Effects of Light Intensity on Early Periderm and Xylem Development in Pinusresinosa, Fraxinuspennsylvanica, and Robiniapseudoacacia

The influence of light intensity on development of the first periderm and xylem in seedlings of Pinusresinosa, Fraxinuspennsylvanica, and Robiniapseudoacacia up to 56 days after seed germination was studied. Dark-grown seedlings of all three species lacked periderm at the end of the experimental periods, emphasizing the essentiality of light for periderm development. P. resinosa seedlings grown for 56 days following germination at 70 ft-c (1 ft-c = 10.76 lm/m2) of light and R. pseudoacacia seedlings grown at 70 or 300 ft-c of light for 30 days following germination failed to develop periderm. Seedlings of F. pennsylvanica grown at 70, 300, 600 or 1200 ft-c of light for 30 days following germination developed periderm. Periderm increments did not differ in 56-day-old P. resinosa seedlings grown at 600 or 1200 ft-c or in 30-day-old F. pennsylvanica seedlings grown at 70 or 300 ft-c of light. Phellogen formed sooner and was more active in 30-day-old R. pseudoacacia seedlings grown at 1200 ft-c than in those grown at 600 ft-c. Time to periderm initiation decreased and phellem increment increased in 30-day-old F. pennsylvanica seedlings with each increase in light intensity from 300 to 1200 ft-c. Secondary xylem was absent in dark-grown seedlings of all three species and in P. resinosa and F. pennsylvanica seedlings grown at 70 ft-c of light. In other treatments xylem increment increased significantly with each increase in light intensity up to 1200 ft-c.