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Growth is a widely used term in plant science and ecology, but it can have different meanings depending on the context and the spatiotemporal scale of analysis. At the meristem level, growth is associated with the production of cells and initiation of new organs. At the organ or plant scale and over short time periods, growth is often used synonymously with tissue expansion, while over longer time periods the increase in biomass is a common metric. At even larger temporal and spatial scales, growth is mostly described as net primary production. Here, we first address the question 'what is growth?'. We propose a general framework to distinguish between the different facets of growth, and the corresponding physiological processes, environmental drivers and mathematical formalisms. Based on these different definitions, we then review how plant growth can be measured and analysed at different organisational, spatial and temporal scales. We conclude by discussing why gaining a better understanding of the different facets of plant growth is essential to disentangle genetic and environmental effects on the phenotype, and to uncover the causalities around source or sink limitations of plant growth.

• Efforts to develop mechanistic tree growth models are hindered by the uncertainty of whether and when tree growth responses to environmental factors are driven by carbon assimilation or by biophysical limitations of wood formation. • In this study, we used multiannual weekly wood-formation monitoring of two conifer species (Larix decidua and Picea abies) along a 900m elevational gradient in the Swiss Alps to assess the biophysical effect of temperature and water potential on wood formation. To this end, we developed a model that simulates the effect of water potential on turgor-driven cambial division, modulated by the effect of temperature on enzymatic activity. • The model reproduced the observed phenology of tracheid production, as well as intra- and interannual tracheid production dynamics of both species along the elevational gradient, although interannual model performance was lower. We found that temperature alone explains the onset of tracheid production, yet water potential appears necessary to predict the ending and the total amount of tracheids produced annually. • We conclude that intra-annual cambial activity is strongly constrained by both temperature and water potential at all elevations, independently of carbon assimilation. At the interannual scale, biophysical constraints likely interact with other factors.

• Photosynthetic stimulation by elevated [CO₂] (e[CO₂]) may be limited by the capacity of sink organs to use photosynthates. In many legumes, N₂-fixing symbionts in root nodules provide an additional sink, so that legumes may be better able to profit from e[CO₂]. However, drought not only constrains photosynthesis but also the size and activity of sinks, and little is known about the interaction of e[CO₂] and drought on carbon sink strength of nodules and other organs. • To compare carbon sink strength, faba bean was grown under ambient (400 ppm) or elevated (700 ppm) atmospheric [CO₂] and subjected to well-watered or drought treatments, and then exposed to 13C pulse-labelling using custom-built chambers to track the fate of new photosynthates. • Drought decreased 13C uptake and nodule sink strength, and this effect was even greater under e[CO₂], and was associated with an accumulation of amino acids in nodules. This resulted in decreased N₂ fixation, and increased accumulation of new photosynthates (13C/sugars) in leaves, which in turn can feed back on photosynthesis. • Our study suggests that nodule C sink activity is key to avoid sink limitation in legumes under e[CO₂], and legumes may only be able to achieve greater C gain if nodule activity is maintained.

In plants, carbon (C) molecules provide building blocks for biomass production, fuel for energy, and exert signalling roles to shape development and metabolism. Accordingly, plant growth is well correlated with light interception and energy conversion through photosynthesis. Because water deficits close stomata and thus reduce C entry, it has been hypothesised that droughted plants are under C starvation and their growth under C limitation. In this review, these points are questioned by combining literature review with experimental and modelling illustrations in various plant organs and species. First, converging evidence is gathered from the literature that water deficit generally increases C concentration in plant organs. The hypothesis is raised that this could be due to organ expansion (as a major C sink) being affected earlier and more intensively than photosynthesis (C source) and metabolism. How such an increase is likely to interact with C signalling is not known. Hence, the literature is reviewed for possible links between C and stress signalling that could take part in this interaction. Finally, the possible impact of water deficit-induced C accumulation on growth is questioned for various sink organs of several species by combining published as well as new experimental data or data generated using a modelling approach. To this aim, robust correlations between C availability and sink organ growth are reported in the absence of water deficit. Under water deficit, relationships weaken or are modified suggesting release of the influence of C availability on sink organ growth. These results are interpreted as the signature of a transition from source to sink growth limitation under water deficit.

[This corrects the article DOI: 10.3389/fpls.2022.1023571.].

