In grapevine, water transport follows apoplastic and symplastic pathways, according to different organs, to their phenological stages, and to strategies for adjustment to environment and cultivation constraints. Apoplastic pathways respond to physical laws regulating both hydraulics of xylem vessels and water evaporation from leaf cavities. Xylem sap flow is affected by a series of conductances, which can be modulated by environmental and physiological factors. Traditionally, root-radial and stomatal conductances have been considered the main controlling factors of water flow. In addition, the hydraulic conductivity of the root (longitudinal) and the shoot can also affect water flow. Longitudinal hydraulic conductivity is affected by changes in average vessel diameter: water stress, leaf shading, low temperature and the downward orientation of the shoot reduce hydraulic conductivity by depressing xylem vessel development. Modifications of vessel size are under auxin control, and have an important role in the adaptation to unfavourable environmental conditions, as smaller vessels are less likely to cavitate and therefore to embolize. On the other hand, smaller vessels have lower conductivity, and this can reduce xylem transport to the leaves from the roots. Symplastic pathways account for cell-to-cell water movements, in response to osmotic driving forces and cell permeability, as regulated by aquaporin expression and activity. The symplastic cells surrounding xylem vessels, the xylem parenchyma for example, could contribute to the regulation of xylem transport by variation of radial cell-to-cell water movement from perivascular tissues to xylem. This, in turn, affects longitudinal xylem sap flow, especially upon critical situations of water balance. Moreover, especially in root tips and leaves, apoplastic water pathways compete with transcellular, aquaporin-controlled water pathways, providing an indirect control mechanism of xylem water flow that is dependent on aquaporin function. Observation focused on water balance at the interface between xylem and surrounding parenchyma provides a tool to explain unclear mechanisms of plant xylem recovery from water deficit. It was found that aquaporins and cell metabolism sensitive to mercury chloride play a significant role in these processes. Water reloading from parenchyma surrounding vessels to xylem has been suggested to take place at some stage of plant rehydration after water stress during embolized-vessel refilling - a mechanism quite similar to that described in the root stele, but occurring in all xylem-fed organs. Xylem refilling can in principle occur, due to the contribution of root pressure pushing embolisms out of the xylem vessels, as it has been reported for seasonal embolism recovery after summer drought and/or after winter freezing in woody plants. More often, however, it was shown that hydraulic conductivity in roots, stems or leaves recovers when xylem pressure is significantly negative. This indicates for mechanisms of embolism repair. On grapevine we proposed a model to describe plant recovery after rehydration based on three main points: embolism repair occurs progressively in shoots and further in roots and in petioles, following an almost full recovery of leaf water potential; hydraulic conductance recovers during diurnal transpiring hours, when formation and repair of embolisms occurs in all plant organs; an abscisic acid (ABA) residual signal in rehydrated leaves hinders stomatal opening even when water potential has recovered, suggesting that an ABA-induced transpiration control promotes gradual embolism repair in rehydrated grapevines. Vitis vinifera cultivation is traditionally (especially in Europe) non-irrigated and spread widely across dry or semi-dry ecosystems. Yield and berry quality strictly depend on the vine adaptability to drought; water stress does not exclusively imply negative effects, but a regulated water stress, which is the base of various agronomic practices - rootstock use, controlled cover crops, tillage, rescue irrigation techniques such as regulated deficit irrigation or partial root-zone drying - has been largely used to balance vine vegetative and reproductive growth with the aim of controlling berry quality. Understanding and manipulating plant-water relations and water-stress tolerance by means of physiology and molecular biology can significantly improve plant productivity and environmental quality and this is clearly a question of wide economic importance in viticulture. Drought signaling among grapevine organs is known to have a dual component: a hydraulic signal controlled by xylem physiology coexists with chemical signals (involving ABA), transported via xylem, phloem and parenchyma pathways. Based on their water potential behavior in response to water stress, grapevine cultivars can be classified in isohydric and anisohydric. Isohydric cultivars are those keeping their leaf water potential above a certain threshold regardless of soil water availability or atmospheric water demand, while anisohydric cultivars are those in which leaf water potential drops with decreasing soil water availability or increasing atmospheric water demand. In isohydric grapevines, leaf water potential rarely drops below -1.5 MPa. This is close to the threshold for severe cavitation in this species, although some cavitation occurs at lower water potentials in petioles, shoot nodes and internodes and roots. Using the daily maximum value of stomatal conductance as an indicator of water stress that allows comparison of plants with iso- and anisohydric behaviors, we defined several stages of photosynthesis regulation in grapevines subjected to progressive soil water stress on the basis of the main causes of