What is the correct adjective suffix form for tropism?

A tropism is a type of biological response where an organism reacts to a particular environmental stimulus. For instance, try searching for gravitropism or phototropism and you'll see what I mean.

I'd like to refer to something as being of a tropical nature, to mean that something has the characteristic of being a tropism. But it doesn't sound well to me, because I'm used to see the word tropical refer to something related to the tropics.

Is tropical the correct syntax for the adjective form of tropism?

Shouldn't it be tropismal instead?

Solution 1:

Trop- basically means "turn", and tropism is presumably named after the way e.g. sunflowers turn towards the sun (they do, don't they?). So the most basic adjective would be tropic, "pertaining to turning". That is my recommendation: it is short, it is a proper translation, it is properly formed, and it is not in use much as an adjective. And it appears to be already used to refer to tropism as a suffix in many words (-tropic).

the flower displayed a tropic response to sunlight

You could use tropical, which would be the same, except that it lacks shortness and it is in use as an adjective meaning something else (the tropics are the circles where the sun "turns" in December and June after having moved away from the equator for a few months, the sun's "location" being understood as that area of the earth's surface that is closest to the sun at mid-day, if I am not mistaken).

Tropismal is possible, and correct, but long. And it is not used in this sense, or at all, so far as I know. The same applies to tropist, another legitimate form.

P.S. I would not recommend gravitropic, because it is a hybrid of Latin (gravis) and Greek (tropeô/trepô). A proper and commonly used Greek suffix meaning "heavy" is bar(y)-, resulting in barotropic, turning towards that which is heavy. However, it appears this word is already in use in a different sense, one related to bar(y)- in the sense of "heavy, pressing upon something", meaning "characterized by or associated with an atmospheric condition in which surfaces of constant pressure coincide with those of constant density". The word geotropic, "earth-turning", as suggested by Sumelic, is a very nice word that is actually so used.

Solution 2:

Searching the Oxford Learner's dictionaries, I found the following words listed:

phototropism:

BrE /ˌfəʊtəʊˈtrəʊpɪzəm/, NAmE /ˌfoʊtoʊˈtroʊpɪzəm/
the action of a plant turning towards or away from light

phototropic:

BrE /ˌfəʊtəʊˈtrɒpɪk/, NAmE /ˌfoʊtoʊˈtrɑːpɪk/
(of a plant or how it grows) turning towards or away from light

There are no entries for "phototropical" or "phototropismal"; it looks like these are not commonly used.

The OED has the following to say about the noun ending -tropism in general:

Pronunciation: Brit. /ˈtrəʊpɪz(ə)m/ , U.S. /ˈtroʊˌpɪzəm/ [...]

1. Chem. Forming nouns denoting types of variation or change in structural or physical properties, as allotropism n., monotropism n., morphotropism n., etc. Usually corresponding to adjectives in -tropic comb. form and nouns in -trope comb. form. Freq. with parallel formations in -tropy comb. form.
2. Chiefly Bot. Forming nouns denoting forms of directional movement or growth in response to a stimulus, as geotropism n., phototropism n., etc. Usually corresponding to adjectives in -tropic comb. form.
3. Pathol. Forming nouns denoting the tendency of a pathogen to infect a specific type of tissue, as neurotropism n., viscerotropism n. at viscero- comb. form 2. Usually corresponding to adjectives in -tropic comb. form.

You can see that for all three listed uses, the corresponding adjectives generally use -tropic.