Birch cuttings (Betula pendula Roth) were grown
in a sand-culture system with two concentrations (0·05, HF and
0·005%, LF) of fertilizer containing macronutrients and
micronutrients, and were exposed to
90/40 nl l−1 O3 (day/night) and
<3 nl l−1 O3 (control) for one
growing season in the field fumigation chambers at Birmensdorf
(Switzerland). Leaves of different ages were analysed for gas exchange,
contents of chlorophyll, protein, and for
metabolites as well as enzyme activities of carbohydrate metabolism.
Ozone reduced net photosynthesis and chlorophyll contents in mature
leaves of both fertilization treatments,
whereas that of protein was only reduced in high-fertilized plants
(HF). However, net photosynthesis, chlorophyll,
and protein increased in young leaves of low-fertilized plants (LF).
The effects of ozone on enzyme activities of
carbohydrate metabolism were most pronounced in leaves of LF plants.
Specific activities of the sucrose-cleaving
enzymes, sucrose synthase and alkaline invertase, were induced, whereas
acid invertase was unchanged.
Extractable activity of sucrose phosphate synthase, which is a key
enzyme of sucrose synthesis, was reduced.
Levels of fructose 2,6-bisphosphate, an inhibitor of sucrose synthesis,
were increased in leaves of O3/LF plants,
but reduced in O3/HF plants. In addition, activities of
enzymes involved in starch metabolism, ADP-glucose
pyrophosphorylase and starch phosphorylase, were lowered in
ozone-treated samples and the ratio of ATP[ratio ]ADP was increased.
It is concluded that chronic ozone exposure leads to an inhibition
of sucrose synthesis and favours sucrose
degradation. This effect is modulated by the nutrient status of the
plants, indicating higher O3 tolerance in HF
plants. Furthermore, as the metabolic responses in the ozone-treated
samples resemble very closely those observed
under end-product inhibition of photosynthesis, we assume that the
O3 effect is mainly due to reduced