Relationship between daily intercepted radiation and tree transpiration in pear

Intercepted radiation can be used as a basis for scheduling irrigation in some fruit tree species. Daily transpiration has been linearly related to daily intercepted radiation in apple canopies. Their ratio, termed transpiratory radiation use efficiency (TRUE), was relatively constant before harvest. Therefore, intercepted radiation and a constant TRUE can be used to estimate the amount of water required for irrigation in apple trees before harvest, serving as a basis for automating irrigation with light sensors. However, it is not known yet whether TRUE is also constant for other fruit tree species. In the case of pear, whole tree transpiration data are scarce, and there are not many works that point out a constant TRUE for this species. To test this hypothesis, we measured whole-day values of intercepted radiation and transpiration, excluding soil evaporation, in 'Conference' pear trees in a large weighing lysimeter 13 days over the growing season under Mediterranean conditions. Our work showed that TRUE was not constant within the season, although transpiration correlated linearly with intercepted radiation, as was observed in apple and in a pear study in Australia. However, our linear relationship did not have a zero intercept, implying that TRUE, which is the slope of this relationship, cannot be constant. Indeed, TRUE increased from spring to midsummer and decreased afterwards, following a pattern similar to intercepted radiation. In conclusion, intercepted radiation and a constant TRUE alone are not considered adequate as a basis for irrigation scheduling in 'Conference' pears grown under Mediterranean conditions. Instead, the linear relationship between daily transpiration and intercepted radiation can be used, provided the non-zero intercept of the relationship is considered.