Norway spruce needle size and cross section shape variability induced by irradiance on a macro- and microscale and CO2 concentration.

Key message: Needle cross-section parameters differ according to needle orientation on a shoot corresponding to irradiance microgradient. Irradiance is a stronger morphogenic factor determining needle size and shape than CO₂ concentration.Abstract: We investigated the effects of irradiance on macroscale and microscale, elevated CO₂ concentration [CO₂], and their interaction on Norway spruce (<italic>Picea abies</italic> L. Karst.) needles. The irradiance macroscale was represented by sun and shade shoots from two vertical positions in a crown and the irradiance microscale corresponded to spatial orientation of individual needles on a shoot (upper, side, and lower needles relative to the shoot axis). Determination of needle cross section shape using generalized Procrustes analysis and principal component analysis provided a novel approach for evaluating needle morphometry. As expected, shade needles on the irradiance macrogradient were flatter and had less cross-sectional area and smaller volume than did sun needles. The irradiance microgradient was detected within both sun and shade shoots, being steeper in sun shoots than shade shoots. On the microscale, the irradiance gradient induced changes in needle size and cross section shape according to needles’ orientation on a shoot. Due to a more favourable light environment the traits of the upper needles within both sun and shade shoots resembled more the sun needle traits. The sun needle volume was significantly larger in the case of elevated [CO₂] as compared to ambient [CO₂]. Irradiance was a stronger morphogenic factor determining needle size and cross section shape compared to CO₂ concentration. We demonstrated that generalized Procrustes analysis can be a very powerful tool in ecophysiological studies for evaluating small-scale, subtle leaf shape changes on an intraspecific level caused by environmental factors. [ABSTRACT FROM AUTHOR]