The mechanisms and characteristics of leaf iridescence in shade plants

• Main Authors: Ping-Yun Tsai, 蔡秉芸
• Other Authors: Chiou-Rong Sheue
• Format: Others
• Language: zh-TW
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/68428251270147917247

碩士 === 國立中興大學 === 生命科學系所 === 105 === Iridescence is a structural color changing from different angles. Foliar iridescence in some shade plants is induced by multilayered structures in modified chloroplasts or cell walls. Iridoplasts are a kind of modified chloroplasts. In this study, Begonia rockii with iridoplasts and Selagenella uncinata with multilayered cell walls were chosen to explore the iridescent differences between these two structures, the effects of light intensity on iridoplasts, and the potential benefits of iridoplasts to shade plants. Iridoplasts within B. rockii and multilayered outer cell walls of S. uncinata were observed with a light microscope (LM) fitted with reflective lights and a spectrometer to understand the differences between their iridescent properties. Iridescent light spots of iridoplasts were in the depths of the adaxial epidermal cells, while those of cell walls were on the surface. As the incident angle changed, iridoplasts showed a greater degree of color change than cell walls, and different iridescent spots of iridoplasts appeared in the cells, while those of cell walls moved continuously over the cell surface. For exploring the effects of light intensity upon iridoplasts, two treatments were conducted. First, a shade treatment was conducted to a B. rockii leaf. After a week, the covered area showed deeper bluish iridescence than the uncovered area. Thus, the former was named bluish area and the latter was greenish area. Observed with a LM, the coverage of bluish spots in the epidermis of the bluish area was higher than that of the greenish area. The ultrastructures revealed with TEM show that the average thickness of iridoplast stroma layers in bluish areas (91.5 ± 1.6 nm) was significantly less than that in greenish areas (106.4 ± 2.5 nm). After the shading was rotated in the second week, the newly covered area became bluish, while the uncovered one became greenish. Second, bluish iridoplasts were observed under the stong light of a LM for about 2.5 hours. During the exposure, their color changed in two stages: (1) changed from blue to transparent, and then reverting to blue; (2) irreversibly changing from blue toward long wavelength colors, then becoming transparent. To predict the reflective and absorptive spectra of iridoplasts from their physical dimensions, Maxwell’s equations were used to simulate the optics of their multilayered structures. For these calculations, the thickness of a repeat unit (a thylakoid layer [T] + a stroma layer [S]) in the multilayered structure was fixed at 190 nm, but T and S were varied. Under this condition, the simulated reflective color remained blue although the T thickness varied from 135 nm to 75 nm and S thickness varied from 5 nm to 25 nm. The simulated absorption of iridoplasts was compared with that of a normal granum structure with the same pigments. The simulated absorption of a blue iridoplast was lower than that of a granum in the blue absorptive region of the chlorophyll a and b absorption spectra, but it was higher in the red absorptive region. The absorption spectra of the bluish and greenish area on a B. rockii leaf were measured to estimate the actual absorption of blue iridoplasts. The actual absorption of iridoplasts of the bluish leaf area was lower than that of the greenish area in the blue active region of the action spectrum of photosynthesis, but it was only slightly higher in the red active region. In addition, the bluish area had fewer starch grain-containing iridoplasts than the greenish area. To understand potential photoprotection from iridoplasts, plastid movements under strong visible light and reflectance of UV were studied. Unlike chloroplasts in mesophylls, which moved away from the light source, iridoplasts in the adaxial epidermis remained situated above the junction between two mesophyll cells under strong visible light, potentially protecting mesophyll layers. The reflectance of iridescent leaves in the UV region was not high. Visible leaf damage and iridoplast disorganization were observed after the leaf was exposed to UV light. This study shows that a comparison of iridescent properties between iridoplasts and multilayered cell walls can be an effective preliminary method to determine the source of iridescence. The flexibility of color change of iridoplasts with light intensity in Begonia leaves is first reported. The thickness of a repeat unit (S + T) is confirmed to primarily determine the color of an iridoplast and this thickness can change mostly because the S thickness varies. Iridoplasts always shelter the chloroplasts in the mesophyll cells, even under strong light, and thus may provide physical photoprotection. Taken altogether, the characteristics of foliar iridescence obtained here provide an advanced basis to better understand iridescent biology and its potential benefits.