Stem photosynthesis not pressurized ventilation is responsible for light-enhanced oxygen supply to submerged roots of alder (Alnus glutinosa)

Abstract

Background and aims: Claims that submerged roots of alder and other wetland trees are aerated by pressurized gas flow generated in the stem by a light-induced thermo-osmosis have seemed inconsistent with root anatomy. Our aim was to seek a verification using physical root-stem models, stem segments with or without artificial roots, and rooted saplings.

Methods: Radial O2 loss (ROL) from roots was monitored polarographically as the gas space system of the models, and stems were pressurized artificially. ROL and internal pressurization were also measured when stems were irradiated and the xylem stream was either CO2 enriched or not. Stem photosynthesis and respiration were measured polarographically. Stem and root anatomy were examined by light and fluorescence microscopy.

Key results: Pressurizing the models and stems to <or=10 kPa, values much higher than those reportedly generated by thermo-osmosis, created only a negligible density-induced increase in ROL, but ROL increased rapidly when ambient O2 concentrations were raised. Internal pressures rose by several kPa when shoots were exposed to high light flux and ROL increased substantially, but both were due to O2 accumulation from stem photosynthesis using internally sourced CO2. Increased stem pressures had little effect on O2 transport, which remained largely diffusive. Oxygen flux from stems in high light periods indicated a net C gain by stem photosynthesis. Chloroplasts were abundant in the secondary cortex and secondary phloem, and occurred throughout the secondary xylem rays and medulla of 3-year-old stems. Diurnal patterns of ROL, most marked when light reached submerged portions of the stem, were modified by minor variations in light flux and water level. Low root temperatures also helped improve root aeration.

Conclusions: Pressurized gas flow to submerged roots does not occur to any significant degree in alder, but stem photosynthesis, using internally sourced CO2 from respiration and the transpiration stream, may play an important role in root aeration in young trees and measurably affect the overall carbon balance of this and other species.

Fig. 1.

Type of assembly used to collect the data on which claims of pressurized gas flow to alder roots and other species were based. Both in the dark and with the stem irradiated, 10 mL quantities of a tracer gas, ethane (100 %), were injected into the top chamber and the lower chamber was sampled periodically. Faster rates of ethane transfer to the lower vessel in the light were interpreted as being due to a thermo-osmotically induced pressure flow from the irradiated shoot. Modified from Grosse et al. (1992).

Fig. 2.

(A) Physical model of the stem–root system to investigate the potential effects on root aeration of (a) gas pressurization per se; (b) partial through-flow convection; and (c) a higher source O₂ concentration. Radial O₂ losses measured in an anaerobic medium with the silicone rubber tip on the tip of the microcapillary slotted through the sleeving Pt electrode. (B) Views of the type of assembly used to study the effects of light on pressurization and O₂ production by stem cuttings. Effects were recorded as ROL measurements on artificial silicone rubber-tipped microcap ‘roots’. Root bases were attached to the stem over cavities prepared by removing lenticels and discs of cork and secondary cortex beneath. The lower vessel was filled with anaerobic medium: 0·05 % agar : water + 7 mm KCl. Xylem of stem cuttings was pre-charged with CO₂-enriched water (4 or 6·9 mm) from a plastic reservoir. Lenticel numbers per cm of stem, 16·3 ± 2·8 s.e. (n = 14); lenticel area (mm² per cm²), 6·2 ± 0·6 s.e. (n = 14). (C) Sectional diagram of an artificial silicone rubber-tipped ‘root’ of the type attached to stem cuttings. The root tip is positioned within a sleeving Pt electrode, and values of diffusive resistances along the path from root base to electrode surface are indicated. (D) The relationship between radial O₂ loss from the artificial roots and O₂ concentration within the root–shoot junction. O₂ electrolysis from which ROL is derived is also shown. (E) Top and side elevation diagrams to show the positioning of cuttings and whole plants in relation to the light, and the heat sink arrangement for measuring the effects of light on ROL and pressurization. (F) Sectional view of apparatus for determining the effect of light on the potential for pressurization in stem cuttings. Container dimensions: internal length 200 mm, internal diameter 55 mm. (G) Sectional view of apparatus for measuring the photosynthetic O₂ production rate of stem cuttings.

