Osmolality and Non-Structural Carbohydrate Composition in the Secondary Phloem of Trees across a Latitudinal Gradient in Europe

Abstract

Phloem osmolality and its components are involved in basic cell metabolism, cell growth, and in various physiological processes including the ability of living cells to withstand drought and frost. Osmolality and sugar composition responses to environmental stresses have been extensively studied for leaves, but less for the secondary phloem of plant stems and branches. Leaf osmotic concentration and the share of pinitol and raffinose among soluble sugars increase with increasing drought or cold stress, and osmotic concentration is adjusted with osmoregulation. We hypothesize that similar responses occur in the secondary phloem of branches. We collected living bark samples from branches of adult Pinus sylvestris, Picea abies, Betula pendula and Populus tremula trees across Europe, from boreal Northern Finland to Mediterranean Portugal. In all studied species, the observed variation in phloem osmolality was mainly driven by variation in phloem water content, while tissue solute content was rather constant across regions. Osmoregulation, in which osmolality is controlled by variable tissue solute content, was stronger for Betula and Populus in comparison to the evergreen conifers. Osmolality was lowest in mid-latitude region, and from there increased by 37% toward northern Europe and 38% toward southern Europe due to low phloem water content in these regions. The ratio of raffinose to all soluble sugars was negligible at mid-latitudes and increased toward north and south, reflecting its role in cold and drought tolerance. For pinitol, another sugar known for contributing to stress tolerance, no such latitudinal pattern was observed. The proportion of sucrose was remarkably low and that of hexoses (i.e., glucose and fructose) high at mid-latitudes. The ratio of starch to all non-structural carbohydrates increased toward the northern latitudes in agreement with the build-up of osmotically inactive C reservoir that can be converted into soluble sugars during winter acclimation in these cold regions. Present results for the secondary phloem of trees suggest that adjustment with tissue water content plays an important role in osmolality dynamics. Furthermore, trees acclimated to dry and cold climate showed high phloem osmolality and raffinose proportion.

Defence syndromes in lodgepole – whitebark pine ecosystems relate to degree of historical exposure to mountain pine beetles

“…Pinus contorta likewise exhibited higher allocation to sugars than starch in Alberta (Goodsman et al ), suggesting this occurs over a broad geographic pattern. Higher allocation to starch versus sugar in the slow‐growing, long‐lived P. albicaulis relative to the fast‐growing, short‐lived P. contorta is consistent with the roles these carbohydrates play in energy storage and drought tolerance, versus rapid energy conversion, respectively (Kaelke et al ; Poorter & Kitajima ; Loehman et al ; Lintunen et al ).…”

Gradients and dynamics of inner bark and needle osmotic potentials in Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies L. Karst)

“…Additionally, in study by Lintunen et al . (), WC had a significant role in generating the osmolality levels of branch inner bark in different latitudes around Europe. Osmolality at full saturation explains in situ osmolality with 30 to 50% regression in branches and stem positions when all the sampling periods are examined together in our study.…”

Leaf carbon and water status control stomatal and nonstomatal limitations of photosynthesis in trees

“…Osmotic potential has been repeatedly demonstrated to decrease during drought stress, in order to maintain the turgor of the mesophyll under decreased water potential (e.g. Sevanto et al, 2014;Salmon et al, 2015;Lintunen et al, 2016). Trees also regulate their leaf osmotic potential to cope with osmotic stress; for example, in the case of in mangroves (Rada et al, 1989) or when growing on saline soil, or to avoid frost damage (Levitt & Scarth, 1936;Charrier et al, 2018).…”