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 CO2 efflux rate (F-CO2) and xylem CO2 concentration ([CO*(2)]). Before and after the drought treatment, diurnal fluctuations in Fco, and [CO*(2)] corresponded well with variations in stem temperature (T,,,). 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, 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 FCO2 and [CO*(2)] disappeared and diurnal fluctuations in F-CO2 and [CO*(2)] corresponded again with variations in T-st

Measurement and simulation of the ventilation rates in a naturally ventilated Azrom type greenhouse in Zimbabwe

A simple greenhouse ventilation model, based on the stack and wind effects (the main driving forces for natural ventilation) was adapted, calibrated and validated using measured air renewal rates in a three span naturally ventilated Azrom type greenhouse in Zimbabwe. Crop transpiration rates were monitored using stem heat balance sap flow gauges installed on the main stems of rose plants to continuously monitor whole-plant transpiration (WPT). This allowed continuous and automatic determination of full scale air renewal and leakage rates using the water vapour balance method. The model was fitted to experimental data of ventilation rates, and discharge and wind effect coefficients were determined. The results show a good fit between measured and predicted values (R2 = 0.80 and 0.82 for winter and summer, respectively), although there is a general over-estimation of the greenhouse air renewal rates, particularly during the night. The model, nevertheless, adequately describes the natural ventilation process in the greenhouse all year round. The model can be used as a design tool to evaluate and optimise the effects of different ventilation configurations and strategies on greenhouse air renewal rates, and as a component in a greenhouse climate model in order to further evaluate the effects of ventilation strategies on the inside greenhouse and crop microclimate, and thus lead to better greenhouse climate control

WATER USE IN OLIVE ORCHARDS ESTIMATED BY PHYSIOLOGIC AND CLIMATIC METHODS IN TUNISIA

This experimental study conducted on irrigated olive trees (cv. Meski) in arid climate in Enfidha-Tunisia. This study aims to estimate the water needs of the Olive Trees for a possible optimization of irrigation.To do this, a calculation of water requirements was performed by physiologic and climatic methods and compared to traditional irrigation practiced by farmer (T0). The physiologic method is based on the xylem sap flow (T1) and the climatic method is based on the reference evapotranspiration (T2).Results showed that the values of the highest transpiration are recorded under 60% ET0 (T2).Under this treatment, the average of transpiration was about 50% of the applied irrigation dose, with soil evaporation average about 15%. These results allowed us to conclude that the physiological method (T1) allows a better estimate of water needs. This method also improves the profitability of the olive tree with a better optimization of water use (save 35% of water)