A quadratic analytical solution of root pressure generation provides insights about bamboo and other species

We derived a steady-state model of whole root pressure generation through the combined action of all parallel segments of fine roots. This may be the first complete analytical solution for root pressure, which can be applied to complex roots/shoots. The osmotic volume of a single root is equal to that of the vessel lumen in fine roots and adjacent apoplastic spaces. Water uptake occurs via passive osmosis and active solute uptake (J_s^*, osmol s-1), resulting in the osmolal concentration Cr (mol·kg-1 of water) at a fixed osmotic volume. Solute loss occurs via two passive processes: radial diffusion of solute Km (Cr-Csoil), where Km is the diffusional constant and Csoil is the soil-solute concentration) from fine roots to soil and mass flow of solute and water into the whole plant from the end of the fine roots. The proposed model predicts the quadratic function of root pressure P_r^2+bP_r+c=0, where b and c are the functions of plant hydraulic resistance, soil water potential, solute flux, and gravitational potential. The present study investigates the theoretical dependencies of Pr on the factors detailed above and demonstrates the root pressure-mediated distribution of water through the hydraulic architecture of a 6.8-m-tall bamboo shoot.

The plant water pump: why water flows uphill of water potential gradients in a root hydraulic anatomy model

<p>Guttation is the exudation of xylem sap from vascular plant leaves. This process is particularly interesting because in its configuration root water uptake occurs against the hydrostatic pressure driving force. Hence, it emphasizes the contribution of another driving force that lifts water in plants: the osmotic potential gradient.</p><p>The current paradigm of root water uptake explains that, due to the endodermal apoplastic barrier, water flows across root radius from the same principles as through selective membranes: driven by the total water potential gradient. This theory relies on the idea that during guttation, osmolites loaded in xylem vessels decrease xylem total water potential, making it more negative than the total soil water potential, and generating water inflow by osmosis as in an osmometer.</p><p>However, this theory fails at explaining experiments in which guttation occurs without sufficient solute loading in root xylem of maize (Enns et al., 1998; Enns et al., 2000) and arrowleaf saltbush (Bai et al., 2007) among others; studies concluding that experimental observations “could not be explained with the current theories in plant physiology”. Such flow rates towards combined increasing pressure potentials and increasing osmotic potentials between separate apoplastic compartments would necessitate an effective root radial conductivity that is negative; a mind bender.</p><p>What piece of hydraulic network would make it possible for water to flow against the total water potential driving force?</p><p>We implemented Steudle’s composite water transport model in the explicit root cross-section anatomical hydraulic network MECHA (Couvreur et al., 2018). All apoplastic, transmembrane and symplastic pathways are interconnected in the network. The results show that while root radial conductivity is particularly sensitive to cell membrane permeability, the combination of conductive plasmodesmata and increased dilution of protoplast osmotic potentials inwards is a key to explain root water flow towards increasing total potentials. A triple cell theory is suggested as new paradigm of root radial flow.</p><p><strong>References</strong></p><p>Bai X-F, Zhu J-J, Zhang P, Wang Y-H, Yang L-Q, Zhang L (2007) Na+ and Water Uptake in Relation to the Radial Reflection Coefficient of Root in Arrowleaf Saltbush Under Salt Stress. Journal of Integrative Plant Biology 49: 1334-1340</p><p>Couvreur V, Faget M, Lobet G, Javaux M, Chaumont F, Draye X (2018) Going with the Flow: Multiscale Insights into the Composite Nature of Water Transport in Roots. Plant Physiology 178: 1689-1703</p><p>Enns LC, Canny MJ, McCully ME (2000) An investigation of the role of solutes in the xylem sap and in the xylem parenchyma as the source of root pressure. Protoplasma 211: 183-197</p><p>Enns LC, McCully ME, Canny MJ (1998) Solute concentrations in xylem sap along vessels of maize primary roots at high root pressure. J. Exp. Bot. 49: 1539-1544</p>

