I was a bit hesitant as to which plant name to put in the title – but to be scientifically correct, the plant(s) discussed here need to be referred to as “Tropaeolum” and not “Nasturtium”. However, to make things confusing, the English common name for Tropaeolum is Nasturtium, but there’s also a Nasturtium genus. As if the English could not have thought of another name…
Scientific name: Tropaeolum (pronounced /trɵˈpiː.ələm/) (genus) 
Common English name: Nasturtium 
Common German name: Kapuzinerkresse
Common French name: Capucine
From my parents’ garden I’m only familiar with T. majus as a decorative climbing/creeping plant. Remarkable are the beautiful flowers that can vary in colour from yellow via orange to deep red on one and the same plant (don’t you wonder how that’s possible??). And the almost round leaves (don’t they remind you of water lily leaves?) are very pretty as well and form a nice contrast to the trumpet-like shaped flowers.
My mother used the T. majus flowers to spice and colour up salads. The flowers have a peppery pungent taste, probably due to the mustard-oil glycosides present throughout (?) the plant . I haven’t tried the leaves so far – which are apparently also edible, both raw and cooked. Almost the entire plant can be consumed: flower buds and unripe seeds can be preserved in vinegar as “capers”. Not to forget the tuber-forming species T. tuberosum, commonly called “mashua”. Its tubers are cultivated in central South-America and the plant grows plentiful even in altitudes of around 3000 und 4000 m AMSL and in marginal soils.
From  I conclude that it would be chemically more correct to speak of the glucosinolates instead of mustard-oil glycosides. 120 different ones are known to occur naturally in plants (are there other important ones that don’t ?). As typical secondary metabolites of almost all plants of the order Brassicales (to which the Tropaeolaceae belong) they are metabolized from amino acids and contain sulfur and nitrogen .
Now even though I unfortunately don’t remember much from my studies of biology, I do remember Professor Leins talking about what happens when you eat raw radish. BTW, he never forgot to point out how well radish goes with a fresh, warm Brezel and a good beer 😉 – which is probably why this story stayed with me…When you bite into your radish or cut it up, you break open cells, tearing apart the order and separate cellular storage places of glucosinolates and the enzyme myrosinase. Myrosinase converts glucosinolates to isothiocyanate, a nitrile, or a thiocyanate with isothiocyanate (mustard oil) being the standard product [4, and literature cited within]. Now the name tells you where the pungent taste comes from!
For the sake of completeness, I’d like to add that is is the specific glucosinolate glucotropaeolin (GTL) that is present in Tropaeolum – whether it is the only one or whether there are further ones that I didn’t find out.
Like many other secondary plant metabolites, also the glucosinolates serve as a plant defense when the plant is being attacked (chewed on…we now know what happens!). More on the role of glucosinolates in insect-plant relationships can be found in a recent review by Hopkins et al., 2009 .
Very vaguely I remember something “chemical” about why we add salt to radish…but can’t remember exactly- but it sure tastes good 🙂
Given the wide (in some parts of the world, e.g. New Zealand also described as invasive) distribution of Tropaeolum today it is noteworthy that they originated from Peru , other sources being more broad and stating the mountainous regions from Mexico to Chile and Argentina . Obviously, it was introduced to Europe, according to [7 and literature stated within] “naturally” – so no famous plant hunter involved.
Last but not least: Medicinal properties
I refrain from saying “medicinal uses” as especially if a plant has been cultivated for a long time it’s very likely that is has been used to treat a lot of illnesses…
– diuretic [8, 9]
– antithrombin activity 
– antibacterial 
– antibiotic 
– used to treat infections of the respiratory tract 
– externally: contact dermatitis 
– mucosal irritations and when used internally, gastrointestinal disorders 
 Podlech, Herbs and Healing Plants of Britain & Europe; Europe, HarperCollins Publishers Ltd, 2008
 Hopkins et al., Annual Review of Entomology, 2009; Vol. 54: 57-83
 Gasparotto et al., J Ethnopharmacol. 2009; 122(3):517-22
 Kleinwächter et al., J Agric Food Chem. 2008;56(23):11165-70
 de Medeiros et al., J Ethnopharmacol. 2000 ;72(1-2):157-65
 Kienholz, Arch Hyg Bakteriol. 1957;141(3):182-97
 Halbeisen, Zentralbl Bakteriol Orig. 1955;164(1-5):220-2
 Schultze, Hippokrates. 1957; 28(8):258-61
 Derrick and Darley, Br J Dermatol. 1997;136(2):290-1.