Fabio Raimondo

Leafminers help us understand leaf hydraulic design

Leaf hydraulics of Aesculus hippocastanum L. were measured over the growing season and during extensive leaf mining by the larvae of an invasive moth (Cameraria ohridella Deschka et Dimic) that specifically destroy the palisade tissue. Leaves showed seasonal changes in hydraulic resistance (R(lamina)) which were related to ontogeny. After leaf expansion was complete, the hydraulic resistance of leaves and the partitioning of resistances between vascular and extra-vascular compartments remained unchanged despite extensive disruption of the palisade by leafminers (up to 50%). This finding suggests that water flow from the petiole to the evaporation sites might not directly involve the palisade cells. The analysis of the temperature dependence of R(lamina) in terms of Q(10) revealed that at least one transmembrane step was involved in water transport outside the leaf vasculature. Anatomical analysis suggested that this symplastic step may be located at the bundle sheath where the apoplast is interrupted by hydrophobic thickening of cell walls. Our findings offer some support to the view of a compartmentalization of leaves into well-organized water pools so that the transpiration stream would involve veins, bundle sheath and spongy parenchyma, while the palisade tissue would be largely by-passed with the possible advantage of protecting cells from short-term fluctuations in water status.

Diurnal changes in embolism rate in nine dry forest trees: relationships with species-specific xylem vulnerability, hydraulic strategy and wood traits

Recent studies have reported correlations between stem sapwood capacitance (C(wood)) and xylem vulnerability to embolism, but it is unclear how C(wood) relates to the eventual ability of plants to reverse embolism. We investigated possible functional links between embolism reversal efficiency, C(wood), wood density (WD), vulnerability to xylem embolism and hydraulic safety margins in nine woody species native to dry sclerophyllous forests with different degrees of iso versus anisohydry. Substantial inter-specific differences in terms of seasonal/diurnal changes of xylem and leaf water potential, maximum diurnal values of transpiration rate and xylem vulnerability to embolism formation were recorded. Significant diurnal changes in percentage loss of hydraulic conductivity (PLC) were recorded for five species. Significant correlations were recorded between diurnal PLC changes and P50 and P88 values (i.e., xylem pressure inducing 50 and 88% PLC, respectively) as well as between diurnal PLC changes and safety margins referenced to P50 and P88. WD was linearly correlated with minimum diurnal leaf water potential, diurnal PLC changes and wood capacitance across all species. In contrast, significant relationships between P50, safety margin values referenced to P50 and WD were recorded only for the isohydric species. Functional links between diurnal changes in PLC, hydraulic strategies and WD and C(wood) are discussed.

Ion-mediated compensation for drought-induced loss of xylem hydraulic conductivity in field-growing plants of Laurus nobilis

Xylem cavitation is a common occurrence in drought-stressed plants. Cavitation-induced embolism reduces xylem hydraulic conductivity (kxylem) and may lead to stomatal closure and reduction of photosynthetic rates. Recent studies have suggested that plants may compensate for kxylem loss through ion-mediated enhancement of the residual water transport capacity of functioning conduits. To test this hypothesis, field-grown laurel (Laurus nobilis L.) plants were subjected to mild drought stress by suspending irrigation. Drought treatment induced a significant increase in xylem embolism compared with control (well watered) plants. Xylem sap potassium concentration ([K+]) increased during the day both in control and water stressed plants. Midday values of sap [K+] were significantly higher in water stressed plants. The recorded increase in sap potassium concentration induced significant enhancement of residual kxylem when solutions with different [K+] were perfused through excised stems sampled in the field and measured in the laboratory. In planta measurements of stem hydraulic conductance revealed no change between water stressed plants and controls. Our data suggest that ion-mediated enhancement of residual kxylem buffered the actual loss of hydraulic conductance suffered by plants during the warmest hours of the day as well as under mild drought stress conditions.

The contribution of vascular and extra-vascular water pathways to drought-induced decline of leaf hydraulic conductance.

