Light Limitation and Foliar Pathogenic Infection Impact Phloem Anatomy and Function in <i>Pinus radiata</i> D. Don

Phloem plays a major role in plant physiology, health and growth. However, little research has addressed the impact of biotic and abiotic stressors on phloem structure and development. This study extends recent interest on stress impact on phloem to further understand its physiological limits by exploring a common combination of stressors within forest systems: reduced light availability and concomitant foliar pathogenic infection. We compared juvenile Pinus radiata D. Don. plants growing under optimal light conditions to plants growing under reduced light availability and exposure to pathogenic infection. We monitored foliar gas exchange and took destructive samples for nonstructural carbohydrate (NSC) analysis and phloem anatomy in spring and early summer. We used software-assisted image analysis to determine cell composition and area of conducting phloem, and a fluid dynamics model to derive phloem hydraulic parameters. Phloem showed environmental plasticity within the same growing season. We found changes in phloem anatomy in shaded and infected plants, including an increased sieve cell density and permeability, and reduced cell wall thickness. While intrinsic phloem hydraulic efficiency was maintained at the tissue level in stressed plants, the reduction in phloem cross-sectional area resulted in an eventual decline in phloem sap flow rate. Thus, phloem cross-sectional area was dynamically adjusted to match reduced translocation requirements. In addition, shaded and infected plants experienced reduced growth and C assimilation, as well as greater necrotic photosynthetic tissue, but showed similar levels of total NSC than control plants. The high levels of NSC observed in our stressed plants are an important finding that suggests that radial growth cessation and, by association, phloem formation impairment are induced by sink limitation instead of reduced carbohydrate supply to the meristem.