Fig. 3.

Stem–root model. The effects on radial O₂ loss of stepwise increases in gas pressure applied at the top of the model with the side arm sealed. ROL is expressed in terms of the O₂ electrolysis current. ROL in ng cm⁻² min⁻¹ = 4·974 × [electrolysis current/surface area of root tip inside the sleeving Pt electrode].

Fig. 4.

Stem–root model. Observed and predicted relationships between ROL and applied pressure. ROL is expressed in terms of the O₂ electrolysis current. The inset shows the observed data plotted with a full (not selectively expanded) y-axis. ROL in ng cm⁻² min⁻¹ = 4·974 × [electrolysis current/surface area of root tip inside the sleeving Pt electrode].

Fig. 5.

Stem–root model. Effects on ROL of stepwise increases in gas pressure with the side arm sealed followed by effects on ROL with the side arm open permitting convective gas flow through the stem capillaries only.

Fig. 6.

Stem–root model, showing (A) the effect on ROL of increasing the partial pressure of the O₂ source to 30·5 % with the side arm sealed and no convection; (B) as for (A) but with the side arm open permitting convection of a 30·5 % O₂ stream through the stem capillaries only; (C) as for (B) but with air (20·7 % O₂) convecting through the stem capillaries. With the y-axis expanded, the inset shows the initial change in ROL with time after exposure to 30·5 % O₂ at (A).

Fig. 7.

Partially submerged alder cutting with artificial roots: effects of light on O₂ concentrations in an artificial root–shoot junction (i.e. in the secondary cortex of stem) and ROL from artificial roots with (A) the submerged part in the dark and emergent stem illuminated with a PAR of 30 µmol m⁻² min⁻¹, (B) the emergent part of the stem illuminated with a PAR of 550 µmol m⁻² min⁻¹ and (C) the emergent and submerged parts of the stem illuminated at 550 µmol m⁻² min⁻¹.

Fig. 8.

A partially submerged alder cutting with artificial roots: effects of light on root–shoot junction (i.e. in the secondary cortex of stem) O₂ concentrations, ROL and pressurization in submerged and emergent parts of a stem. (A) At 200 s, the emergent stem only is exposed to a PAR of 550 µmol m⁻² min⁻¹ (submerged part in the dark); at 3125 s, both emergent and submerged parts are illuminated. (B) Continued from (A) and showing (i) the gradual decline in ROL and stem pressures which began at approx. 6000 s after exposing the emergent and submerged parts to the light, and (ii) the final rapid fall in pressure and ROL when the light was switched off.

Fig. 9.

Partially submerged alder cutting with artificial roots: effects on ROL of applying pressure to the emergent stem—the same stem cutting as for Fig. 8. (A) ROL vs. time and indicating pressure changes followed by a PAR of 550 µmol m⁻² min⁻¹ to emergent and submerged parts. (B) ROL vs. pressure.

Fig. 10.

