Carbon translocation patterns associated with new root proliferation during episodic growth of transplanted Quercus rubra seedlings

Insights of carbon assimilation and allocation in young cork oak (Quercus suber L.) plants using Carbon-14

¹⁴ C methods were applied to young, woody, branched and well-watered cork oak (Quercus suber L.) plants to determine carbon assimilation and its distribution among plant organs. Carbon assimilation rates by attached leaves clamped in a foliar ¹⁴ CO₂ assimilation chamber containing 3.7 × 10⁴  Bq of a portable ventilated diffusion porometer were measured at different ¹⁴ CO₂ pulse-labeling periods (15, 30, 45, 60 and 120 s) in summer. Allocation of recently fixed C by attached leaves within plants was evaluated 7 days after a 60-min of 5.6 MBq of ¹⁴ CO₂ pulse-labeling in late winter. ¹⁴ CO₂ pulse-labeling was separately induced on leaves of a lower branch, two opposite branches at the same lower level, a middle branch and a top branch. ¹⁴ C activity incorporated into the plants was measured by liquid scintillation and autoradiography. Our results show the optimum ¹⁴ CO₂ pulse-labeling period is between 15 and 30 s, which corresponds to 9.81 ± 0.15 and 9.16 ± 0.12 µmol m^-2 s^-1 C assimilation rates in summer, respectively. The investment of current assimilates ranged from 18 to 29% in leaves, 1 to 7% in lateral branches, 0 to 3% in the stem and over 65% in roots, in late winter. Roots displayed the greatest sink strength for the total ¹⁴ C recovered by whole-plants. These results were expected because the trial was done in winter, when cork oak does not produce their leaves. Our results highlight the contribution of current assimilates for growth and maintenance of roots, in young woody plants under Mediterranean climate.

Temperature and moisture dependence of daily growth of Scots pine (Pinus sylvestris L.) roots in Southern Finland

Scots pine (Pinus sylvestris L.) is one of the most important conifers in Northern Europe. In boreal forests, over one-third of net primary production is allocated to roots. Pioneer roots expand the horizontal and vertical root systems and transport nutrients and water from belowground to aboveground. Fibrous roots, often colonized by mycorrhiza, emerge from the pioneer roots and absorb water and nutrients from the soil. In this study, we installed three flatbed scanners to detect the daily growth of both pioneer and fibrous roots of Scots pine during the growing season of 2018, a year with an unexpected summer drought in Southern Finland. The growth rate of both types of roots had a positive relationship with temperature. However, the relations between root elongation rate and soil moisture differed significantly between scanners and between root types indicating spatial heterogeneity in soil moisture. The pioneer roots were more tolerant to severe environmental conditions than the fibrous roots. The pioneer roots initiated elongation earlier and ceased it later than the fibrous roots. Elongation ended when the temperature dropped below the threshold temperature of 4 °C for pioneer roots and 6 °C for fibrous roots. During the summer drought, the fibrous roots halted root surface area growth at the beginning of the drought, but there was no drought effect on the pioneer roots over the same period. To compare the timing of root production and the aboveground organs' production, we used the CASSIA model, which estimates the aboveground tree carbon dynamics. In this study, root growth started and ceased later than growth of aboveground organs. Pioneer roots accounted for 87% of total root productivity. We suggest that future carbon allocation models should separate the roots by root types (pioneer and fibrous), as their growth patterns are different and they have different reactions to changes in the soil environment.

Rainfall variability counteracts N addition by promoting invasive Lonicera maackii and extending phenology in prairie

Although encroaching woody plants have reduced the global extent of grasslands, continuing increases in soil nitrogen availability could slow this trend by favoring resident herbaceous species. At the same time, projected increases in rainfall variability could promote woody encroachment by aligning spatiotemporal patterns of soil moisture availability with the needs of woody species. We evaluated the responses of two deciduous woody species to these simulated environmental changes by planting seedlings of Quercus palustris and Lonicera maackii into tallgrass prairie communities grown under a factorial combination of increased rainfall variability and nitrogen addition. Lonicera maackii growth was reduced 20% by nitrogen addition, and increased rainfall variability led to 33% larger seedlings, despite greater competition for light and soil resources. In contrast, Q. palustris growth showed little response to either treatment. Increased rainfall variability allowed both species to retain their leaves for an additional 6.5 d in autumn, potentially in response to wetter end-of-season shallow soils. Our findings suggest increases in rainfall variability will counteract the inhibitory effect of nitrogen deposition on growth of L. maackii, extend autumn phenology, and promote the encroachment of some woody species into grasslands.

