Carbon Source-sink Relationships Research Articles

Partial asynchrony of coniferous forest carbon sources and sinks at the intra-annual time scale

As major terrestrial carbon sinks, forests play an important role in mitigating climate change. The relationship between the seasonal uptake of carbon and its allocation to woody biomass remains poorly understood, leaving a significant gap in our capacity to predict carbon sequestration by forests. Here, we compare the intra-annual dynamics of carbon fluxes and wood formation across the Northern hemisphere, from carbon assimilation and the formation of non-structural carbon compounds to their incorporation in woody tissues. We show temporally coupled seasonal peaks of carbon assimilation (GPP) and wood cell differentiation, while the two processes are substantially decoupled during off-peak periods. Peaks of cambial activity occur substantially earlier compared to GPP, suggesting the buffer role of non-structural carbohydrates between the processes of carbon assimilation and allocation to wood. Our findings suggest that high-resolution seasonal data of ecosystem carbon fluxes, wood formation and the associated physiological processes may reduce uncertainties in carbon source-sink relationships at different spatial scales, from stand to ecosystem levels.

Comparative transcription profiles reveal that carbohydrates and hormone signalling pathways mediate flower induction in Juglans sigillata after girdling

Flower induction in iron walnut (Juglans sigillata) is a complex and highly regulated process involving multiple signal pathways to ensure flower bud formation in the subsequent year. However, the underlying molecular mechanism of the induction remains unknown. Girdling is an effective method for inducing flower bud formation in iron walnut. In this study, comparative transcriptome profiles between girdled (G) and ungirdled (unG) materials on the same tree were used to identify the signal pathway involved in flower bud induction by girdling in iron walnut. The results showed that genes related to starch and carbohydrate metabolism, photosynthesis, and hormone signals, including pectinesterase (PAE), trehalose 6-photosphatase (TPP), α-aminolase (α-AL), atpF, and auxin response factor ARF9, participated in the induction process after girdling. Expression pattern of genes related to sugar metabolism revealed that sugar was the initial signal for flower induction and that photosynthesis increased after girdling, influenced by the carbon source–sink relationship. Hormone signalling networks including auxin, abscisic acid (ABA), and brassinolide also contributed to flower bud induction of iron walnut. Auxin played an important role in tissue regeneration and flower induction after girdling. ABA directly participated in flower induction. Gene expression pattern similarities between ABA and sugar metabolism signals indicated that ABA may also be involved in sugar regulation, in turn regulating floral induction. In summary, flower induction after girdling in iron walnut is mainly regulated by a polygenic network centred on sugar and hormone signals.

Analysis of the impact of carbon source-sink relationships on flowering patterns reveals that apple tree growth and functioning are determined by mechanisms occurring at the tree and shoot scales

Analysis of the impact of carbon source-sink relationships on flowering patterns reveals that apple tree growth and functioning are determined by mechanisms occurring at the tree and shoot scales

Variations in carbon source-sink relationships in subalpine fir across elevational gradients.

Cold-adapted trees display acclimation in both carbon source and carbon sink capacity to low-temperature stress at their upper elevational range limits. Hence a balanced carbon source-sink capacity might be required for their persistence and survival at the elevational tree limits. The present study examined the spatial dynamics of carbon source-sink relationship in subalpine fir (Abies fargesii) trees along elevational gradients in the northern slope of the temperate region and in the southern slope of the subtropics in terms of climate in the Qinling Mountain range, north-central China. The results showed that non-structural carbohydrate (NSC) concentrations in both the source and sink tissues increased with the increase in elevation. The ratio of carbon source-sink displayed a consistent decreasing trend with the increase in elevation and during growing season, showing that it was lowest at a ratio of 2.93 in the northern slope and at a ratio of 2.61 in the southern slope at the upper distribution elevations in the late growing season. Such variations of carbon source-sink ratio might be attributable to the balance between carbon source and sink activities, which changed seasonally across the elevational distribution range. We concluded that a ratio of carbon source-sink of at least 2.6 might be essential for subalpine fir trees to persist at their upper range limits. Therefore, a sufficient source-sink ratio and a balanced source-sink relationship might be required for subalpine fir trees to survive and develop at their upper elevational distribution limits.

