Photosynthesis of Conifers in Relation to Annual Growth Cycles and Dry Matter Production

Abstract

Observations that deciduous larch species can show annual growth increments equal to or greater than evergreen conifers, and that the saturating light intensity for photosynthesis in needles of Larix leptolepis was almost twice those for several evergreen conifers, led to a study of the photosynthetic mechanism in L. leptolepis. Several features of photosynthesis in L. leptolepis placed this species in an intermediate position between classical C₃ and C₄ plants. Incorporation of ¹⁴C from ¹⁴CO₂ by enzyme preparations of larch needles was eight times greater with PEP as substrate than with ribulose bis phosphate; a chlorophyll a/b ratio of 3.5 was obtained; needles possessed a green starch-containing endodermis but with little orientation of mesophyll cells to this “bundle sheath”; no clear ultrastructural dimorphism was observed between chloroplasts of mesophyll and endodermal cells; a CO₂-compensation point of 20 μl-l⁻¹ was recorded; and the first measurable product of photosynthesis appeared to be malate rather than phospho-glyceric acid. These results are discussed in relation to the deciduous habit of L. leptolepis and its high productivity in comparison with other conifers.