Water and stem girdling affect the tracheids' number more than their shape in Pinus pinaster saplings.

Key message: Primary growth is less sensitive than secondary growth to water and phloem blockage in maritime pine saplings, with variations in radial growth resulting from tracheid production without altering their size. In plants, primary and secondary growth is regulated by internal and external factors. However, apical and cambial meristems may not respond in the same way. In this study, we disentangle the role of carbon and water availability on stem growth (primary and secondary) and on anatomical tracheid features by investigating the effect of stem girdling (phloem blockage vs. no phloem blockage) and water availability (control vs. drought) in three-year-old Pinus pinaster saplings. We found that primary growth was similar among treatments, while secondary growth was affected by both treatments. Tree ring-width increment in saplings subjected to drought corresponded to half of that on control plants, and phloem blockage enhanced ring width under guidelines soil water conditions. These differences in the radial growth resulted mainly from an increase in tracheid production, which also increased with phloem blockage (above the girdling), regardless the water regime. However, the seasonal pattern of tracheid features was mainly explained by individuals and to a less extent by the water regime and girdling. Contrasting seasonal patterns between treatments were found only on cell wall thickness, showing a faster decrease with increasing water constrains in the drought group than in the control one. The absence of treatment effects on primary growth in P. pinaster saplings suggests a trade-off between the priority in plant development and meristem sensitivity to environmental conditions. [ABSTRACT FROM AUTHOR]