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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.

Abortion of reproductive organs is a major limiting factor of yield under water deficit, but is also a trait selected for by evolutionary processes. Youngest reproductive organs must be prone to abortion so older organs can finish their development in case of limited resources. Water deficit increases natural abortion via two developmentally-driven processes, namely a signal from the first fertilized ovaries and a simultaneous arrest of the expansive growth of all ovaries at a precise stage. In maize (Zea mays) subjected to water deficits typically encountered in dryland agriculture, these developmental mechanisms account for 90 % of drought-associated abortion and are irreversible three days after silk emergence. Consistently, transcripts and enzyme activities suggest that the molecular events associated with abortion affect expansive growth in silks whereas ovaries keep a favourable carbon status. Abortion due to carbon starvation is only observed for severe drought scenarios occurring after silking. Both kinetic and genetic evidence indicate that vegetative and reproductive structures share a partly common hydraulic control of expansive growth. Hence, the control of expansive growth of reproductive structures probably has a prominent effect on abortion for mild water deficits occurring at flowering time, while carbon starvation dominates in severe post-flowering drought scenarios.

The growth of fleshy fruits is still poorly understood resulting from the complex integration of water and solute fluxes, cell structural properties, and the regulation of whole plant source-sink relationships. To unravel the contribution of these processes on berry growth, a biophysical grape (Vitis vinifera L.) berry growth module was developed and integrated with a whole-plant functionalstructural model, and was calibrated on two varieties, Cabernet Sauvignon and Sangiovese. The model well captured the variations in growth and sugar accumulation caused by environmental conditions, changes in leaf-to-fruit ratio, plant water status, and varietal differences with obvious future application in predicting yield and maturity under a variety of production contexts and regional climates. Our analyses illustrated that grapevines strive to maintain proper ripening by partially compensating for a reduced source-sink ratio, and that under drought an enhanced berry sucrose uptake capacity can reverse berry shrinkage. Sensitivity analysis highlighted the importance of phloem hydraulic conductance, sugar uptake, and surface transpiration on growth, while suggesting cell wall extensibility and turgor threshold for cell expansion had minor effects. This study demonstrates that this integrated model is a useful tool in understanding the integration and relative importance of different processes in driving fleshy fruit growth.

The elongation of styles and stigma (silks) of maize (Zea mays) flowers is rapid (1–3 mm h−1), occurs over a short period and plays a pivotal role in reproductive success in adverse environments. Silk elongation rate was measured using displacement transducers in 350 plants of eight genotypes during eight experiments with varying evaporative demand and soil water status. Measured time courses revealed that silk elongation rate closely followed changes in soil water status and evaporative demand, with day–night alternations similar to those in leaves. Day–night alternations were steeper with high than with low plant transpiration rate, manipulated via evaporative demand or by covering part of the leaf area. Half times of changes in silk elongation rate upon changes in evaporative demand or soil water status were 10–30 min, similar to those in leaves. The sensitivity of silk elongation rate to xylem water potential was genetically linked to that of leaf elongation rate. Lines greatly differed for these sensitivities. These results are consistent with a common hydraulic control of expansive growth in vegetative and reproductive structures upon changes in environmental conditions via a close connection with the xylem water potential. They have important implications for breeding, modelling and phenotyping.

Expansive plant growth during water deficit is common in temperate and extreme climates. Understanding how the turgor pressure, P, behaves during water deficit is essential for a better understanding of expansive growth rate, v. Here, validated biophysical equations together with dimensional analyses are used to simulate water deficit and determine the behavior of P and v. A dimensionless number, Πpw, helps simplify the biophysical equations and interpret the results. The magnitude of Πpw increases as water deficit severity increases. Analyses reveal that both P and v decrease curvilinearly as Πpw increases. Simple mathematical relationships between P, v, and Πpw, are derived, providing a clear and quantitative understanding of how P and v change as water deficits become more severe. Additionally, it is shown how the results of these analyses can be used to assess P and v of roots growing in water deficit.

Turgor pressure provides the force needed to stress and deform the cell walls of plants, algae, and fungi during expansive growth. However, turgor pressure plays another subtle but equally important role in expansive growth of walled cells: it connects the two biophysical processes of water uptake and wall deformation to ensure that the volumetric rates of water uptake and enlargement of the cell wall chamber are equal. In this study, the role of turgor pressure as a ‘connector’ is investigated analytically by employing validated and established biophysical equations. The objective is to determine the effect of ‘wall loosening’ on the magnitude of turgor pressure. It is known that an increase or decrease in turgor pressure and/or wall loosening rate increases or decreases the expansive growth rate, respectively. Interestingly, it is shown that an increase in the wall loosening rate decreases the turgor pressure slightly, thus reducing the effect of wall loosening on increasing the expansive growth rate. Other analyses reveal that reducing the rate of water uptake results in a larger decrease in turgor pressure with the same increase in wall loosening rate, which further reduces the effect of wall loosening on increasing the expansive growth rate.