Reduced translocation of current photosynthate precedes changes in gas exchange for Quercus rubra seedlings under flooding stress

Northern red oak (Quercus rubra L.) seedlings are frequently planted on suboptimal sites in their native range in North America, subjecting them to environmental stresses, such as flooding, for which they may not be well adapted. Members of the genus Quercus exhibit a wide range of responses to flooding, and responses of northern red oak to flooding remain inadequately described. To better understand the physiological effects of root system inundation in post-transplant northern red oak seedlings and the effects of flooding on endogenous patterns of resource allocation within the plant, we observed the effects of short-term flooding initiated at the linear shoot growth stage on net photosynthetic rates, dark respiration, chlorophyll fluorescence (Fv/Fm) and translocation of (13)C-labeled current photosynthate. Downward translocation of current photosynthate declined after 4 days of flooding and was the first measured physiological response to flooding; net photosynthetic rates decreased and dark respiration rates increased after 7 days of flooding. Short-term flooding did not affect maximal potential efficiency of photosystem II (Fv/Fm). The finding that decreased downward translocation of (13)C-labeled current photosynthate preceded reduced net photosynthesis and increased dark respiration during flooding suggests the occurrence of sink-limited photosynthesis under these conditions.

Fast growth involves high dependence on stored resources in seedlings of Mediterranean evergreen trees

BACKGROUND AND AIMS

The carbon (C) and nitrogen (N) needed for plant growth can come either from soil N and current photosynthesis or through remobilization of stored resources. The contribution of remobilization to new organ growth on a whole-plant basis is quite well known in deciduous woody plants and evergreen conifers, but this information is very limited in broadleaf evergreen trees. This study compares the contribution of remobilized C and N to the construction of new organs in spring, and assesses the importance of different organs as C and N sources in 1-year-old potted seedlings of four ecologically distinct evergreen Mediterranean trees, namely Quercus ilex, Q. coccifera, Olea europaea and Pinus hapelensis.

METHODS

Dual (13)C and (15)N isotope labelling was used to unravel the contribution of currently taken up and stored C and N to new growth. Stored C was labelled under simulated winter conditions. Soil N was labelled with the fertilization during the spring growth.

KEY RESULTS

Oaks allocated most C assimilated under simulated winter conditions to coarse roots, while O. europaea and P. halepensis allocated it to the leaves. Remobilization was the main N source (>74 %) for new fine-root growth in early spring, but by mid-spring soil supplied most of the N required for new growth (>64 %). Current photosynthesis supplied >60 % of the C in new fine roots by mid-spring in most species. Across species, the proportion of remobilized C and N in new shoots increased with the relative growth rate. Quercus species, the slowest growing trees, primarily used currently acquired resources, while P. halepensis, the fastest growing species, mainly used reserves. Increases in the amount of stored N increased N remobilization, which fostered absolute growth both within and across species. Old leaves were major sources of remobilized C and N, but stems and roots also supplied considerable amounts of both in all species except in P. halepensis, which mainly relied on foliage formed in the previous growing season to supply stored resources.

CONCLUSIONS

Seedlings of Mediterranean evergreen trees have distinct C and N storage physiologies, with relative growth rate driving the contribution of remobilized resources to new growth. These differences may reduce competition and facilitate species coexistence.

Are above- and below-ground phenology in sync?

