Can the hotness of the chilli be enhanced by nurture?

Found a wiki page that might help you with this.
http://en.wikipedia.org/wiki/Chili_pepper
Reading the intensity values is eye opening!! HOT!!

Hi UKSlim. I planted my chili plant out in the garden in May and it has been happy there until the recent frosts (I harvested a while ago though). The fruit are small and have quite a kick to them but I did nothing to them and suspect that the heat is down to variety.

Heat is pretty much down to variety. Smaller fruiting varieties tend to be hotter. Hungarian Hotwax, for instance, despite the name, has large fairly mild chillies, and Jalapenos are relatively mild too. We have some I'm going to plant in the New Year that have seeds you have to handle wearing rubber gloves - can't remember the name though, I'll check. They should be ripe by the time you come to visit!!

I have read somewhere that if you give the plant some stress ie dont water it in the greenhouse, and give it extra heat that this will increase the hotness of the chilli. I also understand that the heat comes from the seeds, which we often dont eat.

Try and get plants from a nursery rather than garden centre - you will get more info and more choice. There's the South Devon Chili Farm down this way - i don't know where you are but they might post. I've had some great - and hot ones from them! Try Aji Lemon, bright yellow - devilishly hot - but calms down once its pickled. Or Twilight - a lovely small bushy plant with fruits in red, orange and purple, tiny but very powerful, this is an easy one to overwinter, keep above 5 degrees, and give it a good prune. Good luck!

You cannot change the "heat" of your chillis. The "heat" comes, not from the seeds, but from the "juice", which is called capsaicin. The hotness of the capsaicin varies from variety to variety, and is measured in Scoville Heat Units (SHUs).

When my chillies turn red I leave them on the plant for a few more weeks. I found this definately helps.
Probably allows the fruit to mature.
p.s. I'm no expert. I'm more of a "nearly novice".
I like chillies, however just the smell of a meal with some chilli in it can have me brake out in a sweat! When I eat a mild chilli I can reproduce a shroud of Turin facemask several times throughout the meal.

Have a good grow,
Singeing Tongue.

The range of heat achievable by a given pepper is largely determined by its variety. Consider though, that as with a lot of genetic factors, whether a given plant or fruit is at the high end or low end of a given range can and does vary with nurture. Typically longer and hotter days during growing will make peppers hotter. The same variety grown in Arizona for example, will be a bit hotter than it would be if it were grown in Minnesota (and the latter is not out of reach if you start indoors early). I have also discovered that depriving your plants of water for a few days prior to harvesting will kick up the heat a notch over the same fruit saturated shortly before picking - this is probably related to dilution from hydration. That said, a weak cayenne will still almost always be hotter than a strong jalapeno...Although some varieties overlap in potential scovilles.