Drought and the diurnal patterns of stem CO2 efflux and xylem CO2 concentration in young oak (Quercus robur).

A young potted oak (Quercus robur L.) tree was subjected to drought by interrupting the water supply for 9 days. The tree was placed in a growth chamber in which daily patterns of temperature and radiation were constant. The effects of drought on the water and carbon status of the stem were examined by measuring stem sap flow rate, stem water potential, stem diameter variations, stem CO(2) efflux rate (F(CO2)) and xylem CO(2) concentration ([CO(2)]). Before and after the drought treatment, diurnal fluctuations in F(CO2) and [CO(2)] corresponded well with variations in stem temperature (T(st)). Daytime depressions in F(CO2) did not occur. During the drought treatment, F(CO2) still responded to stepwise changes in temperature, but diurnal fluctuations in F(CO2) were no longer correlated with diurnal fluctuations in T(st). From the moment daily growth rate of the stem became zero, diurnal fluctuations in F(CO2) became closely correlated with diameter variations, exhibiting clear daytime depressions. The depressions in F(CO2) were likely the result of a reduction in metabolic activity caused by the lowered daytime stem water status. Xylem [CO(2)] showed clear daytime depressions in response to drought. When the tree was re-watered, F(CO2) and [CO(2)] exhibited sharp increases, coinciding with an increase in stem diameter. After resumption of the water supply, daytime depressions in F(CO2) and [CO(2)] disappeared and diurnal fluctuations in F(CO2) and [CO(2)] corresponded again with variations in T(st).