Drought-Induced Root Pressure in Sorghum bicolor

Root pressure, also manifested as profusive sap flowing from cut stems, is a phenomenon in some species that has perplexed biologists for much of the last century. It is associated with increased crop production under drought, but its function and regulation remain largely unknown. In this study, we investigated the initiation, mechanisms, and possible adaptive function of root pressure in six genotypes of Sorghum bicolor during a drought experiment in the greenhouse. We observed that root pressure was induced in plants exposed to drought followed by re-watering but possibly inhibited by 100% re-watering in some genotypes. We found that root pressure in drought stressed and re-watered plants was associated with greater ratio of fine: coarse root length and shoot biomass production, indicating a possible role of root allocation in creating root pressure and adaptive benefit of root pressure for shoot biomass production. Using RNA-Seq, we identified gene transcripts that were up- and down-regulated in plants with root pressure expression, focusing on genes for aquaporins, membrane transporters, and ATPases that could regulate inter- and intra-cellular transport of water and ions to generate positive xylem pressure in root tissue.

Connecting the dots between root cross-section images and modelling tools to create a high resolution root system hydraulic maps in Zea mays.

Root hydraulic properties play a central role in the global water cycle, agricultural systems productivity, and ecosystem survival as they impact the global canopy water supply. However, the available experimental methods to quantify root hydraulic conductivities, such as the root pressure probing, are particularly challenging and their applicability on thin roots and small root segments is limited. There is a gap in methods enabling easy estimations of root hydraulic conductivities across a diversity of root types and at high resolution along root axes. In this case study, we analysed Zea mays (maize) plants of the var. B73 that were grown in pots for 14 days. Root cross-section data were used to extract anatomical measurements. We used the Generator of Root Anatomy in R (GRANAR) model to generate root anatomical networks from anatomical features. Then we used the Model of Explicit Cross-section Hydraulic Anatomy (MECHA) to compute an estimation of the root axial and radial hydraulic conductivities (kx and kr, respectively), based on the generated anatomical networks and cell hydraulic properties from the literature. The root hydraulic conductivity maps obtained from the root cross-sections suggest significant functional variations along and between different root types. Predicted variations of kr along the root axis were strongly dependent on the maturation stage of hydrophobic barriers. The same was also true for the maturation rates of the metaxylem. The different anatomical features, as well as their evolution along the root type add significant variation to the kr estimation in between root type and along the root axe. Under the prism of root types, anatomy, and hydrophobic barriers, our results highlight the diversity of root radial and axial hydraulic conductivities, which may be veiled under low-resolution measurements of the root system hydraulic conductivity. While predictions of our root hydraulic maps match the range and trend of measurements reported in the literature, future studies could focus on the quantitative validation of hydraulic maps. From now on, a novel method, which turns root cross-section images into hydraulic maps will offer an inexpensive and easily applicable investigation tool for root hydraulics, in parallel to root pressure probing experiments.

Loss and recovery of leaf hydraulic conductance: Root pressure, embolism, and extra-xylary resistance

Vascular networks in plant leaves must provide for the safe and efficient transport of water, nutrients, and energy; however, the conditions whereby these networks lose and regain conductive capacity are still poorly understood. We measured the loss and recovery of leaf hydraulic conductance (Kleaf) in a tropical monocotyledon (Bambusa vulgaris) and dicotyledon species (Bauhinia blakeana) using Rehydration Kinetics Method (RKM) as well as a recently developed optical method. We found that both species lost ca 88% of their maximal Kleaf (measured by RKM) before any embolization was detected in their conductive elements (via optical observation). This suggests that the majority of loss in Kleaf, as measured with RKM, was associated with resistances other than embolization. Furthermore, embolism in B. vulgaris, a species known to generate root pressure, was reversed when rehydrated under positive pressure (120 kPa), but not under atmospheric pressure. In contrast, embolism was not reversed in B. blakeana under either elevated or atmospheric pressure. However, reductions in Kleaf that was not associated with embolization was recovered by this species when rehydrated under atmospheric conditions, whereas B. vulgaris did not exhibit any recovery under the same conditions. We suggest that root pressure is an adaptive mechanism allowing for the reversal of embolism and the recovery of extraxylary conductance. The absence of embolism reversal in B. blakeana suggests that embolization may be permanent without neutral or positive xylem pressure, but that the recovery of extraxylary conductance can be regained routinely in this species in the absence of root pressure.