photosynthesis decline. Knowledge about chemical and hydraulic root signals induced by soil water stress has stimulated new irrigation strategies. Regulated deficit irrigation (RDI) and partial root-zone drying (PRD) were developed to improve yield-to-irrigation ratio. PRD is designed to expose part of the root system to drying soil in order to produce the root drought signal, while the remaining roots in wet soil can maintain water supply and therefore leaf water potential. PRD enhances root hydraulic conductance in fruit trees; during PRD treatment, roots show higher uptake capacity than in whole root-zone irrigation treatment. Putative aquaporin stimulation by ABA produced by PRD (or RDI) may be involved. Prolonged exposure of roots to drying soil may cause anatomical changes as epidermis suberization, collapse of cortex, and loss of secondary roots. Alternate watering, after a period of soil drying, may improve this situation by inducing new secondary roots. In the field during a typical drying cycle (10-15 days), only roots near to soil surface feel dry soil whereas deeper roots extract water from wetter soil layers. This may be reduce the synthesis of drought chemical signals, like ABA, and hence may be reduce PRD effect. Also the water redistribution process from wet to dry roots (the so-called ‘hydraulic lift’) in response to water potential gradients can contribute to decrease of ABA biosynthesis. This phenomenon has been observed in several grapevines subjected to dry soil conditions or PRD treatment. Abscisic acid production in drying roots and its translocation and accumulation in the shoot drive several physiological mechanisms in grapevine leaves. In grape berries, ABA is considered a promoter of ripening as its concentration increases at the beginning of véraison. Exogenous ABA, used to promote ripening in berries, induces the accumulation of regulatory genes of anthocyanin biosynthesis of grape, leading to an increase of anthocyanin accumulation in berry skins. During ripening, the accumulation of sugars in berries requires the coordinated expression of sucrose transporters, invertases, and monosaccharide transporters. The expression of the glucose transporter homologue (VvHT1, V. vinifera hexose transporter 1), isolated from grape berries at véraison, is regulated by sugars and ABA. Phloem influx into the berry is accompanied by a decrease in cell turgor, which influences the expression of many genes at the onset of ripening. A regulated water stress, lowering cell turgor, particularly if applied pre-véraison, may induce an increase in sugar influx and ABA, influencing several key steps of the phenylpropanoid biosynthetic pathway involved in the biosynthesis of anthocyanins, proanthocyanidins and flavonols, increases in water-deficit conditions.
A hydraulic world: from plant water transport to vineyard irrigation requirements.
LOVISOLO, Claudio
2010-01-01
Abstract
In grapevine, water transport follows apoplastic and symplastic pathways, according to different organs, to their phenological stages, and to strategies for adjustment to environment and cultivation constraints. Apoplastic pathways respond to physical laws regulating both hydraulics of xylem vessels and water evaporation from leaf cavities. Xylem sap flow is affected by a series of conductances, which can be modulated by environmental and physiological factors. Traditionally, root-radial and stomatal conductances have been considered the main controlling factors of water flow. In addition, the hydraulic conductivity of the root (longitudinal) and the shoot can also affect water flow. Longitudinal hydraulic conductivity is affected by changes in average vessel diameter: water stress, leaf shading, low temperature and the downward orientation of the shoot reduce hydraulic conductivity by depressing xylem vessel development. Modifications of vessel size are under auxin control, and have an important role in the adaptation to unfavourable environmental conditions, as smaller vessels are less likely to cavitate and therefore to embolize. On the other hand, smaller vessels have lower conductivity, and this can reduce xylem transport to the leaves from the roots. Symplastic pathways account for cell-to-cell water movements, in response to osmotic driving forces and cell permeability, as regulated by aquaporin expression and activity. The symplastic cells surrounding xylem vessels, the xylem parenchyma for example, could contribute to the regulation of xylem transport by variation of radial cell-to-cell water movement from perivascular tissues to xylem. This, in turn, affects longitudinal xylem sap flow, especially upon critical situations of water balance. Moreover, especially in root tips and leaves, apoplastic water pathways compete with transcellular, aquaporin-controlled water pathways, providing an indirect control mechanism of xylem water flow that is dependent on aquaporin function. Observation focused on water balance at the interface between xylem and surrounding parenchyma provides a tool to explain unclear mechanisms of plant xylem recovery from water deficit. It was found that aquaporins and cell metabolism sensitive to mercury chloride play a significant role in these processes. Water reloading from parenchyma surrounding vessels to xylem has been suggested to take place at some stage of plant rehydration after water stress during embolized-vessel refilling - a mechanism quite similar to that described in the root stele, but occurring in all xylem-fed organs. Xylem refilling can in principle occur, due to the contribution of root pressure pushing embolisms out of the xylem vessels, as it has been reported for seasonal embolism recovery after summer drought and/or after winter freezing in woody plants. More