Three-year-old cloned saplings of Cryptomeria japonica D. Don were grown in a growth cabinet that allowed automatic control of temperature, relative humidity, and light conditions, and were irrigated once daily. The lighting period in the growth cabinet was controlled, with 14 h of light and 10 h of darkness (lights on at 08:00 and lights off at 22:00). This study investigated the relationship between sap flow rates and swelling and shrinkage of the stems of the cloned saplings. The improved stem heat-balance method and a strain gauge were used to measure swelling and shrinkage.
The sap flow rate reacted to light and darkness immediately, increasing and decreasing just after lights-on and lights-off, respectively. The tangential strain on the surface of the inner bark exhibited a reaction that followed but opposed the reaction of the sap flow to lighting conditions.
Based on the diurnal changes, sap flow rate rates over 24 hours could be divided into four periods: Zone A_1 began with lights-on, when the sap flow rate increased, and lasted about 2 hours. In the following zone, A_2, the sap flow rate remained almost constant at 1.3 g/min for about 10 hours, and then declined for about 2 hours as lights out approached. In Zone B, the first dark period, the sap flow declined, quickly at first and then more slowly, for about 4 hours, until the start of the second dark period, Zone C, when the the sap flow rate became almost constant at 0.05 g/min and remained so for about six hours.
As noted above, close correlations were observed between sap flow rates and tangential strain in all periods. The first derivative of each sap flow rate and the corresponding tangential strain were calculated, and the results indicated a negative correlation between the two variables in all periods.
In particular, the relationship between the first derivative values exhibited a highly negative correlation in Zones A_1 and B, expressed as a primary formula. Sap flow was found to continue for some time after lights-out, and this compensated for reduced evaporative effects, albeit at a slow rate, over 4 hours.
The total sap flow rates during both the light and dark periods were measured. The total sap flow rate in the dark was only about 10% of that in the light, disregarding transpiration in the dark for convenience. Thus, the total sap flow rate responsible for swelling of the stem was about 10% of that consumed in evaporation during the light period.