Small hydrostatic pressures overcome the occlusion by air emboli in cut rose stems

Cut flowering rose shoots (Rosa hybrida L., cv. Sweet Promise) were dehydrated in air for 24 h (at 20 °C and 60 % relative humidity) and then placed in vases in which the water column was held at 3, 7, 10, or 18 cm. The rate of bud opening was positively correlated with column height. The rate of water uptake was much lower in stems placed at 3 cm than in those placed at 18 cm, showing that a small difference in hydrostatic pressure (1.5 kPa) was able to overcome an inhibition of water uptake. Sealing the stem surface with Parafilm and laboratory grease did not change the response. At 18 cm column height, therefore, the water did not enter the stem through the surface, e.g. the scars of removed leaves and thorns. Stems that were air-dehydrated only for 30 min, after which all xylem conduits opened by cutting are filled with air, had a high rate of water uptake, irrespective of column height. In contrast to 24 h of dehydration, Sonia stems that had been dehydrated for only 30 min showed high water ascent in the xylem conduits opened by cutting, and virtual absence of xylem cavitation. The effect of small hydrostatic pressures may, therefore, be accounted for by higher water ascent in xylem conduits opened by cutting, and/or refilling of cavitated conduits.