Experimental open-field day-length-extension method and estimation of the effective light period using solar altitude

We developed an open-field day-length-extension method for estimating the effect of a temperature increase on crop growth. We chose a pair of experimental sites (Sapporo and Morioka) with the necessary difference in air temperature. To estimate the temperature effect alone, day length at the southern site was extended to equal that at the northern site by supplemental light treatment. In the day-length-extension treatment, the start time of the lighting at dawn and the end time at dusk were set on the basis of the threshold light intensity at the northern site (the threshold light intensity at which the crop senses light and shows a photoperiodic response). We used the threshold solar altitude corresponding to the threshold light intensity to determine the start and end times of supplemental lighting. The threshold solar altitude, however, depends on weather conditions. To cancel the effect of weather, we used the threshold solar altitude of a fine day even on cloudy or rainy days. Measurement of photon flux density at the two sites showed that the difference in the threshold solar altitude on a fine day between the sites was small, suggesting that the same threshold value can be widely used.