Effect of foehn on nighttime sap flow of soybean

Abstract

Foehn winds cause damage known as white head injury to paddy rice; and this damage is more severe at night than during the day. However, little has been reported about the nighttime effects of the foehn on other crops. We investigated the foehn's effects on soybean plants, and compared day and nighttime responses. We measured sap flow, which is assumed to equal transpiration, by the heat balance method. The gauge conductance of sap flow sensors during the nighttime foehn was calibrated against values obtained in the pre or post-foehn period. Stomatal conductance was calculated using sap flow data and meteorological data. At night, sap flow during foehns was greater than that in non-foehn conditions. During the foehns in 2007 and 2008, average nighttime sap flows were 4.3 g h-1 (2007) and 4.0 g h-1 (2008). The cumulative sap flow through the night during the foehns in 2007 and 2008 were 10.5% and 7.0% of their respective daytime values. In 2007, sap flow increased linearly with increasing vapor pressure deficit (Da) when Da exceeded 0.7 kPa. In 2008, however, sap flow increased rapidly when Da exceeded 0.8 kPa. These different responses of sap flow to Da in 2007 and 2008 were attributed to differences in regulation of stomatal conductance, which was low in 2007 but varied markedly in 2008. The reason for these differences may be the different soil moisture conditions in 2007 and 2008. © 2010, The Society of Agricultural Meteorology of Japan. All rights reserved.