Analysis of Non-Structural Carbohydrates and Xylem Anatomy of Leaf Petioles Offers New Insights in the Drought Response of Two Grapevine Cultivars

Xylem plasticity of root, stem, and branch in Cunninghamia lanceolata under drought stress: implications for whole-plant hydraulic integrity

Introduction

A better understanding of xylem hydraulic characteristics in trees is critical to elucidate the mechanisms of forest decline and tree mortality from water deficit. As well as temperate forests and forests growing in arid regions, subtropical and tropical forests are also predicted to experience an increased frequency and intensity of climate change-induced drought in the near future.

Methods

In this study, 1-year-old Cunninghamia lanceolata seedlings (a typical subtropical species in southern China) were selected for a continuous controlled drought pot experiment of 45 days duration. The experimental treatments were non-drought (control), light drought, moderate drought and severe drought stress, which were 80%, 60%, 50%, and 40%, respectively of soil field maximum moisture capacity.

Results

The hydraulic conductivity, specific conductivity and water potential of roots, stems, and branches of C. lanceolata all decreased with the prolonging of drought in the different drought intensities. The relative decrease in these hydraulic values were greater in roots than in stems and branches, indicating that roots are more sensitive to drought. Root tracheid diameters normally reduce to ensure security of water transport with prolonged drought, whilst the tracheid diameters of stems and branches expand initially to ensure water transport and then decrease to reduce the risk of embolism with continuing drought duration. The pit membrane diameter of roots, stems and branches generally increased to different extents during the 15-45 days drought duration, which is conducive to enhanced radial water transport ability. The tracheid density and pit density of stems generally decreased during drought stress, which decreased water transport efficiency and increased embolism occurrence. Correlation analysis indicated that anatomical plasticity greatly influenced the hydraulic properties, whilst the relationships varied among different organs. In roots, tracheid diameter decreased and tracheid density increased to enhance water transport security; stems and branches may increase tracheid diameter and pit membrane diameter to increase hydraulic conductivity ability, but may increase the occurrence of xylem embolism.

Discussion

In summary, under drought stress, the xylem anatomical characteristics of C. lanceolata organs were highly plastic to regulate water transport vertically and radially to maintain the trade-off between hydraulic conductivity efficiency and safety.

Starch depletion in the xylem and phloem ray parenchyma of grapevine stems under drought

While nonstructural carbohydrate (NSC) storage can support long-lived woody plants during abiotic stress, the timing and extent of their use are less understood, as are the thresholds for cell mortality as NSCs and water supplies are consumed. Here, we combine physiological and imaging tools to study the response of Vitis riparia to a 6-week experimental drought. We focused on the spatial and temporal dynamics of starch consumption and cell viability in the xylem and phloem of the stem. Starch dynamics were further corroborated with enzymatic starch digestion and X-ray microcomputed tomography imaging. Starch depletion in the stems of droughted plants was detected after 2 weeks and continued over time. We observed distinct differences in starch content and cell viability in the xylem and phloem. By the end of the drought, nearly all the starch was consumed in the phloem ray parenchyma (98 % decrease), and there were almost no metabolically active cells in the phloem. In contrast, less starch was consumed in the xylem ray parenchyma (30 % decrease), and metabolically active cells remained in the ray and vessel-associated parenchyma in the xylem. Our data suggest that the higher proportion of living cells in the phloem and cambium, combined with smaller potential NSC storage area, rapidly depleted starch, which led to cell death. In contrast, the larger cross-sectional area of the xylem ray parenchyma with higher NSC storage and lower metabolically active cell populations depleted starch at a slower pace. Why NSC source-sink relationships between xylem and phloem do not allow for a more uniform depletion of starch in ray parenchyma over time is unclear. Our data help to pinpoint the proximate and ultimate causes of plant death during prolonged drought exposure and highlight the need to consider the influence of within-organ starch dynamics and cell mortality on abiotic stress response.