Water transport is an integral part of the process of growth by cell expansion and accounts for most of the increase in cell volume characterizing growth. Under water deficiency, growth is readily inhibited and growth of roots is favoured over that of leaves. The mechanisms underlying this differential response are examined in terms of Lockhart's equations and water transport. For roots, when water potential (Ψ) is suddenly reduced, osmotic adjustment occurs rapidly to allow partial turgor recovery and re‐establishment of Ψ gradient for water uptake, and the loosening ability of the cell wall increases as indicated by a rapid decline in yield‐threshold turgor. These adjustments permit roots to resume growth under low Ψ. In contrast, in leaves under reductions in Ψ of similar magnitude, osmotic adjustment occurs slowly and wall loosening ability either does not increase substantially or actually decreases, leading to marked growth inhibition. The growth region of both roots and leaves are hydraulically isolated from the vascular system. This isolation protects the root from low Ψ in the mature xylem and facilitates the continued growth into new moist soil volume. Simulations with a leaky cable model that includes a sink term for growth water uptake show that growth zone Ψ is barely affected by soil water removal through transpiration. On the other hand, hydraulic isolation dictates that Ψ of the leaf growth region would be low and subjected to further reduction by high evaporative demand. Thus, a combination of transport and changes in growth parameters is proposed as the mechanism co‐ordinating the growth of the two organs under conditions of soil moisture depletion. The model simulation also showed that roots behave as reversibly leaky cable in water uptake. Some field data on root water extraction and vertical profiles of Ψ in shoots are viewed as manifestations of these basic phenomena. Also discussed is the trade‐off between high xylem conductance and strong osmotic adjustment.

A cell-growth-based algorithm is presented based on the biomechanics of plant cells in tissues to help explain the typical differences in cellular architecture found between different pome fruit species, cultivars and tissues. The cell was considered as a closed thin-walled structure, maintained in tension by turgor pressure. The cell walls of adjacent cells were modelled as parallel and linearly elastic elements, which obeyed Hooke’s law. A Voronoi tessellation was used to generate the initial topology of the cells. Cell expansion then resulted from turgor pressure acting on the yielding cell wall material. To find the sequence positions of each vertex of the cell walls, and thus, the shape of the cells with time, a system of differential equations for the positions and velocities of each vertex were established and solved using a Runge–Kutta fourth and fifth order (ODE45) method. The model was used to generate realistic 2D fruit tissue structures composed of cells of random shapes and sizes, cell walls and intercellular spaces. Comparison was made with fruit tissue micrographs. The virtual tissues can be used for numerical simulation of heat and mass transfer phenomena or mechanical deformation during controlled atmosphere storage of fresh pome fruit.

Cells of algae, fungi, and plants have walls and exhibit expansive growth which can increase their volume by as much as 10,000 times. Expansive growth is central to their morphogenesis, development, and sensory responses to environmental stimuli. Equations describing the biophysical processes of the water uptake rate and the wall deformation rate have been derived, validated, and established. A significant amount of research provides insight into the molecular underpinnings of these processes. What is less well known are the relative magnitudes of these processes and how they compare during expansive growth and with walled cells from other species. Here, dimensionless numbers (Π parameters) are used to determine the magnitudes of the biophysical processes involved in the expansive growth rate of cells from algae (Chara corallina), fungi (Phycomyces blakesleeanus), and plants (Pisum satinis L.). It is found for all three species that the cell’s capability for the water uptake rate is larger than the wall plastic deformation rate and much larger than the wall elastic deformation rate. Also, the wall plastic deformation rates of all three species are of similar magnitude as their expansive growth rate even though the stress relaxation rates of their walls are very different. It is envisioned that dimensionless numbers can assist in determining how these biophysical processes change during development, morphogenesis, sensory responses, environmental stress, climate change, and after genetic modification.

A considerable amount of research has been conducted to determine how cell walls are loosened to produce irreversible wall deformation and expansive growth in plant and algal cells. The same cannot be said about fungal cells. Almost nothing is known about how fungal cells loosen their walls to produce irreversible wall deformation and expansive growth. In this study, anoxia is used to chemically isolate the wall from the protoplasm of the sporangiophores of Phycomyces blakesleeanus. The experimental results provide direct evidence of the existence of chemistry within the fungal wall that is responsible for wall loosening, irreversible wall deformation and elongation growth. In addition, constant-tension extension experiments are conducted on frozen-thawed sporangiophore walls to obtain insight into the wall chemistry and wall loosening mechanism. It is found that a decrease in pH to 4.6 produces creep extension in the frozen-thawed sporangiophore wall that is similar, but not identical, to that found in frozen-thawed higher plant cell walls. Experimental results from frozen-thawed and boiled sporangiophore walls suggest that protein activity may be involved in the creep extension.