Approach towards the control of rose flower opening by light environment

The opening of a flower is a phenomenon where sugars and water accumulate in the petals (a sink organ), causing the petal cells to swell dramatically in response. Cell division in the rose petals ends early, during the bud stage, and any later growth is caused by hypertrophy of individual petal cells.4–6 Rose petals are also hypothesized to show differences in cellular structure and mechanical strength which lead to different growth rates in adaxial and abaxial epithelial cells, causing the petals to bend outward.7,8 In addition, cellular hypertrophy is thought to occur through the accumulation of sugars in vacuoles within the cell, which raises the osmotic pressure inside the cell relative to outside, and through increased distensibility of the cell wall.9–11 Together, these changes cause water to flow into the cell. Even in cut flowers, there is a correlation between ease of opening after harvest and the soluble sugar content of the bud: varieties with high sugar content at the bud stage open well after cutting even without sugar supplementation, while those with low sugar content are arrested in a partially open state unless they are given sugar.12,13 This example clearly shows that sugars have a large impact on flower opening. Previous research in flower petals has shown that the sugar-metabolizing invertase enzymes are involved in sugar accumulation in vacuoles,14 while cell wall proteins such as expansins and endotransglycosylase/hydrolase (XTH) are involved in cell wall distensibility.15–18 In addition, it was shown that flower opening in tulips is caused by reversible phosphorylation of aquaporins, a family of water-permeable channels.19–21 Aquaporins are presumably involved in the flow of water into rose petal cells as well. Thus, hypertrophy of petal cells is thought to require 1) increased osmotic pressure within the cell, 2) relaxation of the cell wall, and 3) the flow of water into the cell (Figure 1).22 Figure 1 Flower opening and cell enlargement of rose petal cell. A: Roseflower opening is a process of irreversible petal growth and reflection in which existing cells expand and fresh and dry weights increase; B: Sugar accumulation in vacuole, cell wall loosening, and subsequent water flow into cell are thought