Summary Although the analysis of flux data has increased our understanding of the interannual variability of carbon inputs into forest ecosystems, we still know little about the determinants of wood growth. Here, we aimed to identify which drivers control the interannual variability of wood growth in a mesic temperate deciduous forest. We analysed a 9‐yr time series of carbon fluxes and aboveground wood growth (AWG), reconstructed at a weekly time‐scale through the combination of dendrometer and wood density data. Carbon inputs and AWG anomalies appeared to be uncorrelated from the seasonal to interannual scales. More than 90% of the interannual variability of AWG was explained by a combination of the growth intensity during a first ‘critical period’ of the wood growing season, occurring close to the seasonal maximum, and the timing of the first summer growth halt. Both atmospheric and soil water stress exerted a strong control on the interannual variability of AWG at the study site, despite its mesic conditions, whilst not affecting carbon inputs. Carbon sink activity, not carbon inputs, determined the interannual variations in wood growth at the study site. Our results provide a functional understanding of the dependence of radial growth on precipitation observed in dendrological studies.

The yield of sugar beet has continuously increased in the past decades. The question arises, whether this progress will continue in the future. A key factor for increasing yield potential of the crop is breeding progress. It was related to a shift in assimilate partitioning in the plant toward more storage carbohydrates (sucrose), whereas structural carbohydrates (leaves, cell wall compounds) unintendedly declined. The yield potential of sugar beet was estimated at 24 t sugar ha . For maximum yield, sufficient growth factors have to be available and the crop has to be able to fully utilize them. In sugar beet, limitations result from the lacking coincidence of maximum irradiation rates and full canopy cover, sink strength for carbon assimilation and high water demand, which cannot be met by rainfall alone. After harvest, sugar losses during storage occur. The paper discusses options for a further increase in yield potential, like autumn sowing of sugar beet, increasing sink strength and related constraints. It is prospected that yield increase by further widening the ratio of storage and structural carbohydrates will come to its natural limit as a certain cell wall stability is necessary. New challenges caused by climate change and by prolonged processing campaigns will occur. Thus breeding for improved pathogen resistance and storage properties will be even more important for successful sugar beet production than a further increase in yield potential itself.

Climate change has profoundly altered the timing of phenological events, disrupting ecosystem functions and carbon dynamics. However, photosynthetic duration is often overestimated, particularly in autumn, as greenness and photosynthesis tend to decouple. This highlights the need for a more precise understanding of autumn photosynthetic phenology to refine global carbon assessments. In this study, we estimated the end-of-photosynthesis date (EOP) across the Northern Hemisphere (>30° N) from 2001 to 2021 using satellite-derived solar-induced chlorophyll fluorescence and flux tower observations. We then analyzed EOP’s spatiotemporal variations and identified its primary biome-specific drivers. Our results reveal a widespread advancement of EOP, except in water-limited biomes, with the most pronounced shifts in colder regions such as boreal forests and tundra. These shifts are mainly driven by the carryover effects of earlier photosynthetic onset and early-season warming, which are likely influenced by leaf lifespan and sink capacity. Nutrient limitation may further exacerbate sink constraints, particularly in colder biomes. The effects of earlier photosynthetic onset and early-season warming counteract the delaying influence of late-season warming on EOP. The significant advancement of early-stage EOP (EOP75) indicates that early-season drivers have a stronger effect than late-season delays, while late-stage EOP (EOP25) remains nearly unchanged due to a balance of opposing forces. Consequently, the limited photosynthetic gains from an earlier or unchanged EOP may fail to offset respiratory carbon losses, potentially weakening the Northern Hemisphere’s carbon sink capacity and complicating future carbon-climate feedback projections.

Down-regulation of photosynthesis is among the most common responses observed in C3 plants grown under elevated atmospheric CO2 concentration ([CO2]). Down-regulation is often attributed to an insufficient capacity of sink organs to use or store the increased carbohydrate production that results from the stimulation of photosynthesis by elevated [CO2]. Down-regulation can be accentuated by inadequate nitrogen (N) supply, which may limit sink development. While there is strong evidence for down-regulation of photosynthesis at elevated [CO2] in enclosure studies most often involving potted plants, there is little evidence for this when [CO2] is elevated fully under open-air field treatment conditions. To assess the importance of sink strength on the down-regulation of photosynthesis and on the potential of N to mitigate this down-regulation under agriculturally relevant field conditions, two tobacco cultivars ( Nicotiana tabacum L. cv. Petit Havana; cv. Mammoth) of strongly contrasting ability to produce the major sink of this crop, leaves, were grown under ambient and elevated [CO2] and with two different N additions in a free air [CO2] (FACE) facility. Photosynthetic down-regulation at elevated [CO2] reached only 9% in cv. Mammoth late in the season likely reflecting sustained sink strength of the rapidly growing plant whereas down-regulation in cv. Petit Havana reached 25%. Increased N supply partially mitigated down-regulation of photosynthesis in cv. Petit Havana and this mitigation was dependent on plant developmental stage. Overall, these field results were consistent with the hypothesis that sustained sink strength, that is the ability to utilize photosynthate, and adequate N supply will allow C3 crops in the field to maintain enhanced photosynthesis and therefore productivity as [CO2] continues to rise.