Sorry, photosynthesis has more in common chemically with respiration than digestion. Plants don't 'shit and piss' their metabolic waste any more than we 'shit and piss' carbon dioxide from our lungs.
Oh looky, a real authority on the subject:
Autotrophic plants
use light energy to make all the organic compounds they
require from carbon dioxide, water, and about 14 mineral nutrients they absorb
from their environment. Therefore, they don't really produce metabolic wastes
to the extent that heterotrophs do. Heterotrophs digest food. Plants do not
need to digest with the exception of a few species of carnivorous plants that
obtain mineral nutrients by digesting insects and small animals.
The term food is not even a term that should be applied to autotrophic plants.
Food is something that is digested and provides energy. Plants are food for
animals and other heterotrophs but they do not produce food for themselves.
Heterotrophs get energy and organic nutrients (amino acids, fatty acids,
carbohydrates, vitamins) from other organisms (food).
Autotrophic plants get
energy from light and synthesize their organic nutrients from carbon dioxide
and water.
Probably the only metabolic waste product of autotrophic plants is oxygen,
which readily diffuses out of the stomata on leaves or lenticels on
photosynthetic stems with secondary growth.
Nonphotosynthetic plant parts and photosynthetic plant parts at night produce
excess carbon dioxide which might be considered a waste product. However,
carbon dioxide is essential to plants for photosynthesis. Overall plants absorb
much more carbon dioxide than they excrete.
Plants do sometimes accumulate mineral nutrients or salts that they put in
their cell's central vacuole. Certain salt-tolerant plants (called halophytes)
sometimes excrete salt onto their leaves.
Plant cytoplasm maintains a low calcium level by transporting calcium into the
central vacuole. The calcium often forms calcium oxalate crystals in the
central vacuole. However, those sharp crystals may function in protecting the
plant from herbivores so they probably aren't waste products.
Plants produce thousands of secondary compounds such as caffeine, latex, and
nicotine, that were once thought to be waste products. However, the current
view is that secondary compounds have functions in the plant. Many secondary
compounds deter herbivores.
David Hershey, Faculty, Botany, NA
http://www.madsci.org/posts/archives/2002-02/1013824497.Bt.r.html