The effect of vapor pressure deficit on leaf area and water transport in flower stems of soil-less culture rose

The effect of indoor air relative humidity on leaf area and water transport of flower stems in a greenhouse rose crop (Rosa X hybrida cv Mercedes) grown on a soil-less substrate was studied on 2-year-old plants, in a freely draining irrigated system ensuring a high leaching fraction. In one compartment of the greenhouse, the roof opened when the air temperature reached 28 °C. In the other, an evaporative wet pad and fans were operated at 28 °C.The wet pad treatment decreased vapor pressure deficit (VPD). A maximum VPD difference of 1.45 kPa between the two compartments occurred during the noon-hours on a warm day with high atmospheric evaporative demand. On days with moderate evaporative demand, the wet pad was either not operated or when operated, produced VPD differences smaller than 1.45 kPa. Wet pad treatment decreased the transpiration rate per unit leaf area of the flower stem. On a typical summer day, with high evaporative demand, mean maximum water loss per unit leaf area was 2.63 ± 0.13 and 1.79 ± 0.09 kg m−2 day−1 for the high and low VPD compartments, respectively. However, low VPD decreased only slightly mean daily water flow per stem: 104.3 ± 6.9 g at low VPD versus 112.4 ± 8.7 g at high VPD (not significant at p > 0.05), despite the significant differences (p > 0.05) in VPD between the greenhouse compartments. The wet pad treatment mitigated leaf water potential drop at noon-time. The results suggest that rose flower stems adapt to high VPD by decreasing leaf area for maintaining high sap flow rate per unit area.