(A) Transverse section of sector of a 1-year-old alder stem: red autofluorescence under blue light is due to chloroplasts in secondary cortex, secondary phloem, secondary xylem and medulla. Scale bar = 200 µm. (B) As (A) but at higher magnification showing individual chloroplasts fluorescing red in secondary phloem and in secondary xylem ray cells. Scale bar = 25 µm. (C) Sector of a 3-year-old alder stem showing red autofluorescence from chloroplasts in medulla and the inner two annual rings. Scale bar = 200 µm. (D) Transverse section of a 3-year-old alder stem through a lenticel. Some algal cells are fluorescing red on the outer surface of the flaking lenticel. Some red fluorescence is seen from chloroplasts in secondary cortex and secondary phloem. The pink colour in banded tissues of lenticel is not fluorescence but is due to anthocyanins. Scale bar = 100 µm. (E) Transverse section of a 1-year-old alder stem with transmitted white light. Chloroplasts (green) in secondary xylem rays and medulla. Scale bar = 200 µm. (F) Three-year-old alder sapling (plant 1) prepared for ROL and pressure measurements and for applying above ambient O₂ concentrations to the shoot base. Note the sleeving Pt electrodes around the tips of two of the adventitious roots. An acrylic chamber around the shoot base and the pressure sampling/delivery point on the emergent part of the stem are visible. (G) As (F) showing the submerged stem base and sleeving Pt electrodes on two adventitious roots. (H) Transverse section of 16 cm long adventitious alder root at 15 mm from the base showing aerenchyma (aer) in the primary cortex and well developed secondary xylem (sx) and secondary phloem (sp). Arrows point to non-aerenchymatous gas spaces (black) visible in the secondary cortex (sc) and running radially in the secondary rays. Scale bar = 200 µm. (I) As (H) but at 49 mm from the base. Aerenchyma (aer) visible in the primary cortex and early normal development of secondary xylem (sx), secondary phloem (sp) and secondary cortex (sc). Lignified tissues, including primary xylem (px) stained red using phloroglucinol and concentrated HCl. Scale bar = 100 µm. (J) Root system of a 3-year-old alder after 4 months in a waterlogged soil. Adventitious roots produced from the submerged stem base during this period have grown to a length of up to 36 cm. The white tips are visible, as are their thick woody bases protruding at right angles to the stem. (K) Transverse section 40 mm from the tip of longest adventitious root shown in (J) (length = 36 cm). Lignified primary xylem (px) stained red using phloroglucinol and concentrated HCl but arrows point to abnormal development of secondary xylem with large thin-walled non-lignified cells. Scale bar = 100 µm.

Fig. 11.

Alder sapling (plant 1) showing the effects of light on ROL from a 13·5 cm long intact root. (A) ROL was unaffected by north light (50 µmol m⁻² s⁻¹) or supplementary light (550 µmol m⁻² s⁻¹) on an emergent shoot only but rose rapidly when emergent and submerged parts were exposed to a PAR of 550 µmol m⁻² min⁻¹ followed by a steady decline. ROL returned rapidly to background (approximately zero) when the light was switched off. (B) Continuation from (A) showing a similar pattern but lower ROL peak after exposure of emergent and submerged parts to 550 µmol m⁻² s⁻¹ PAR. (C) Continuation from (B) showing two further cycles of light application and a further diminished ROL peak despite some CO₂ enrichment of the bathing medium.

Fig. 12.

An alder sapling (plant 1) showing the effects of exposing the emergent stem base to pure O₂ followed by exposure of emergent and submerged parts to a PAR of 550 µmol m⁻² min⁻¹.

Fig. 13.

An alder sapling (plant 1) showing the effects on ROL from an 18 cm long root of (A) cooling submerged parts from 22 to 11 °C—emergent parts and top 5 cm of submerged parts illuminated but only by north light at approx. 50 µmol m⁻² s⁻¹, and (B) allowing the submerged parts to warm up again to room temperature (approx. 22 °C).

Fig. 14.

An alder sapling (plant 1). Observed and predicted effects of increasing pressurization applied to the connector on an emergent shoot 50 mm above the water line. The root system is held at 11 °C, the emergent parts and the top 5 cm of submerged parts receiving only north light at approx. 50 µmol m⁻² min⁻¹.

Fig. 15.

An alder sapling (plant 2). Effects of different light regimes on ROL; the root system is held at approx. 11 °C. Initial reading with submerged parts in the dark and the emergent parts receiving north light at approx. 37 µmol m⁻² s⁻¹ followed by (A) the emergent parts exposed to a PAR of 550 µmol m⁻² s⁻¹ and (B) emergent parts and the top 5 cm of submerged parts receiving 550 µmol m⁻² s⁻¹.

Fig. 16.

An alder sapling (plant 2). Diurnal patterning in root ROL modified by additional fluctuations caused by variations in light flux during the day and water level changes. Monitored roots were initially apprrox. 12 cm long. The basal 5·5 cm of submerged stem were unshaded and the plant was exposed to north light from the laboratory window (PAR ≤70 µmol m⁻² s⁻¹). Roots grew slowly for the duration of the experiment. Root chamber temperature was 11–12 °C.