Contrasting Responses of Two Grapevine Cultivars to Drought: The Role of Non-structural Carbohydrates in Xylem Hydraulic Recovery

Xylem embolism is one of the possible outcomes of decreasing xylem pressure when plants face drought. Recent studies have proposed a role for non-structural carbohydrates (NSCs) in osmotic pressure generation, required for refilling embolized conduits. Potted cuttings of grapevine Grenache and Barbera, selected for their adaptation to different climatic conditions, were subjected to a drought stress followed by re-irrigation. Stem embolism rate and its recovery were monitored in vivo by X-ray micro-computed tomography (micro-CT). The same plants were further analyzed for xylem conduit dimension and NSC content. Both cultivars significantly decreased Ψpd in response to drought and recovered from xylem embolism after re-irrigation. However, although the mean vessel diameter was similar between the cultivars, Barbera was more prone to embolism. Surprisingly, vessel diameter was apparently reduced during recovery in this cultivar. Hydraulic recovery was linked to sugar content in both cultivars, showing a positive relationship between soluble NSCs and the degree of xylem embolism. However, when starch and sucrose concentrations were considered separately, the relationships showed cultivar-specific and contrasting trends. We showed that the two cultivars adopted different NSC-use strategies in response to drought, suggesting two possible scenarios driving conduit refilling. In Grenache, sucrose accumulation seems to be directly linked to embolism formation and possibly sustains refilling. In Barbera, maltose/maltodextrins could be involved in a conduit recovery strategy via the formation of cell-wall hydrogels, likely responsible for the reduction of conduit lumen detected by micro-CT.

Diversification of Vascular Occlusions and Crystal Deposits in the Xylem Sap Flow of Five Tunisian Grapevines

Xylem vessels are essential pivotal organs in bulk hydraulic flow through the whole woody plant. However, environmental constraints generate disagreements in xylem structures, which are characterized by air emboli and occlusions formations, compromising water conductivity in grapevines. The aim of this work was to explore xylem morphology dynamics through the xylem sap flow of five Tunisian grapevine cultivars during the natural bleeding sap periods of 2019, 2021, and 2022. In fact, Sakasly, Khamri, Hencha, Razegui1, and Razegui2 rain-fed grapevine cultivars revealed differential responses towards xylem sap movement. The results demonstrated that the xylem sap flow was significantly more abundant in 2019 than 2021 and 2022 bleeding sap campaigns. A variation was revealed between the cultivars regarding the xylem sap flow. In fact, Sakasly gave the best xylem flow during the three campaigns. Razegui1 and Razegui2 registered approximately similar xylem sap flow, while Hencha and Khamri present the lowest sap fluxes during the three campaigns. Moreover, several vascular occlusions forms were identified from stem cross sections using environmental scanning electron microscopy (ESEM), including tyloses, gels, starch, and gum deposits. The highest occlusion number was observed in Sakasly, Razegui1, and Razegui2 cultivars. Among different biogenic calcium shapes, several were observed for the first time in grapevine, including multi-faceted druse, cubic, crystalline sand, styloids, spherical, or drop-like structures. Considering their lower flow and totally blocked vessels, both Hencha and Khamri confirmed their susceptibility to environmental constraints. However, Sakasly, Razegui1, and Razegui2 cultivars presented higher tolerance according to their sap flow and xylem morphology.

The Leaf Trichome, Venation, and Mesophyll Structural Traits Play Important Roles in the Physiological Responses of Oak Seedlings to Water-Deficit Stress

