Variations in needle morphology and anatomy as related to variations in photosynthetic potential within loblolly pine (Pinus taeda L.) canopies

Abstract

Variations in needle characteristics were studied in relation to photosynthetic potential for foliage from six positions within the canopies of three Loblolly Pine trees. The six positions represented two distinct lightning conditions, open and closed grown, and three vertical positions, top, middle and bottom on the canopies of the three trees. Only foliage of the first flush of the current year’s growth was studied. Photosynthetic rates of each shoot were obtained at six light intensities (50-10,000 ft. candles) and expressed per unit of foliage surface area, per unit of foliage weight and per million stomates. Needle characteristics were determined for the whole shoots by measuring needle length, surface area, weight, and number of stomates on needle subsamples from each shoot. Needles were shown to be significantly larger with more stomates per unit area when produced under the higher light intensities in the open-grown canopy portions. Needles from the top of the tree were larger than needles from the bottom. Closed grown needles were thinner with more stomates per unit weight. All of the needle characteristics studied showed large variations within the canopy. The least variable characteristics was the ratio of needle area to (needle weight)^2/3>. Photosynthetic rates for foliage from the six canopy positions showed differences which were dependent on the units of expression used. The differences in response rate between foliage from open and closed-grown conditions and from top, middle, and bottom position were non-significant. Photosynthetic rates were equally variable throughout the three units of expression used. This indicated that no one characteristic was the most accurate index of photosynthetic potential. Foliage weight was recommended as the most suitable index because it was measured most easily.