Coexistent Heteroblastic Needles of Adult Pinus canariensis C.Sm. ex DC. in Buch Trees Differ Structurally and Physiologically

Abstract

Great variation in shape and size between primary (juvenile) and secondary (adult) needles, so-called leaf-heteroblasty, occurs in several Pinus species. Most of them loss primary needles during the juvenile-to-adult transition of the tree. An exception to this is Pinus canariensis (a Canary Islands endemism) in which basal resprouting twigs of adult trees frequently wear both primary and secondary needles. Taking advantage of this extraordinary study-case-species, we conducted an exhaustive comparison of both needle types through quantitative analyses of needle anatomy, photochemical performance, gas exchange, and resistance to extreme dehydration and to extreme needle temperature. We hypothesized that primary needles would show lower investment to leaf structure but higher photosynthetical efficiency. Primary needles had less stomatal density and thicker and less wettable cuticles. In cross section, primary needles showed smaller structural fraction (e.g., percent of hypodermis, endodermis and vascular tissue) and higher fraction of photosynthetic parenchyma. Significant differences between primary and secondary needles were not found in net carbon assimilation not in their leaf mass area values. Interestingly, secondary needles showed higher electron transport rate, and they were additionally much more efficient in retaining water under severe and controlled desiccant conditions. When subjected to extreme temperatures (−10° to +50 °C), primary needles recovered better their photochemical efficiency than secondary needles, after +46° and +48 °C heat-shock treatments. Our results indicate that both needle types broaden the diversity of physiological responses against environmental constrains in basal twigs of adult P. canariensis trees. Considering that this is a fire-resistant and resprouting species, this advantage could be particularly useful after a drastic environmental change such a fire or a gap opening in the forest.

The Effect of Hot Wind on Needle and Stem Water Status: Response Strategies in Resprouting and Non-Resprouting Pine Species

“…and are typically higher at the leaf level due to the impact of decreasing the diameter of the tracheary elements [54]. This effect could be particularly noticeable in P. canariensis due to a delayed heteroblastic change [34] and primary needles with a lesser-developed vascular tissue than secondary needles [55]. Thus, leaves can be considered 'hydraulic bottlenecks' [56,57].…”

“…Leaf hydraulic resistances may result in large water potential gradients between the xylem and other tissues in transpiring leaves [59]. The presence of endodermis and transfusion tissue surrounding vascular tissues in needles [55,60,61] may contribute to generating and sustaining such water potential gradients together with an uneven water distribution over the leaf, as has been observed in other species [62]. In addition, increased vulnerability to cavitation in leaves could be driven by an early loss of hydraulic conductance in extra-xylary pathways rather than xylem embolism [63].…”

Heteroblastic Foliage Affects the Accumulation of Non-Structural Carbohydrates and Biomass in Pinus massoniana (Lamb.) Seedlings

“…Moreover, primary needles had greater mesophyll and chloroplast volume fractions than that of secondary needles, implying they have an overall greater photosynthetic activity [37]. Nevertheless, the secondary needles had a higher fraction of water conducting (xylem), nutrient transporting (phloem), and transfusion parenchyma than that of primary needles, improving the compensation mechanism of material transport capacity [38]. We hypothesized that heteroblastic foliage would exhibit different photosynthetic mechanisms under similar environmental conditions.…”

Gestión forestal para la adaptación al cambio global. Evaluación de tratamientos selvícolas para la diversificación y regeneración de masas de repoblación

“…Las procedencias atlánticas de P. pinaster tienen asociados mayores crecimientos que las mediterráneas ante las mismas condiciones de cultivo, lo que se debe a su mayor proporción de acículas juveniles, que son fotosintéticamente más eficientes que las adultas (de la Mata et al, 2014). Fernández-Marín et al (2021) encontraron el mismo resultado en el caso de P. canariensis. Sin embargo, parece que en las procedencias mediterráneas, una mayor proporción de acículas adultas indica un mejor status de establecimiento, pues las plántulas que se encuentran más estresadas suelen retrasar la producción de acícula adulta (de la Mata et al, 2014).…”