Re: Citrus Trees

Dear Enrico -

To start with, I will answer the part of your question to which I don't have
a good answer.  That is, why isn't your citrus tree attacked by pests when 
you put it outside?  While I don't really know the answer to this question,
I will give a guess that you are right - that pests that usually attack 
citrus plants aren't found in Canada because Canada is quite outside the 
range of plants in the citrus family (although the citrus family is rather
widespread, it is mainly found in tropical and subtropical areas of the 
world).  Plants in the citrus family produce a variety of substances that
make them unattractive to herbivores.  Many produce calcium oxalate crystals
in their leaves, which make them cut into the flesh of animals that might
want to chew on them.  Most produce not only terpenoid substances, but also
alkaloids, coumarins, and phenolic compounds, none of which are very
palatable.  Probably it takes a very specific sort of herbivore to get past
all these barriers.

Now to move on to questions I can actually answer:  What do plants in the 
citrus genus have in common?  The genus _Citrus_ is by far the most 
economically important and well known genus in the family Rutaceae.  It
is composed of 60 known species, most of which are cultivated.  The trait
that unites the genus is its fruit, which is called a hesperidium.  A very
commonly known hesperidium is an orange.  It comes from a superior ovary, 
has a leathery rind that contains aromatic oil glands, and a placenta of
enlarged, juice filled cells.  This sort of fruit is found only in the
citrus genus.

The lemon smell that you get from the leaves is produced by specialized cells
that secrete ethereal oils.  These cells are found not only on the leaves, 
but also on the fruit.  Sometime when you're eating an orange, notice the
pores on the outside of the skin.  Squeeze the skin a little and juice 
should squirt from the pores.  Now take a leaf off your citrus plant and
hold it up to the light.  You should see small translucent spots all over
the leaf.  These are the leafy version of the pores on the skin of the fruit.
If your tree should ever flower, you will notice that the flowers smell
a lot like the leaves, only much more intensely.  On some plants, even the
petals bear these specialized cells.

I hope this answers your questions.  Sorry I couldn't answer your first
question more specifically.  Perhaps if you resubmit your question, you
will be able to find someone more well versed in plant pathology than I.

Pam VanderWiel