Globally, root production accounts for 33-67% of terrestrial net primary productivity and influences decomposition via root production and turnover, carbon (C) allocation to mycorrhizal fungi and root exudation. As recognized above ground, the timing of phenological events affects terrestrial C balance, yet there is no parallel understanding for below-ground phenology. In this paper we examine the phenology of root production and its relationship to temperature, soil moisture, and above-ground phenology. Synthesizing 87 observations of whole-plant phenology from 40 studies, we found that, on average, root growth occurs 25 ± 8 d after shoot growth but that the offset between the peak in root and shoot growth varies > 200 d across biomes (boreal, temperate, Mediterranean, and subtropical). Root and shoot growth are positively correlated with median monthly temperature and mean monthly precipitation in boreal, temperate, and subtropical biomes. However, a temperature hysteresis in these biomes leads to the hypothesis that internal controls over C allocation to roots are an equally, if not more, important driver of phenology. The specific mechanisms are as yet unclear but they are likely mediated by some combination of photoassimilate supply, hormonal signaling, and growth form.

(11)C-PET imaging reveals transport dynamics and sectorial plasticity of oak phloem after girdling

Carbon transport processes in plants can be followed non-invasively by repeated application of the short-lived positron-emitting radioisotope (11)C, a technique which has rarely been used with trees. Recently, positron emission tomography (PET) allowing 3D visualization has been adapted for use with plants. To investigate the effects of stem girdling on the flow of assimilates, leaves on first order branches of two-year-old oak (Quercus robur L.) trees were labeled with (11)C by supplying (11)CO2-gas to a leaf cuvette. Magnetic resonance imaging gave an indication of the plant structure, while PET registered the tracer flow in a stem region downstream from the labeled branches. After repeated pulse labeling, phloem translocation was shown to be sectorial in the stem: leaf orthostichy determined the position of the phloem sieve tubes containing labeled (11)C. The observed pathway remained unchanged for days. Tracer time-series derived from each pulse and analysed with a mechanistic model showed for two adjacent heights in the stem a similar velocity but different loss of recent assimilates. With either complete or partial girdling of bark within the monitored region, transport immediately stopped and then resumed in a new location in the stem cross-section, demonstrating the plasticity of sectoriality. One day after partial girdling, the loss of tracer along the interrupted transport pathway increased, while the velocity was enhanced in a non-girdled sector for several days. These findings suggest that lateral sugar transport was enhanced after wounding by a change in the lateral sugar transport path and the axial transport resumed with the development of new conductive tissue.

Remobilization of acorn nitrogen for seedling growth in holm oak (Quercus ilex), cultivated with contrasting nutrient availability

The relative contribution of nitrogen (N) reserves from seeds or uptake by the roots to the growth and N content of young seedlings has received little attention. In this study, we investigated the contribution of N from the acorn or uptake by the roots to the N content of holm oak (Quercus ilex L.) seedlings and determined if remobilization of acorn N was affected by nutrient availability in the growing media. Q. ilex seedlings were cultivated for 3 months, until the end of the second shoot flush of growth, with three N fertilization rates: 8.6 mM N, 1.4 mM N or no fertilization. Fertilizer N was enriched in (15)N. Between 62 and 75% of the N contained in high and low fertilized seedlings, respectively, at the end of the second flush of growth was derived from the acorn. However, the dependence on acorn N was greater during the early root growth and first shoot flush of growth and decreased during the second shoot flush of growth, with root uptake contributing 32-54% of plant new N in this latter developmental stage in high and low fertilized plants, respectively. Fertilization rate did not affect the amount of N taken up during the earliest developmental stages, but it increased it during the second shoot flush of growth. Fertilization increased the mass of the shoot segment formed during the second shoot flush of growth and reduced the root mass, with no effect on whole plant growth. Remobilization of acorn N was faster in unfertilized plants than in fertilized plants. It is concluded that the holm oak seedlings depend greatly upon acorn N until the end of the second shoot flush of growth, that significant root N uptake starts at the beginning of the second shoot flush of growth and that acorn N remobilization is a plastic process that is accelerated under extremely low substratum nutrient content.