In this study, we investigated the effects of water-deficit stress on the leaf anatomical traits, physiological traits, and stem starch content in Quercus acutissima Carruth and Quercus serrata Murray by subjecting their seedlings to well-watered (WW) and water-deficit stress (WS) treatments. The water stress-induced changes in trichome density, trichome-to-stomata ratio, mesophyll thickness, vein density, vein distance, vein loopiness, vessel diameter, transpiration (E), stomatal conductance (gs), water use efficiency (WUE), and starch content were analyzed between two time points. While trichome density did not vary between treatments in Q. acutissima, it dramatically increased in Q. serrata (62.63–98.96 trichomes mm−2) at the final week. The WS-treated seedlings had a thicker palisade mesophyll (162.85–169.56 µm) than the WW-treated samples (118.56–132.25 µm) in both species. The vein density and loopiness increased significantly in the WS-treated Q. serrata seedlings. Small-sized vessels (10–50 µm) were more frequent in the WS than the WW in Q. serrata. The E, gs, WUE, and starch content declined significantly in the WS-treated seedlings compared with WW-treated samples in both species. Further, principal component analysis revealed significant relationships between anatomical and physiological traits, particularly in the WS-treated seedlings of Q. serrata. The coordinated changes in leaf anatomical traits, physiological traits, and stem starch content indicate an important role in the survival of Q. acutissima and Q. serrata seedlings in water-deficit stress environments, although Q. serrata may show higher survivability under prolonged water stress than Q. acutissima.

Physiological and Biochemical Dynamics of Pinus massoniana Lamb. Seedlings under Extreme Drought Stress and during Recovery

In recent years, global forests have been facing an increase in tree mortality owing to increasing droughts. However, the capacity for plants to adjust their physiology and biochemistry during extreme drought and subsequent recovery is still unclear. Here, we used 1.5-year-old Pinus massoniana Lamb. seedlings and simulated drought conditions to achieve three target stress levels (50%, 85%, and 100% loss of stem hydraulic conductivity (PLC)), followed by rehydration. Needle water status, gas exchange, and biochemical parameters were assessed during drought and recovery. The results showed that drought had significantly negative impacts on needle water status and gas exchange parameters, with gas exchange declining to 0 after PLC85 was achieved. Soluble protein concentration (SPC), soluble sugar concentration (SSC), malondialdehyde (MDA) content, superoxide dismutase (SOD) activity, and needle water-use efficiency showed fluctuations. The activity of antioxidant enzymes and the values of osmotic regulators were then gradually decreased as the physiological and biochemical functions of seedlings were disturbed. Seedlings showed a stronger ability to recover from PLC50 than PLC85 and PLC100. We conclude that the physiological and biochemical recovery of P. massoniana seedlings is more likely to be inhibited when plants experience increasing drought stress that induces 85% and greater loss of hydraulic conductance.

Morpho-Anatomical Traits and Soluble Sugar Concentration Largely Explain the Responses of Three Deciduous Tree Species to Progressive Water Stress

A better understanding of plant drought responses is essential to improve plant water use efficiency, productivity, and resilience to ever-changing climatic conditions. Here, we investigated the growth, morpho-anatomical, physiological, and biochemical responses of Quercus acutissima Carruth., Quercus serrata Murray, and Betula schmidtii Regel to progressive water-stress. Seedlings were subjected to well-watered (WW) and water-stressed (WS) conditions while regularly monitoring the soil volumetric water content, stem diameter (SD), height, biomass, stomatal conductance (gs), intercellular CO2 concentration (Ci), and leaf relative water content (RWC). We also investigated the variation in stomatal pore (SP) area, specific leaf area (SLA), root xylem vessel diameter (VD), and total soluble sugar (TSS) concentration between treatments. After 2 months, WS significantly suppressed SD growth of Q. acutissima and B. schmidtii but had no impact on Q. serrata. Total biomass significantly declined at WS-treated seedlings in all species. WS resulted in a smaller SLA than WW in all species. The SP of WS-treated seedlings of Q. acutissima and B. schmidtii significantly decreased, whereas it increased significantly with time in Q. serrata. Larger vessels (i.e., >100 to ≤ 130) were more frequent at WS for Q. acutissima and B. schmidtii, whereas smaller vessels (i.e., >40 to ≤ 90) were more frequent at WS than at WW for Q. serrata after 8 weeks. Tylosis was more frequent at WS than WW for Q. serrata and B. schmidtii at eighth week. WS seedlings showed lower gs, Ci, and RWC compared with WW-treated ones in Q. acutissima and B. schmidtii. TSS concentration was also higher at WS-treated seedlings in two Quercus species. Overall, principal component analysis (PCA) showed that SLA and SP are associated with WS seedlings of Q. serrata and B. schmidtii and the tylosis frequency, TSS, and VD are associated with WS seedlings of Q. acutissima. Therefore, water-stressed plants from all species responded positively to water stress with increasing experimental duration and stress intensity, and that is largely explained by morpho-anatomical traits and soluble sugar concentration. The present study should enhance our understanding of drought-induced tree growth and short-term tree-seedling responses to drought.