Drought stress can impair leaf hydraulic conductance (Kleaf), but the relative contribution of changes in the efficiency of the vein xylem water pathway and in the mesophyll route outside the xylem in driving the decline of Kleaf is still debated. We report direct measurements of dehydration-induced changes in the hydraulic resistance (R=1/K) of whole leaf (Rleaf), as well as of the leaf xylem (Rx) and extra-vascular pathways (Rox) in four Angiosperm species. Rleaf, Rx, and Rox were measured using the vacuum chamber method (VCM). Rleaf values during progressive leaf dehydration were also validated with measurements performed using the rehydration kinetic method (RKM). We analysed correlations between changes in Rx or Rox and Rleaf, as well as between morpho-anatomical traits (including dehydration-induced leaf shrinkage), vulnerability to embolism, and leaf water relation parameters. Measurements revealed that the relative contribution of vascular and extra-vascular hydraulic properties in driving Kleaf decline during dehydration is species-specific. Whilst in two study species the progressive impairment of both vascular and extra-vascular pathways contributed to leaf hydraulic vulnerability, in the other two species the vascular pathway remained substantially unaltered during leaf dehydration, and Kleaf decline was apparently caused only by changes in the hydraulic properties of the extra-vascular compartment.

Plant performance on Mediterranean green roofs: interaction of species-specific hydraulic strategies and substrate water relations

Mediterranean native shrubs can be very useful for green roofs in hot and arid regions. Our data show that both Arbutus unedo L. and Salvia officinalis L. could be successfully utilized, although the choice of species should be based on the drought-resistant strategy relative to the desired technical performance of the green roof. Moreover, substrate selection was found to have a crucial role in the success of green roof installations in the Mediterranean area.

Leaf hydraulic architecture and water relations of three ferns from contrasting light habitats

Leaf hydraulic architecture and water relations of three fern species were measured. The species selected were adapted either to deeply shaded (Woodwardia radicans), moderately shaded (Dryopteris affinis) or moderately sunny (Polystichum setiferum) habitats, as confirmed by microclimatic measurements performed in the field. Leaf water potential (Ψleaf) was lower and leaf conductance to water vapour (gL) was higher in P. setiferum than in the shade-adapted ferns. Leaf osmotic potential and water potential at the turgor loss point were lower in the sun-adapted species than in the other ferns. Leaf hydraulic resistance (Rleaf) was lowest in P. setiferum and Rleaf was correlated with gL across species. Low Rleaf was coordinated with low rachis hydraulic resistance (Rrachis). Low values of Rrachis in P. setiferum were not due to the presence of wide xylem conduits as checked on the basis of anatomical measurements, but to increased radial permeability of vascular bundles. This was a consequence of the absence of endodermis surrounding the vascular bundles in P. setiferum, which was observed in the rachis of shade-adapted species. We conclude that hydraulic adjustment of fern fronds is a key component of adaptation of pteridophytes to contrasting light habitats.

Mistletoes and mutant albino shoots on woody plants as mineral nutrient traps

Background and Aims Potassium, sulphur and zinc contents of mistletoe leaves are generally higher than in their hosts. This is attributed to the fact that chemical elements which are cycled between xylem and phloem in the process of phloem loading of sugars are trapped in the mistletoe, because these parasites do not feed their hosts. Here it is hypothesized that mutant albino shoots on otherwise green plants should behave similarly, because they lack photosynthesis and thus cannot recycle elements involved in sugar loading. Methods The mineral nutrition of the mistletoe Scurrula elata was compared with that of albino shoots on Citrus sinensis and Nerium oleander. The potential for selective nutrient uptake by the mistletoe was studied by comparing element contents of host leaves on infected and uninfected branches and by manipulation of the haustorium–shoot ratio in mistletoes. Phloem anatomy of albino leaves was compared with that of green leaves. Key Results Both mistletoes and albino leaves had higher contents of potassium, sulphur and zinc than hosts or green leaves, respectively. Hypothetical discrimination of nutrient elements during the uptake by the haustorium is not supported by our data. Anatomical studies of albino leaves showed characteristics of release phloem. Conclusions Both albino shoots and mistletoes are traps for elements normally recycled between xylem and phloem, because retranslocation of phloem mobile elements into the mother plant or the host is low or absent. It can be assumed that the lack of photosynthetic activity in albino shoots and thus of sugars needed in phloem loading is responsible for the accumulation of elements. The absence of phloem loading is reflected in phloem anatomy of these abnormal shoots. In mistletoes the evolution of a parasitic lifestyle has obviously eliminated substantial feeding of the host with photosynthates produced by the mistletoe.

