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Permanent wilting point
Permanent wilting point (PWP) or wilting point (WP) is defined as the minimal point of soil moisture the plant requires not to wilt. If moisture decreases to this or any lower point a plant wilts and can no longer recover its turgidity when placed in a saturated atmosphere for 12 hours. The physical definition of the wilting point is defined as the water content at −1500 J/kg (or −15 bars) of suction pressure, or negative hydraulic head.

Field determination of PWP
Materials and Methods
Results from the 1997 study were very similar to the results found in the 1996 study (Norton and Silvertooth, 1997). In order to accomplish the stated objectives two separate studies were established, one at the University of Arizona Maricopa Agricultural Center on a Casa Grande sandy loam and the other was at the University of Arizona Marana Agricultural Center on a Pima clay loam. Four blocks of 12, 40 inch rows extending the full length of the irrigation run (approximately 600 feet) were established at each location. Deltapine 33B was planted on 19 March and 17 April at Maricopa and Marana respectively. Two neutron probe access tubes were placed down the center of each block spaced approximately 200 feet apart at both locations. Neutron probe readings were conducted before and after each irrigation. Irrigations were terminated at different stages of growth for each of the four blocks. The first block received only a water-up at irrigation at Maricopa and only a pre-irrigation at Marana. The second block was terminated just prior to first bloom and received no further irrigations. The third block was terminated after peak bloom receiving no more irrigations. The fourth block was taken into cut-out before irrigations were terminated. provides a list of irrigation events for all treatments at each location. PWP was identified visually for each block. PWP is defined by Taiz and Zeiger (1991) as the point at which the plant does not recover overnight from a water stress induced wilt. This was the criteria used to identify PWP. Soil water content was then identified at that point for each block using neutron probe readings.
Results and Discussion
At the Maricopa location, values for volumetric soil water content varied according to stage of growth. Treatment 1, which reached PWP just prior to first bloom still had a high volumetric water content below 2 feet. As treatments 2 and 3 were brought to PWP, more of the soil water below 2 feet was extracted. These results demonstrate the importance of realizing from what depths the crop is extracting water at different stages of the season. Water extraction patterns at Marana differed slightly than those at Maricopa. An increase in depth of extraction was not observed as the crop increased in stage of growth. Several irrigations at Marana did not completely fill the profile. Water penetration during these irrigation events reached to approximately three feet. Since the fourth and fifth feet were very seldom replenished the effect of increasing depth of extraction was not observed. and list the volumetric soil water content associated with the identification of PWP for each of the three treatments at Maricopa and Marana respectively. Also shown in these tables is the calculated PAW based upon FC and PWP for each of the three treatments. The first and most obvious result to notice is the differences in the amount of PAW between Maricopa and Marana. Marana, Pima clay loam soil, has a much higher water holding capacity than the sandy loam at Maricopa. A comment should be made regarding PAW values at lower depths at Marana. It is possible that values for PAW are overestimated due to the extremely low water contents at the fourth and fifth foot depths (particularly treatment 2). If irrigations were being scheduled based on percent depletion of PAW an optimum irrigation regime would call for irrigations more frequently at Maricopa than Marana. This study will be continued at both Maricopa and Marana for the 1998 growing season.