Crop yield response to water - Cra

ox 10 (CONTINUED)The major limitation of using SWP for irrigation management is its labor requirement, alimitation that greatly increases for the predawn measurements, because the measurementsrequire trips to the fields, and they cannot be automated. Alternative techniques to thepressure chamber have been proposed, such as measuring the diurnal trunk diameter changeswith dendrometers. This technique provides continuous records automatically and it has beentested in a few species, although it is not widely used outside research activities. Anothershortcoming of plant-based scheduling approaches is that they do not provide quantitativeinformation on how much water should be applied, as the soil water monitoring and thewater budget methods offer. Research information on SWP of the various tree and vinespecies is available to diagnose the relative level of water status in an orchard, and this is oneof the most useful applications of SWP measurements. Box 11 provides indicative values forsome tree species and further details are given in the specific crop chapters. Both evaporativedemand and time of the season may affect the reference SWP values.Canopy temperatureTree canopy temperature is another indicator of water stress. The underlying mechanism isthat the closure of stomata induced by water deficits causes an increase in canopy temperaturebecause of a lower transpirational cooling. On a clear day, plant canopies well supplied withwater are cooler than the surrounding air by several degrees. As water supply is restrictedand water stress is imposed, the canopies warm up as transpiration is reduced, and theirtemperatures can be equal or higher than that of the air. In a given environment, canopytemperature increases with the severity of water stress. The development of the infraredthermometer (IRT) made it possible to measure canopy temperature remotely, withoutphysical contact with the plant and this made it possible to use canopy temperatures forirrigation management. As an example, the difference between canopy and air temperaturemay range from -3 ºC when canopies are well watered to +3 ºC under significant stress, whichoffers a wide window to detect stress.Yield Response to Water of Fruit Trees and Vines 275