Rhythmic changes in water uptake, transpiration and water potential of cutroses as affected by photoperiods

Freshly harvested 'Bridal Pink' roses (Rosa hybrida Hort.) were held in dei onized water and placed in a room at 20 degrees C under continuous dark, li ght, or a 12-hr photoperiod. Cut flower weight, water uptake, transpiration , petal water potential, and bud opening stage were measured at 4-hr interv als. Under continuous dark conditions, water uptake and transpiration rates were slow, ranging around 1 g . 100 gfw(-1) . hr(-1). Because transpiration did not exceed the water uptake throughout the three-day experimental period, a steady increase in fresh weight resulted. Petal water potential ranged ar ound -0.2 MPa, decreased slightly (becoming more negative) after 7:00. Unde r continuous light, water uptake and transpiration rates changed cyclically with increases between 7:00 and 19:00. Transpiration rate began to exceed the water uptake at 7:00 similar to 11:00 on Day 3; concurrently, fresh wei ght, which had been increasing began to decrease. Petal water potential dec reased between 7:00 and 19:00 and increased from 19:00. But the tendency fo r water potential was to gradually decline with time. Although bud opening stage was advanced early in the morning under both dark and light condition s, it proceeded faster under latter. Diurnal periodicity was detected in so me parameters obtained under the light conditions by using Fourier analysis . Fluctuations in the water uptake and transpiration rates closely correspond ed to diurnal periodicity under a 12-hr photoperiod, being elevated in the light and depressed in the dark. These diurnal changes in water uptake and transpiration were greatest during the third and fourth light periods. Tran spiration exceeded water uptake in the fourth light period and thereafter. Thus, fresh weight initially increased but decreased in the light and incre ased again in the dark. But the tendency for fresh weight was to gradually decline with time. Petal water potential decreased in the light and increas ed in the dark, but the recovery in the dark became insufficient so that th e petals finally wilted completely. Bud opening stage advanced immediately after the beginning of light periods except for Day 1 and 2 during which it lagged.