Differences in hydraulic traits of grapevine rootstocks are not conferred to a common Vitis vinifera scion

Cultivars of grapevine are commonly grafted onto rootstocks to improve resistance against biotic and abiotic stress, however, it is not clear whether known differences in hydraulic traits are conferred from rootstocks to a common scion. We recently found that Vitis riparia and Vitis champinii differed in drought-induced embolism susceptibility and repair, which was related to differences in root pressure generation after rewatering (Knipfer et al. 2015). In the present study, we tested whether these and other physiological responses to drought are conferred to a common V. vinifera scion (Cabernet Sauvignon) grafted on V. riparia and V. champinii rootstocks. We measured xylem embolism formation/repair using in vivo microCT imaging, which was accompanied with analysis of leaf gas exchange, osmotic adjustment and root pressure. Our data indicate that differences in scion physiological behaviour for both rootstock combinations were negligible, suggesting that the sensitivity of Cabernet Sauvignon scion to xylem embolism formation/repair, leaf gas exchange and osmotic adjustment is unaffected by either V. riparia or V. champinii rootstock in response to drought stress.

Whole-Genome and Expression Analyses of Bamboo Aquaporin Genes Reveal Their Functions Involved in Maintaining Diurnal Water Balance in Bamboo Shoots

Water supply is essential for maintaining normal physiological function during the rapid growth of bamboo. Aquaporins (AQPs) play crucial roles in water transport for plant growth and development. Although 26 PeAQPs in bamboo have been reported, the aquaporin-led mechanism of maintaining diurnal water balance in bamboo shoots remains unclear. In this study, a total of 63 PeAQPs were identified, based on the updated genome of moso bamboo (Phyllostachys edulis), including 22 PePIPs, 20 PeTIPs, 17 PeNIPs, and 4 PeSIPs. All of the PeAQPs were differently expressed in 26 different tissues of moso bamboo, based on RNA sequencing (RNA-seq) data. The root pressure in shoots showed circadian rhythm changes, with positive values at night and negative values in the daytime. The quantitative real-time PCR (qRT-PCR) result showed that 25 PeAQPs were detected in the base part of the shoots, and most of them demonstrated diurnal rhythm changes. The expression levels of some PeAQPs were significantly correlated with the root pressure. Of the 86 sugar transport genes, 33 had positive co-expression relationships with 27 PeAQPs. Two root pressure-correlated PeAQPs, PeTIP4;1 and PeTIP4;2, were confirmed to be highly expressed in the parenchyma and epidermal cells of bamboo culm, and in the epidermis, pith, and primary xylem of bamboo roots by in situ hybridization. The authors’ findings provide new insights and a possible aquaporin-led mechanism for bamboo fast growth.

Tipburn Severity and Calcium Distribution in Lisianthus (Eustoma Grandiflorum (Raf.) Shinn.) Cultivars under Different Relative Air Humidity Conditions

Tipburn is a major problem for the production of lisianthus (Eustoma grandiflorum (Raf.) Shinn.) cultivars. Relative air humidity is regarded as a key environmental factor affecting tipburn severity in commercial crops. However, there are limited studies comparing the occurrence of tipburn and calcium (Ca) distribution in lisianthus cultivars under different relative air humidity conditions. Accordingly, we investigated the effect of relative air humidity on tipburn severity, transpiration rate, and Ca content in seven lisianthus cultivars. Under a high humidity treatment (70%), only two cultivars (“Voyage pink” (VP) and “Azuma-no-kaori” (AK)) showed significantly higher tipburn severity than those under a low humidity treatment (50%), which suggests that high humidity conditions do not always increase tipburn severity in lisianthus. Transpiration rates of all cultivars, except for AK, were either significantly lower under the high humidity treatment than under the low humidity treatment, or did not vary significantly between the treatments. In contrast, total Ca concentrations in all cultivars, except for “Piccolosa snow” (PS), were significantly higher under the high humidity treatment than under the low humidity treatment. These results suggest that Ca acquisition and distribution in lisianthus cultivars are strongly influenced by Ca uptake from root pressure.