Popular Science: Light as a driver of the shape variety of Norway spruce needles
At Bílý Kříž in the Beskydy Mountains, the experimental site of the Academy of Sciences, scientists took 300 needles from 10 trees growing in special glass domes, half of them coming from trees growing seven years at elevated CO2 concentrations (700 ppm). Each needle was measured and microphotographs of three cross sections were taken. Altogether they had 900 pictures for the following analyses. In addition, they selected 10 sun and 10 shade spruce shoots to analyse the irradiation gradient.
The cross-section shape of the needles was evaluated using a Procrustes analysis, a method based on the examination of the shape using landmarks. Thanks to this, they could compare the cross-sectional shapes with needles originating from the shady and sunny parts of shoots located in different positions on the shoots (top, side, bottom) and growing in domes under different concentrations of CO2. They found that in the frame of a tree, the shade needles were flatter and the cross section had a smaller area than the sun needles. The sun needles were more bulky than the shaded ones at both CO2 concentrations. But what was different in the two atmospheric conditions was the cross-sectional area; while the sun needles were larger at higher CO2 concentrations, in the case of the shaded needles it was just the opposite. It can be summarized that the needle flatness is the main trait that is directly determined by irradiation. The needles on the bottom of the shoot were flatter than the ones on the side, and these were flatter than the ones of the top. The irradiation therefore has a greater effect on needle shaping than atmospheric CO2.
The scientists evaluated the self-shading of needles in the laboratory using an optical spectrometer. The results of the measurements of light penetration through the shoots showed a significant gradient of irradiation caused by the needles' self-shading. In a shoot there is a microgradient in the intensity of the irradiance. The gradient is particularly pronounced on sun shoots which have needles distributed more evenly around the entire twig compared to shade shoots from shaded positions. Scientists have concluded that the difference in the size and shape of the needles on the shoots is caused by different irradiation which is likely to lead to different physiological processes such as light absorption or carbon absorption. These conclusions from observations on individual shoots are probably valid even within the entire tree crown.
Scientists, based on their results, recommend the Procrustes analysis as an appropriate tool for investigating changes in the shape of plant organs induced by changes in environmental conditions.
Kubínová, Z., Janáček, J., Lhotáková, Z., Šprtová, M., Kubínová, L., Albrechtová, J. (2018). Norway spruce needle size and cross section shape variability induced by irradiance on macro- and microscale and CO2 concentration. Trees 32: 231–244. http://doi.org/10.1007/s00468-017-1626-3.