This article is from the Ozone Depletion: UV Radiation and its Effects FAQ, by Robert Parson firstname.lastname@example.org with numerous contributions by others.
Generally (though not exclusively) harmful, but hard to quantify.
Many experiments have studied the response of plants to UV-B radiation,
either by irradiating the plants directly or by filtering out some
of the UV in a low-latitude environment where it is naturally high.
The artificial UV sources do not have the same spectrum as solar
radiation, however, while the filtering experiments do not
necessarily isolate all of the variables, even when climate
and humidity are controlled by growing the plants in a greenhouse.
Out of some 200 agricultural plants tested, more than half show
sensitivity to UV-B increases. The measured effects vary markedly
from one species to another; some adapt very readily while others are
seriously damaged. Even within species there are marked differences;
for example, one soybean variety showed a 25% growth reduction under a
simulated ozone depletion of 16%, whereas another variety showed no
significant yield reduction. The general sense seems to be that
ozone depletion amounting to 10% or more could seriously affect
agriculture. Smaller depletions could have a severe impact on local
ecosystems, but very little is known about this at present.
I have not investigated the literature on this in detail, not
being a biologist. Interested readers should consult [Tevini and
Teramura], [Bornman and Teramura], or the book by [Tevini] and
the references therein. If any botanist out there would like to write
a summary for this FAQ, please let me know.