Effects of NaCl addition to the growing medium on plant hydraulics and water relations of tomato

This work reports on experimental evidence for the role of ion-mediated changes of xylem hydraulic conductivity in the functional response of Solanum lycopersicum L. cv. Naomi to moderate salinity levels. Measurements were performed in fully developed 12-week-old plants grown in half-strength Hoagland solution (control, C-plants) or in the same solution added with 35mM NaCl (NaCl-plants). NaCl-plants produced a significantly less but heavier leaves and fruits but had similar gas-exchange rates as control plants. Moreover, NaCl-plants showed higher vessel multiple fraction (FVM) than control plants. Xylem sap potassium and sodium concentrations were significantly higher in NaCl-plants than in control plants. When stems were perfused with 10mM NaCl or KCl, the hydraulic conductance of NaCl plants was nearly 1.5 times higher than in control plants. Accordingly, stem hydraulic conductance measured in planta was higher in NaCl- than in control plants. Our data suggest that tomato plants grown under moderate salinity upregulate xylem sap [Na+] and [K+], as well as sensitivity of xylem hydraulics to sap ionic content, thus, increasing water transport capacity.

Hydraulic connections of leaves and fruit to the parent plant in Capsicum frutescens (hot pepper) during fruit ripening

BACKGROUND AND AIMS The hydraulic architecture and water relations of fruits and leaves of Capsicum frutescens were measured before and during the fruiting phase in order to estimate the eventual impact of xylem cavitation and embolism on the hydraulic isolation of fruits and leaves before maturation/abscission. METHODS Measurements were performed at three different growth stages: (1) actively growing plants with some flowers before anthesis (GS1), (2) plants with about 50 % fully expanded leaves and immature fruits (GS2) and (3) plants with mature fruits and senescing basal leaves (GS3). Leaf conductance to water vapour as well as leaf and fruit water potential were measured. Hydraulic measurements were made using both the high-pressure flow meter (HPFM) and the vacuum chamber (VC) technique. KEY RESULTS The hydraulic architecture of hot pepper plants during the fruiting phase was clearly addressed to favour water supply to growing fruits. Hydraulic measurements revealed that leaves of GS1 plants as well as leaves and fruit peduncles of GS2 plants were free from significant xylem embolism. Substantial increases in leaf petiole and fruit peduncle resistivity were recorded in GS3 plants irrespective of the hydraulic technique used. The higher fraction of resistivity measured using the VC technique compared with the HPFM technique was apparently due to conduit embolism. CONCLUSIONS The present study is the first to look at the hydraulics of leaves and fruits during growth and maturation through direct, simultaneous measurements of water status and xylem efficiency of both plant regions at different hours of the day.

Impairment of leaf hydraulics in young plants of Citrus aurantium (sour orange) infected by Phoma tracheiphila

Phoma trachephila (Petri) Kantschaveli et Gikachvili causes dieback of several Citrus species. The impact of this fungus on leaf hydraulics was studied in Citrus aurantium L. (sour orange) with the aim of identifying the primary mechanism of damage to leaves. Leaves inoculated with a conidial suspension were measured for conductance to water vapor (gL) and specific hydraulic conductance (Kleaf) every 3 days after inoculation. The earliest symptom of infection consisted of vein chlorosis. Functional vein density (FVD) was monitored and microscopic observations were made of major vein conduits. Impairment of vein hydraulics started 25 days after inoculation with a losses of Kleaf of 40% and gL of ~60%. Most minor veins within chlorotic areas were no longer functioning and some conduits of the major veins showed digested interconduit pits leading to vein cavitation. The close Kleaf-FVD relationship revealed that vein impairment caused drop of Kleaf and, consequently, of gL at chlorotic areas. Leaf infection was focused to veins that were first forced to embolise and then invaded by fungal hyphae. The vein embolism due to the Phoma amplifies the native dominant hydraulic resistance of leaf veins, and leads ultimately to early shedding of infected leaves.