A study was conducted at the Embrapa Semi-Árido, Petrolina-PE, Brazil, with the aim of understanding the mechanism by which the umbu tree (Spondias tuberosa Arr. Cam.) maintains its diurnal internal water balance in dry and wet seasons. The results obtained were based on the measuring of leaf water potential and its components using the pressure chamber and hygrometric chambers / microvoltmeter. Under dry conditions, the lowest values on water potential and osmotic potential were observed around 8:00 h, reaching, respectively, --0. 97 MPa and --1.17 MPa, resulting a turgor pressure of 0.2 MPa. The lowest turgor pressure occurred at 16:00 h when 0.16 MPa was detected but full recovery was not observed by the end of the light period. During the raining season, however, the lowest water potential was --1.55 MPa at 14:00h and -1.57 MPa at 14:00 h giving a turgor pressure of 0.02 MPa. Recovery was fast and, by the end of the day, plant water status was similar to the value observed at predawn. These results suggest the umbu tree presents two strategies for maintaining a favorable internal water balance, regarding the environmental situations studied. Under dry conditions the diurnal balance would be maintained at the expenses of water stored in the tubers and by restricted transpiration. During the rainy season, the diurnal water balance may have been mediated by a short term osmotic adjustment as judged by the observed afternoon range between water potential and osmotic potential.
water potential; osmotic potential; turgor pressure; osmotic adjustment; Spondias tuberosa
