LAUSR

Abstract A common effect of cold temperatures on plants is dehydration. When the temperature decreases below some specific thresholds, olive trees, as other sensitive crops, reduce their leaf water potential and transpiration rates even when soil water content is not limiting. The aim of our trial was to study the relationships between climatic conditions and plant water status, to elucidate the different roles played by soil and air temperature. Olive plants were randomly divided in heated and non-heated soils and plant water status was weekly determined. In heated plants, soil temperature was maintained above 15 °C, while in non-heated soils, plants were maintained in field conditions. The effect of air temperature was determined by analyzing the plant’s water status in heated plants, while the effect of soil temperature was studied through the plant water status differences between heated and non-heated soil treatments. Plant water status in the heated treatment fit into the vapour pressure deficit model, according to previous works conducted in warm conditions. Plant water status differences showed great variations in short periods of time, an unusual event with warm temperatures. In this study, two thresholds of soil temperature were found: above 10 °C, it had no effect on plant water status; between 10 °C and 6.5 °C, there was a linear relationship between plant water status and soil temperature, and below 6.5 °C, wind and relative humidity determined the plant water status. Our hypothesis is that low soil temperature produces a high resistance to water movement, while wind and relative humidity control the desiccation process in the leaf, so the great variations in plant water status are only at the leaf level and can therefore change quickly.