066404/'


Da Fuq?

The best dwc grower DD has the res at 64 ,has no pathogen issues and here is what he gets--

Seems like you know why a hot root zone works cause you read about it.. but you don't know how to hit those levels safely in your rez without breeding bad stuff. first thing to remember is its not the hot rootzone that increases growth but a colder canopy and hot root zone. Plants see no real growth increase if the roots are 73 degrees and the canopy is 73. so properly chilling you're room and heating your roots is important. Unfortunately roots need oxygen as well and heating your rez water decreases the ability to hold oxygen. So its not so much about the root slim but a combination of dissolved oxygen problems and root slim choking even more oxygen from plants. You could try a biobucket or a germicidal lamp with frequent water changes but that still doesn't remove the problems of oxygen deprivation. You could trying flooding the rez like a flood and drain system with easy drying media like pea rocks while putting bubbling airstones at the bottom to create a dwc/flood drain system with germicidal lamps cleaning the solution in between floods and the rocks can be used as a thermal heat sink to keep root zone warm..

But why make things harder than they have to be get a big pot and wrap a heat blanket or something around it using the soil as heat sink and water normally while keeping room temps optimal ... so much easier

Thanks for the info. Those buds look amazing. I cranked my reservoir temp down to 64. Their roots look like they're loving it already.

is it going to matter in soil the temps you water with?