often, however, it was shown that hydraulic conductivity in roots, stems or leaves recovers when xylem pressure is significantly negative. This indicates for mechanisms of embolism repair. On grapevine we proposed a model to describe plant recovery after rehydration based on three main points: embolism repair occurs progressively in shoots and further in roots and in petioles, following an almost full recovery of leaf water potential; hydraulic conductance recovers during diurnal transpiring hours, when formation and repair of embolisms occurs in all plant organs; an abscisic acid (ABA) residual signal in rehydrated leaves hinders stomatal opening even when water potential has recovered, suggesting that an ABA-induced transpiration control promotes gradual embolism repair in rehydrated grapevines. Vitis vinifera cultivation is traditionally (especially in Europe) non-irrigated and spread widely across dry or semi-dry ecosystems. Yield and berry quality strictly depend on the vine adaptability to drought; water stress does not exclusively imply negative effects, but a regulated water stress, which is the base of various agronomic practices - rootstock use, controlled cover crops, tillage, rescue irrigation techniques such as regulated deficit irrigation or partial root-zone drying - has been largely used to balance vine vegetative and reproductive growth with the aim of controlling berry quality. Understanding and manipulating plant-water relations and water-stress tolerance by means of physiology and molecular biology can significantly improve plant productivity and environmental quality and this is clearly a question of wide economic importance in viticulture. Drought signaling among grapevine organs is known to have a dual component: a hydraulic signal controlled by xylem physiology coexists with chemical signals (involving ABA), transported via xylem, phloem and parenchyma pathways. Based on their water potential behavior in response to water stress, grapevine cultivars can be classified in isohydric and anisohydric. Isohydric cultivars are those keeping their leaf water potential above a certain threshold regardless of soil water availability or atmospheric water demand, while anisohydric cultivars are those in which leaf water potential drops with decreasing soil water availability or increasing atmospheric water demand. In isohydric grapevines, leaf water potential rarely drops below -1.5 MPa. This is close to the threshold for severe cavitation in this species, although some cavitation occurs at lower water potentials in petioles, shoot nodes and internodes and roots. Using the daily maximum value of stomatal conductance as an indicator of water stress that allows comparison of plants with iso- and anisohydric behaviors, we defined several stages of photosynthesis regulation in grapevines subjected to progressive soil water stress on the basis of the main causes of photosynthesis decline. Knowledge about chemical and hydraulic root signals induced by soil water stress has stimulated new irrigation strategies. Regulated deficit irrigation (RDI) and partial root-zone drying (PRD) were developed to improve yield-to-irrigation ratio. PRD is designed to expose part of the root system to drying soil in order to produce the root drought signal, while the remaining roots in wet soil can maintain water supply and therefore leaf water potential. PRD enhances root hydraulic conductance in fruit trees; during PRD treatment, roots show higher uptake capacity than in whole root-zone irrigation treatment. Putative aquaporin stimulation by ABA produced by PRD (or RDI) may be involved. Prolonged exposure of roots to drying soil may cause anatomical changes as epidermis suberization, collapse of cortex, and loss of secondary roots. Alternate watering, after a period of soil drying, may improve this situation by inducing new secondary roots. In the field during a typical drying cycle (10-15 days), only roots near to soil surface feel dry soil whereas deeper roots extract water from wetter soil layers. This may be reduce the synthesis of drought chemical signals, like ABA, and hence may be reduce PRD effect. Also the water redistribution process from wet to dry roots (the so-called ‘hydraulic lift’) in response to water potential gradients can contribute to decrease of ABA biosynthesis. This phenomenon has been observed in several grapevines subjected to dry soil conditions or PRD treatment. Abscisic acid production in drying roots and its translocation and accumulation in the shoot drive several physiological mechanisms in grapevine leaves. In grape berries, ABA is considered a promoter of ripening as its concentration increases at the beginning of véraison. Exogenous ABA, used to promote ripening in berries, induces the accumulation of regulatory genes of anthocyanin biosynthesis of grape, leading to an increase of anthocyanin accumulation in berry skins. During ripening, the accumulation of sugars in berries requires the coordinated expression of sucrose transporters, invertases, and monosaccharide transporters. The expression of the glucose transporter homologue (VvHT1, V. vinifera hexose transporter 1), isolated from grape berries at véraison, is regulated by sugars and ABA. Phloem influx into the berry is accompanied by a decrease in cell turgor, which influences the expression of many genes at the onset of ripening. A regulated water stress, lowering cell turgor, particularly if applied pre-véraison, may induce an increase in sugar influx and ABA, influencing several key steps of the phenylpropanoid biosynthetic pathway involved in the biosynthesis of anthocyanins, proanthocyanidins and flavonols, increases in water-deficit conditions.
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