Growth and carbon balance are differently regulated by tree and shoot fruiting contexts: an integrative study on apple genotypes with contrasted bearing patterns.

In plants, carbon source-sink relationships are assumed to affect their reproductive effort. In fruit trees, carbon source-sink relationships are likely to be involved in their fruiting behavior. In apple, a large variability in fruiting behaviors exists, from regular to biennial, which has been related to the within-tree synchronization vs desynchronization of floral induction in buds. In this study, we analyzed if carbon assimilation, availability and fluxes as well as shoot growth differ in apple genotypes with contrasted behaviors. Another aim was to determine the scale of plant organization at which growth and carbon balance are regulated. The study was carried out on 16 genotypes belonging to three classes: (i) biennial, (ii) regular with a high production of floral buds every year and (iii) regular, displaying desynchronized bud fates in each year. Three shoot categories, vegetative and reproductive shoots with or without fruits, were included. This study shows that shoot growth and carbon balance are differentially regulated by tree and shoot fruiting contexts. Shoot growth was determined by the shoot fruiting context, or by the type of shoot itself, since vegetative shoots were always longer than reproductive shoots whatever the tree crop load. Leaf photosynthesis depended on the tree crop load only, irrespective of the shoot category or the genotypic class. Starch content was also strongly affected by the tree crop load with some adjustments of the carbon balance among shoots since starch content was lower, at least at some dates, in shoots with fruits compared with the shoots without fruits within the same trees. Finally, the genotypic differences in terms of shoot carbon balance partly matched with genotypic bearing patterns. Nevertheless, carbon content in buds and the role of gibberellins produced by seeds as well as the distances at which they could affect floral induction should be further analyzed.

Understanding crop physiology to select breeding targets and improve crop management under increasing atmospheric CO2 concentrations

The present overview paper reviews knowledge on plant metabolism under elevated atmospheric CO2 concentrations (e[CO2]) with regard to underpinning options for the management of crop production systems and the selection of crop traits beneficial for future conditions.Better understanding of intra-specific variability in responses to e[CO2] is of great importance to breed or select best possible genotypes for future conditions. Yield increases per 100μLL−1 increase in [CO2] varied between none and over 30% among varieties of important crops. Carbon source–sink relationships are believed to play a major role in determining the ability of a plant to utilise e[CO2] and avoid downward acclimation of photosynthesis upon prolonged e[CO2] exposure. Corresponding traits (e.g. tillering capacity, stem carbohydrate storage capacity, or seed size and numbers) are currently under investigation in Free Air Carbon dioxide Enrichment (FACE) facilities, such as AGFACE (Australian Grains FACE).The stimulatory effect of e[CO2] on plant growth is dependent on adequate nutrient supply. For example, N concentrations in plant tissues generally decrease under e[CO2], which in leaves is commonly related to a decrease in Rubisco concentration and activity, and therefore linked to photosynthetic downward acclimation. This effect is also of direct concern for food production where decreased N and protein content can have negative effects on product quality (e.g. grain protein). Plant nutrient metabolism appears to adjust to a new physiological equilibrium under e[CO2] which limits the extent to which nutrient application can ameliorate the situation. What the control points are for an adjustment of plant N metabolism is unclear. Rubisco metabolism in leaves, N assimilation, N translocation or N uptake are all potential key steps that may be inhibited or downregulated under e[CO2]. To achieve the best possible growth response whilst maintaining product quality, it is important to understand plant nutrient metabolism under e[CO2].Comparatively little is known about mechanisms of potential changes in plant stress tolerance under e[CO2]. Defence metabolites such as antioxidants are, in part, directly linked to primary carbohydrate mechanism and so potentially impacted by e[CO2]. It is unknown whether photoprotective and antioxidative defence systems, key to plant stress tolerance, will be affected, and if so, whether the response will be strengthened or weakened by e[CO2]. Better understanding of underlying principles is particularly important because it is virtually impossible to test all possible stress factor combinations with e[CO2] in realistic field settings.

