A Proposed Drought Response Equation Added to the Münch-Horwitz Theory of Phloem Transport

Goeschl, J. D.; Han, Lifeng

doi:10.3389/fpls.2020.505153

Cited by 4 publications

(3 citation statements)

References 34 publications

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“…Interestingly, in a meta‐analysis of maize phloem sap composition (sap collected by exudation) across 17 lines, sucrose was negatively correlated to grain specific weight, while other sugars (such as arabinose) were negatively correlated to grain production (grain biomass per plant) (Yesbergenova‐Cuny et al, 2016). Phloem dynamics modelling suggests that under drought, (i) the decrease in phloem turgor pressure induces a decline in (un)loading transporter capacity, ensuring that phloem sucrose concentration is in the steady‐state (and thus does not increase to unrealistic values), (ii) this effect in turn allows proper sucrose export from leaves and limits sap viscosity, despite a small decline in phloem sap velocity (Goeschl & Han, 2020; Stanfield & Bartlett, 2022). Taken as a whole, sucrose concentration might have increased but likely, did not change dramatically under nonirrigated conditions.…”

Section: Discussionmentioning

confidence: 99%

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Revisiting yield in terms of phloem transport to grains suggests phloem sap movement might be homeostatic

Tcherkez

Holloway‐Phillips

Lothier

et al. 2023

Plant Cell & Environment

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Phloem sap transport, velocity and allocation have been proposed to play a role in physiological limitations of crop yield, along with photosynthetic activity or water use efficiency. Although there is clear evidence that carbon allocation to grains effectively drives yield in cereals like wheat (as reflected by the harvest index), the influence of phloem transport rate and velocity is less clear. Here, we took advantage of previously published data on yield, respiration, carbon isotope composition, nitrogen content and water consumption in winter wheat cultivars grown across several sites with or without irrigation, to express grain production in terms of phloem sucrose transport and compare with xylem water transport. Our results suggest that phloem sucrose transport rate follows the same relationship with phloem N transport regardless of irrigation conditions and cultivars, and seems to depend mostly on grain weight (i.e., mg per grain). Depending on the assumption made for phloem sap sucrose concentration, either phloem sap velocity or its proportionality coefficient to xylem velocity change little with environmental conditions. Taken as a whole, phloem transport from leaves to grains seems to be homeostatic within a narrow range of values and following relationships with other plant physiological parameters across cultivars and conditions. This suggests that phloem transport per se is not a limitation for yield in wheat but rather, is controlled to sustain grain filling.

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Section: Discussionmentioning

confidence: 99%

“…(ii) this effect in turn allows proper sucrose export from leaves and limits sap viscosity, despite a small decline in phloem sap velocity (Goeschl & Han, 2020;Stanfield & Bartlett, 2022). Taken as a whole, sucrose concentration might have increased but likely, did not change dramatically under nonirrigated conditions.…”

Section: Effects Of Water Availabilitymentioning

confidence: 99%

Revisiting yield in terms of phloem transport to grains suggests phloem sap movement might be homeostatic

Tcherkez

Holloway‐Phillips

Lothier

et al. 2023

Plant Cell & Environment

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show abstract

“…The unloading rates depended on the sucrose concentration in the phloem unloading zone, not the concentration gradient between the unloading zone and sink tissue. This also captures passive unloading dynamics for species that enzymatically convert unloaded sucrose to starch (Goeschl and Han, 2020), or use active uptake processes to remove sucrose immediately surrounding the phloem unloading zone into storage vacuoles (Saftner et al, 1983). (2) Unloading did not constrain sucrose export, and all sucrose that reached the unloading zone was exported in the same timestep (i.e., the maximum unloading rate V maxU >> V maxL ).…”

Section: Model Assumptionsmentioning

confidence: 99%

Coordination Between Phloem Loading and Structure Maintains Carbon Transport Under Drought

Stanfield

Bartlett

2022

Front. Plant Sci.

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Maintaining phloem transport under water stress is expected to be crucial to whole-plant drought tolerance, but the traits that benefit phloem function under drought are poorly understood. Nearly half of surveyed angiosperm species, including important crops, use sucrose transporter proteins to actively load sugar into the phloem. Plants can alter transporter abundance in response to stress, providing a potential mechanism for active-loading species to closely regulate phloem loading rates to avoid drought-induced reductions or failures in phloem transport. We developed an integrated xylem-phloem-stomatal model to test this hypothesis by quantifying the joint impacts of transporter kinetics, phloem anatomy, and plant water status on sucrose export to sinks. We parameterized the model with phloem hydraulic resistances and sucrose transporter kinetic parameters compiled from the literature, and simulated loading regulation by allowing loading rates to decline exponentially with phloem pressure to prevent excessive sucrose concentrations from inducing viscosity limitations. In the absence of loading regulation, where loading rates were independent of phloem pressure, most resistance values produced unrealistic phloem pressures owing to viscosity effects, even under well-watered conditions. Conversely, pressure-regulated loading helped to control viscosity buildup and improved export to sinks for both lower and higher resistant phloem pathways, while maintaining realistic phloem pressures. Regulation also allowed for rapid loading and export in wet conditions while maintaining export and viable phloem pressures during drought. Therefore, we expect feedbacks between phloem pressure and loading to be critical to carbon transport in active-loading species, especially under drought, and for transporter kinetics to be strongly coordinated with phloem architecture and plant water status. This work provides an important and underexplored physiological framework to understand the ecophysiology of phloem transport under drought and to enhance the genetic engineering of crop plants.

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Exploring optimal stomatal control under alternative hypotheses for the regulation of plant sources and sinks

2021

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Experimental evidence that nonstomatal limitations to photosynthesis (NSLs) correlate with leaf sugar and/or leaf water status suggests the possibility that stomata adjust to maximise photosynthesis through a trade-off between leaf CO 2 supply and NSLs, potentially involving source-sink interactions. However, the mechanisms regulating NSLs and sink strength, as well as their implications for stomatal control, remain uncertain.We used an analytically solvable model to explore optimal stomatal control under alternative hypotheses for source and sink regulation. We assumed that either leaf sugar concentration or leaf water potential regulates NSLs, and that either phloem turgor pressure or phloem sugar concentration regulates sink phloem unloading.All hypotheses led to realistic stomatal responses to light, CO 2 and air humidity, including conservative behaviour for the intercellular-to-atmospheric CO 2 concentration ratio. Sugarregulated and water-regulated NSLs are distinguished by the presence/absence of a stomatal closure response to changing sink strength. Turgor-regulated and sugar-regulated phloem unloading are distinguished by the presence/absence of stomatal closure under drought and avoidance/occurrence of negative phloem turgor. Results from girdling and drought experiments on Pinus sylvestris, Betula pendula, Populus tremula and Picea abies saplings are consistent with optimal stomatal control under sugar-regulated NSLs and turgor-regulated unloading.Our analytical results provide a simple representation of stomatal responses to aboveground and below-ground environmental factors and sink activity.

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A Proposed Drought Response Equation Added to the Münch-Horwitz Theory of Phloem Transport

Cited by 4 publications

References 34 publications

Revisiting yield in terms of phloem transport to grains suggests phloem sap movement might be homeostatic

Revisiting yield in terms of phloem transport to grains suggests phloem sap movement might be homeostatic

Coordination Between Phloem Loading and Structure Maintains Carbon Transport Under Drought

Exploring optimal stomatal control under alternative hypotheses for the regulation of plant sources and sinks

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