Physiological attributes of three- and four-needle fascicles of loblolly pine (Pinus taeda L.)

Key message Fascicle types differed morphologically but had similar photosynthetic capacity on a surface area basis. In Pinus species, fascicles can develop with a different number of needles than what is typical. For example, Pinus taeda fascicles typically have three needles, but sometimes have two or four. Although differing fascicle morphology could be a response to changes in the environment designed to optimize carbon gain or minimize water loss, we are unaware of any work comparing physiological differences between fascicles with different numbers of needles. We compared the physiological and morphological characteristics of three- and four-needle fascicles of a loblolly pine clone with an abnormally high abundance of four-needle fascicles to better understand whether differences in needle morphology affected photosynthetic capacity or transpiration. Three- and four-needle fascicles had equal length, diameter, and volume, but four-needle fascicles had significantly greater surface area, mass, and tissue density. Equal fascicle total volume resulted in smaller per-needle volume in four-needle fascicles compared to three-needle fascicles. On a unit surface area basis, light-saturated net assimilation, stomatal conductance and transpiration were similar between the three- and four-needle fascicles although the maximum rate of carboxylation was significantly greater in four-needle fascicles. On a per-fascicle basis, four-needle fascicles had greater transpiration, stomatal conductance, and maximum rate of light-saturated net assimilation. Our results suggest that several factors, including increased tissue density and stomatal density, offset the reduction in needle volume in four-needle fascicles, resulting in similar levels of gas exchange per unit surface area in three- and four-needle fascicles.