Nitrogen and carbon source–sink relationships in trees at the Himalayan treelines compared with lower elevations

No single hypothesis or theory has been widely accepted for explaining the functional mechanism of global alpine/arctic treeline formation. The present study tested whether the alpine treeline is determined by (1) the needle nitrogen content associated with photosynthesis (carbon gain); (2) a sufficient source-sink ratio of carbon; or (3) a sufficient C-N ratio. Nitrogen does not limit the growth and development of trees studied at the Himalayan treelines. Levels of non-structural carbohydrates (NSC) in trees were species-specific and site-dependent; therefore, the treeline cases studied did not show consistent evidence of source/carbon limitation or sink/growth limitation in treeline trees. However, results of the combined three treelines showed that the treeline trees may suffer from a winter carbon shortage. The source capacity and the sink capacity of a tree influence its tissue NSC concentrations and the carbon balance; therefore, we suggest that the persistence and development of treeline trees in a harsh alpine environment may require a minimum level of the total NSC concentration, a sufficiently high sugar:starch ratio, and a balanced carbon source-sink relationship.

Belowground carbon pools and processes in different age stands of Douglas-fir

Forest floor material and soil organic matter may act as both a source and a sink in global CO2 cycles. Thus, the ecosystem processes controlling these pools are central to understanding the transfers of carbon (C) between the atmosphere and terrestrial systems. To examine these ecosystem processes, the effect of stand age on temporal carbon source-sink relationships was examined in 20-year-old, 40-year-old and old-growth stands of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) in the Cascade Mountains of south-central Washington State. Belowground C and nitrogen (N) storage and soil respiration were measured. In addition, nylon mesh bags containing homogenized soils from each site were buried at the respective sites to quantify root ingrowth and potential C sequestration and loss. The sites supporting the 20- and 40-year-old stands had soil C stores reflecting the C contributions from logging residue, coarse woody debris and stumps left after harvest. Because the N-fixer red alder (Alnus rubra Bong.) comprised 33% of the 40-year-old stand, this site had significantly greater concentrations and pools of N in the forest floor than sites without red alder. This N-rich site had consistently lower soil CO2 efflux rates during the growing season than the sites supporting the 20-year-old and old-growth stands. Estimated annual soil C efflux was 1367, 883 and 1194 g m-2 for the sites supporting the 20-, 40- and old-growth stands, respectively. These values are higher than previously reported values. Root ingrowth was significantly less in the 40-year-old stand than in the 20-year-old stand, and both young stands showed markedly less fine root growth than the old-growth stand. At the sites supporting the young stands, C and N were lost from the soil bags, whereas there was an increase in C and N in the soil bags at the site supporting the old-growth stand. The fine root growth and soil respiration data support the hypothesis that belowground C allocation decreases with increasing fertility. Quantification of the source-sink relationship of soil C at the three stands based on litterfall, relative root ingrowth and soil respiration measurements was compromised because of significant CO2 flux from decaying organic matter in the young stands.

Temperature sensitivity of soil respiration and its effects on ecosystem carbon budget: nonlinearity begets surprises

Nonlinearity is a salient feature in all complex systems, and it certainly characterizes biogeochemical cycles in ecosystems across a wide range of scales. Soil carbon emission is a major source of uncertainty in estimating the terrestrial carbon budget at the ecosystem level and above. Due to the lack of consideration of the nonlinearity in temperature sensitivity of soil respiration, several commonly used ecosystem models produce substantially different estimates of soil respiration with the same or similar model input. In this paper we demonstrated that the response of soil respiration to changes in temperature sensitivity is nonlinear and, thus, that the oversimplified formulations may significantly reduce the accuracy of ecosystem models in predicting carbon fluxes. To alleviate this problem, we have developed a general model of temperature sensitivity of soil respiration that explicitly considers this nonlinearity. The model was supported by our field measurements from a forest ecosystem, and used to assess the uncertainty in estimating the soil CO 2 efflux with several commonly used ecosystem models. Our results indicated that the variations and nonlinearity of the soil respiration-temperature relationship and its dependence on moisture may have important implications for ecosystem carbon modeling at regional and global scales. In other words, ‘small causes’ may lead to ‘large effects’ in complex ecosystems in terms of carbon dynamics. In particular, when the variability in temperature sensitivity of soil respiration was incorporated in the several commonly used ecosystem models, the carbon source-sink relationship for terrestrial ecosystems under future global warming scenarios became dramatically different from those reported previously. Thus, we advocate that confidence limits are both necessary and feasible for simulated carbon budget from ecosystem models. Based on field measurements and model simulations, our study provides useful information for computing such confidence limits. In addition, our new model of temperature sensitivity of soil respiration seems more general and yet realistic, and can improve the accuracy of ecosystem models in predicting carbon fluxes at large scales.