Sucrose Metabolism and Transport in Grapevines, with Emphasis on Berries and Leaves, and Insights Gained from a Cross-Species Comparison

In grapevines, as in other plants, sucrose and its constituents glucose and fructose are fundamentally important and carry out a multitude of roles. The aims of this review are three-fold. First, to provide a summary of the metabolism and transport of sucrose in grapevines, together with new insights and interpretations. Second, to stress the importance of considering the compartmentation of metabolism. Third, to outline the key role of acid invertase in osmoregulation associated with sucrose metabolism and transport in plants.

Multi-seasonal water-stress memory versus temperature-driven dynamic structural changes in grapevine

Perennial plants perpetually adapt to environmental changes in complex and yet insufficiently understood manner. We aimed to separate the intra-seasonal temperature effects on structure and function from perennial and annual water stress effects. This study focused on grapevine (Vitis vinifera L. 'Cabernet Sauvignon') petioles, which being a continuously produced organ, represent the current status of the plant. Field-grown mature plants subjected to multi-annual irrigation treatments (severe water stress, mild water stress and non-stressed) throughout the growing season were compared with greenhouse-grown plants under three temperature regimes (22, 28 and 34 °C). Physiological and functional anatomy parameters were measured. A generalized additive model (GAM) based on meteorological and lysimeter-based field data was applied to determine the relative influence of various meteorological parameters on evapotranspiration (ETc) during the growing season in the field experiment. At the beginning of the growing season, in May, petioles in the severe stress treatment showed a stress-related structure (decreased length, safer hydraulic structure and increased lignification), though having high values of stem water potential (SWP). As the season progressed and temperatures increased, all water availability treatments petioles showed similar changes, and at the end of season, in August, were structurally very similar. Those changes were independent of SWP and were comparable to high temperature-induced changes in the greenhouse. In contrast, stems hydraulic structure was strongly influenced by water availability. Regression analyses indicated a relationship between petioles xylem structure and stomatal conductance (gs), whereas gs (but not SWP) was temperature-dependent. The GAM showed that ETc was mainly dependent on temperature. Our results indicate a perennial water-stress memory response, influencing the petiole structure at the beginning of the following season. Intra-seasonally, the petiole's structure becomes independent of water status, whereas temperature drives the structural changes. Thus, ongoing climate change might disrupt plant performance by purely temperature-induced effects.

Abscisic Acid Mediates Drought and Salt Stress Responses in Vitis vinifera-A Review

The foreseen increase in evaporative demand and reduction in rainfall occurrence are expected to stress the abiotic constrains of drought and salt concentration in soil. The intensification of abiotic stresses coupled with the progressive depletion in water pools is a major concern especially in viticulture, as most vineyards rely on water provided by rainfall. Because its economical relevance and its use as a model species for the study of abiotic stress effect on perennial plants, a significant amount of literature has focused on Vitis vinifera, assessing the physiological mechanisms occurring under stress. Despite the complexity of the stress-resistance strategy of grapevine, the ensemble of phenomena involved seems to be regulated by the key hormone abscisic acid (ABA). This review aims at summarizing our knowledge on the role of ABA in mediating mechanisms whereby grapevine copes with abiotic stresses and to highlight aspects that deserve more attention in future research.