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.
Highlights
The site is characterized by a semiarid Mediterranean climate with a mean annual precipitation of 368 mm, where the drought period lasts from June to August, and precipitation is concentrated in late autumn and winter
Morphological and anatomical study of P. canariensis needle pairs taken from the same resprouting twigs of adult (60–70 y old) trees, led us to identify and quantify main differences between primary and secondary needles
Frequency distribution of tracheid diameter followed a similar pattern in Forests 2021, 12, x FOR PEER REVIEWboth needle types with tracheids of 3–4 μm ø being responsible for ca. 50% of the9total of 20 conductive area (Figure S1)
Summary
Heteroblasty has evolved multiple times during plant evolution in different unrelated taxa [2,3], and some exceptional cases such as New Zealand flora reach in heteroblastic plants (e.g., Pseudopanax crassifolius) [5], Acacia species, or the European Ivy (Hedera helix) have received deep scientific attention [2]. Another remarkable example is found within conifers as several species show a large heteroblastic modification between juvenile (primary) and adult (secondary) needles [6,7]
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