Structure and chlorophyll fluorescence of heteroblastic foliage affect first-year growth in Pinus massoniana Lamb. seedlings

Pine seedlings exhibit heteroblastic foliage (primary and secondary needles) during seedling development. However, few trials have studied how heteroblastic foliage influences pine seedling growth by seasonal variation. This study first investigated the anatomical differences between the primary and secondary needles of one-year-old Pinus massoniana seedlings. We measured chlorophyll fluorescence (ChlF) and evaluated the photoprotective mechanisms and light energy partitioning of these heteroblastic leaves from September to November. The results showed that the primary needles, as juvenile foliage, had a greater fraction of mesophyll tissue and stomata. In addition, the primary needles had two vascular bundles, and shorter distance from xylem and phloem to mesophyll cells, exhibiting a luxury growth strategy of rapidly obtaining high returns. The ChlF parameters indicated that the primary needles maintained a relatively high level of photoprotection by thermal dissipation (nonphotochemical quenching (NPQ)) and nonregulated energy dissipation (Y(NO)). The secondary needles, representing mature foliage, had greater area of xylem and phloem tissues. The contents of Chl b and carotenoids (Car) significantly increased in November, promoting φPo and photoprotection, which suggested that the secondary needles were more resistant to low temperatures. During the whole light response process of secondary needles, the increases in the electron transfer rate (ETR) and light energy utilization efficiency (α) helped to increase the actual photosynthetic quantum yield (Y(II)) by reducing energy dissipation by decreasing the proportion of regulated energy dissipation (Y(NPQ)) and Y(NO). Given the sensitivity of this heteroblastic foliage to environmental changes, the practical use and extension of P. massoniana for afforestation purposes should be carried out with caution.