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

Abstract

Preconditions of phloem transport in conifers are relatively unknown. We studied the variation of needle and inner bark axial osmotic gradients and xylem water potential in Scots pine and Norway spruce by measuring needle and inner bark osmolality in saplings and mature trees over several periods within a growing season. The needle and inner bark osmolality was strongly related to xylem water potential in all studied trees. Sugar concentrations were measured in Scots pine, and they had similar dynamics to inner bark osmolality. The sucrose quantity remained fairly constant over time and position, whereas the other sugars exhibited a larger change with time and position. A small osmotic gradient existed from branch to stem base under pre-dawn conditions, and the osmotic gradient between upper stem and stem base was close to zero. The turgor in branches was significantly driven by xylem water potential, and the turgor loss point in branches was relatively close to daily minimum needle water potentials typically reported for Scots pine. Our results imply that xylem water potential considerably impacts the turgor pressure gradient driving phloem transport and that gravitation has a relatively large role in phloem transport in the stems of mature Scots pine trees.

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

“…Granot et al, 2013;Kelly et al, 2013Kelly et al, , 2014Kelly et al, , 2017 to play such a central role in regulating leaf physiological processes in response to changes in sugar metabolism. Under the well-supported assumption that changes in leaf osmolality reflect changes in sugar concentration (Warren et al, 2007;Paljakka et al, 2017 but see Discussion for more details) and that hexokinase would play a similar role in trees as in the previously studied plants, hexokinase would be a good candidate for the coordination mechanism underlying the simultaneous changes in stomatal and nonstomatal limitations in our study (see Fig. 8).…”

“…Talbott & Zeiger, ; Huang & Jander, ). Soluble sugars likely represent a large share of the osmotically active compounds in the leaves of the tree species in our study (Morgan, ; Paljakka et al , ), but ions such as of sodium and potassium are also likely to constitute part of the measured osmolytes (in particular due to the freezing treatment of the samples). Leaf sugar concentration has previously been associated with increasing NSLs in some trees and herbaceous plants, both in C 3 and C 4 metabolism (e.g.…”

An update on phloem transport: a simple bulk flow under complex regulation

“…The scaling of SE conductivity with tree height was shown within a single tree ( Figure 2A ; 57 , 58 ), within a species 59 , and across species ( Figure 2B ; 55 , 60 ), confirming that resistance decreases to accommodate mass flow in larger trees. Furthermore, it was recently shown in mature, field-grown Scots pine trees that there is an osmotic pressure gradient along the phloem pathway from leaves to the stem base ( Figure 2C ; 61 ). The osmotic pressure gradient, supported by gravity, was calculated to be large enough to overcome the xylem water pressure potential and establish a phloem turgor pressure gradient that drives mass flow according to the Münch mechanism at all times across the diel cycle 61 .…”

“…Furthermore, it was recently shown in mature, field-grown Scots pine trees that there is an osmotic pressure gradient along the phloem pathway from leaves to the stem base ( Figure 2C ; 61 ). The osmotic pressure gradient, supported by gravity, was calculated to be large enough to overcome the xylem water pressure potential and establish a phloem turgor pressure gradient that drives mass flow according to the Münch mechanism at all times across the diel cycle 61 . Taken together, these results confirm that Münch-type pressure flow works even in the tallest angiosperm and gymnosperm trees, although transport speed might be up to 10 times lower than in herbaceous plants 62 .…”

Leaf Carbon Export and Nonstructural Carbohydrates in Relation to Diurnal Water Dynamics in Mature Oak Trees

“…trees are reported to have higher export rates during the day than at night (Davey et al, 2006), whereas one-year-old Eucalyptus (Eucalyptus globulus) trees that reach a minimum (midday) leaf C of 21.4 MPa have greater export rates at night than during the day (Quick et al, 1992). Additionally, for mature Scots pine (Pinus sylvestris) trees, phloem transport rate, estimated based on osmotic potentials, decreases during the day (Paljakka et al, 2017). Here, we document carbon export patterns in mature (.20 m tall) red oak (Quercus rubra) trees, in the context of water relations, to shed light on how leaf water status affects phloem functioning.…”