A new range of plant watering tools is being sold in the UK.
It claims to increase plant growth and nutrient uptake by passing water through a 'nano-bubble oxygenating chamber'. See
www.purerain.co.uk
What do you think?

Mike/Citrange

Nanobubbles??? As someone who knows a little about nanotechnology, I would say their description sounds awfully like marketing hype...probably 5% actual technology and 95% hype. Having said that hydroponics system a can have water aerators to prevent the water the systems use from becoming stagnant. You don't need nanobubbles to aerate water.

I completely agree that using the word 'nanobubbles' is just jumping on the nano-technology bandwagon.
But I do water all my potted citrus with rainwater that has been stored - on average probably several weeks. After that runs dry in summer, it's mains water. So is this water severely lacking oxygen? Would mixing in air bubbles make any difference to my plants?

I believe rain water 'catches' nitrogen not oxygen when it rains. The nitrogen after a rain is what I have always though made everything green up so pretty. Snow does the same thing for the spring in areas that get snow in the spring and winter. The air we breath is about 78% nitrogen. Tom

They should have use aeration instead of oxygenation. Nitrogen fixation in rainfall is normally due to lightning fusing N(2) and O(2) to create nitrates which the rainfall picks up. Atmospheric nitrogen is in the form of N(2), and is not readily usable by plants. I seriously doubt that they have a spark in their gadgets to create a usable form.

Great point !

I think you're wrong about oxygen/nitrogen in water.
Oxygen from the air does dissolve in water, nitrogen does not - unless, as you say, it has somehow been converted into nitrate form. The amount of oxygen that can be dissolved in water reduces with rising temperatures. So perhaps somewhere between 50F and 90F there is an optimum for best root growth with maximum dissolved oxygen?
Now I've been reading a bit on the subject, I've seen articles recommending adding hydrogen peroxide to your container watering. Mostly for cannabis growers, but presumably relevant to citrus too. See
http://www.quickgrow.com/gardening_articles/hydrogen_peroxide_horticulture.html
Anyone got any strong feelings about this???

Air is composed of several gasses (roughly 78% nitrogen, 21% oxygen, and traces of other gasses such as argon and carbon dioxide).

All of these gasses dissolve in water and the maximum solubility of each is temperature dependent. The nitrogen in air is a diatomic gas composed of two atoms of nitrogen and is for all intents and purposes useless to plants. Nitrogen gas becomes available to plants after it is processed by nitrogen fixing organisms such as nitrogen fixing bacteria in the soil, which convert nitrogen gas to ammonia. Plants directly use the oxygen (again a diatomic gas) and carbon dioxide in air for cellular respiration and carbon fixation, respectively.

Rainwater may contain some low concentration of nitrates (most likely in the form of nitric acid)... think acid rain. The source of this nitrogen is either electrical discharge from lightening (as babranch mentioned) or from pollution (e.g. NOx gasses).

These purerain devices surely do not add any more oxygen to the water than could be accomplished by simple agitation of the water.

The claims of hydrogen peroxide I believe are valid on increasing root growth by oxygenating the soil and oxidizing (killing) soil pathogens. I don't see the purerain tools doing this. Hydrogen peroxide works so well because of how unstable it is. When applied, it quickly oxidizes due to the atoms needing to lose the extra negative charge. Most commercial products sold for this purpose are actually peracetic acid, which is a combination of hydrogen peroxide and acetic acid. They are typically around 27% hydrogen peroxide while store shelf hydrogen peroxide is only 3%.

Scott_6B is right about the bacteria fixing nitrogen. But there are several different types of bacteria that do different things with nitrogen. One type (nitrogen fixing) fixes atmospheric nitrogen into ammoniacal, one (nitrifying) changes ammoniacal into nitrite, one (nitrifying) turns nitrite into nitrate, and the last (denitrifying) changes nitrate into diatomic nitrogen gas. Beside denitrification, some forms like ammoniacal, can be lost through volatilization.

The problems that I have with the purerain are the claims that it increases plant growth with "nanobubble technology". If it isn't adding more nitrogen into the soil then how is extra air making a significant difference? How would it be any different than what the water breaker at the end of the wand is doing?