Minisymposium 20: Global Change

Abs # 34002: Ultraviolet-B radiation protection mechanism in the leaves of pecan (Carya illinoensis)

Presenter:	Qi, Yadong , qyadong@subr.edu
Authors	Qi, Yadong  (A)   Bai, Shuju  (B)   Vogelmann, Thomas C (C)   Heisler, Gordon M (D)
Affiliations:	(A): Southern University Agricultural Research and Extension Center, Baton Rouge, LA 70813 (B): Department of Computer Science, Southern University, Baton Rouge, LA 70813 (C): Botany Department, University of Vermont, Burlington, VT 05405 (D): USDA Forest Service, Northeast Forest Experimental Station, Syracuse, NY 13210

We have recently completed a 5-year project assessing ultraviolet-B (UV-B, 280-320nm) radiation tolerance involving 35 southern tree species. This paper reports the results of pecan (Carya illinoensis). The light penetration from the adaxial surfaces of the mature leaves of pecan was measured using a fiber optic microprobe system at four representative wavelengths: UV-B at 310nm, ultraviolet-A (UV-A) at 360 nm, blue light at 430nm, and red light at 680nm. The thickness of the leaf adaxial epidermal layer was averaged 10um and total leaf thickness was 220um. The patterns of the light attenuation by the leaf tissues exhibited strong wavelength dependence. The leaf epidermal layer was chiefly responsible for absorbing the UV-A and UV-B radiation. About 99% of 310nm radiation was steeply attenuated within the first 5um of the adaxial epidermis; thus, very little UV-B radiation transmitted to the mesophyll tissues where contain photosynthetic sensitive sites. The adaxial epidermis attenuated 97% of the UV-A radiation and the mean depth of 360nm penetration was 33um. The epidermis also absorbed about 80% of the blue light and 57% of the red light; however, the blue and red wavelengths penetrated much deeper into the mesophyll tissues and were gradually attenuated by the leaves. The mean penetration depths were 140um for 430nm and 190um for 680nm light. The mesophyll tissues attenuated averagely 20% of the blue light and 42% of the red light, which were available for photosynthesis use. Since the epidermal layer absorbed nearly all UV-B radiation, it acted as an effective filter screening out the harmful radiation and protecting photosynthetically sensitive tissues from the UV-B damage. Therefore, the epidermal function of the UV-B screening effectiveness can be regarded as one of the UV-B protection mechanisms in pecan. In addition, we investigated the seasonal changes in leaf UV-B absorbing compounds concentration, whole leaf optical properties, and leaf anatomy in pecan. Leaf UV-B absorbing compounds concentration showed a significant increase with exposure to the increased ambient UV-B/solar radiation during the early part of the growing season. The increased concentration of leaf UV-B absorbing compounds due to exposure to UV-B is also considered as one of the strategies to resist UV-B in pecan.