Daily time course of whole-shoot gas exchange rates in two drought-exposedMediterranean shrubs

Effects of drought on water relations, whole-shoot gas-exchange characteris tics, and pigment and zeatin concentrations were investigated in the Medite rranean shrubs rosemary (Rosmarinus officinalis L.) and lavender (Lavandula stoechas L.). Two-year-old, greenhouse-grown plants were placed in a whole -shoot gas-exchange measurement system and subjected to 10 days of drought, resulting in severe water stress, and then re-watered for 5 days in order to study their recovery. Water stress resulted in a significant decline in maximum whole-shoot not CO2 assimilation rates (A(n)) for both species that was associated with reductions in leaf area and stomatal conductance. Beca use shoot dark respiration rate (R-d) was less sensitive to water stress th an A(n), shoot R-d/A(n) ratio increased from about 15 to 95% during water s tress. No major changes in chlorophyll and carotenoid concentrations of ros emary leaves were observed during the experiments, but chlorophyll and caro tenoid concentrations fell significantly in water-stressed lavender leaves. Zeatin concentrations were higher in rosemary leaves than in lavender leav es during water stress. After re-watering, whole-shoot A(n) and R-d rapidly recovered to their pre-drought rates.