Soil temperature, nitrogen mineralization, and carbon source–sink relationships in boreal forests

Boreal forests contain large quantities of soil carbon, prompting concern that climatic warming may stimulate decomposition and accentuate increasing atmospheric CO2 concentrations. While soil warming increases decomposition rates, the accompanying increase in nutrient mineralization may promote tree growth in these nutrient-poor soils and thereby compensate for the increased carbon loss during decomposition. We used a model of production and decomposition to test this hypothesis. In black spruce (Piceamariana (Mill.) B.S.P.), white spruce (Piceaglauca (Moench) Voss), and paper birch (Betulapapyrifera Marsh.) forests, decomposition increased with the soil warming caused by a 5 °C increase in air temperature. However, increased nitrogen mineralization promoted tree growth, offsetting the increased carbon loss during decomposition. In the black spruce forest, increased tree production was maintained for the 25 years of simulation. Whether this can be maintained indefinitely is unknown. In the birch forest, tree production decreased to prewarming levels after about 10 years. Our analyses examined only the consequences of belowground feedbacks that affect ecosystem carbon uptake with climatic warming. These analyses highlight the importance of interactions among net primary production, decomposition, and nitrogen mineralization in determining the response of forest ecosystems to climatic change.

Carbon Source‐Sink Relationships within Narrow‐Row Cotton Canopies1

Cotton (Gossypium hirsutum L.) is an indeterminate crop in which vegetative and reproductive sinks compete for photosynthetic assimilates. The objective of this study was to evaluate the carbon source sink relationship in cotton canopies with differing leaf types grown under narrow‐row culture and to determine if measurement of specific leaf weight (SLW) can be used as a simplified evaluation of photosynthetic rate within cotton canopies. Three near‐isogenic lines of ‘Stoneville 7A’ (normal, okra, and superokra) cotton which differed in blade area/leaf were grown in narrow‐row culture at a plant density of 10 plants m‐2. During the 3rd (29 July) and 5th (13 August) weeks of flowering, whole plants were exposed to 14CO2 and evaluated at three height intervals (0 to 30 cm, 30 to 55 cm, and above 55 cm) for leaf area index (LAI), 14CO2 uptake, and biomass distribution. Okra and superokra leaf types generally had higher 14CO2 uptake within the canopy on a leaf area basis, but lower LAI than normal leaf plants. Uptake of 14CO2 among leaf types on a land area basis was different only on 13 August in the top interval where the ranking was normal > okra > superokra. By 13 August, the top interval accounted for 81, 77, and 69% of the canopy 14Cassimilate production, but contained only 29, 28, and 32% of the fruit dry weight for normal, okra, and superokra leaf plants, respectively. Mutant leaf types had a larger percentage of dry weight as fruit and less as leaves and petioles compared to normal leaf plants. This study suggests that vegetative growth is favored at the expense of reproductive growth due to high LAI at the top of normal leaf canopies when grown in narrow‐row culture. However, the mutant leaf types tend to have less biomass resulting in only equivalent yields to normal leaf plants when grown under full‐season culture. Significant correlation coefficients between C uptake and SLW ranged between 0.72 and 0.89 depending upon time of SLW measurement and sampling date, but were not altered by leaf type. Use of SLW is a simple method for estimating photosynthetic activity of leaves within cotton canopies.