Robert Savé

The reflectance at the 950–970 nm region as an indicator of plant water status

Abstract We present new remote sensing indices of plant water status: the ratio between the reflectance at 970 nm, one of the water absorption bands, and the reflectance at a reference wavelength, 900 nm (R970/R9000; the first derivative minimum in this near-infrared region (dNIRminimum ) and the wavelength where this minimum is found ( λNIRminimum). In order to evaluate them, we carried out three experiments. Daily irrigated gerbera plants were allowed to dry until almost wilting and then daily irrigation was restarted; pepper and bean plants were grown for four months submitted to two different irrigation treatments; and bean detached leaves were submitted to progressive dehydration whereas pressure-volume curves were being carried out. In gerbera plants, the trough about 950–970 nm decreased as the drought was increasing. Therefore, the R970/R900 index and the dNIRminimum closely tracked the changes in relative water content (RWC), leaf water potential, stomatal conductance and the foliage-air temperat...

Cell wall elasticity and Water Index (R970 nm/R900 nm) in wheat under different nitrogen availabilities

Abstract New data showing the importance of cell wall elasticity in the expression of the Water Index (WI) (R970/R900) as an indicator of plant water status are presented. We studied wheat under five different nitrogen (N) fertilization treatments (0 to 200kg ha-1) to test N-availability effects on tissue structural changes such as cell wall composition and elasticity, and the corresponding effects on WI. Although water was amply supplied in all the plots, Wl, CWSI (Crop Water Stress Index) and ST (canopy minus air temperature) were all highest in the N-0 (no fertilization) treatment. Plants in this treatment had the most xeromorphic characteristics. Their specific leaf weight (SLAV) and leaf cellulose contents were higher and their cell wall elasticities were lower than in the other N treatments. WI was a useful water status indicator (even of mild water stress conditions) at ground level when elasticity was low, WI may also become an indicator of cell wall elasticity of leaves when plant water status is...

Seasonal changes of whole root system conductance by a drought-tolerant grape root system

The role of root systems in drought tolerance is a subject of very limited information compared with above-ground responses. Adjustments to the ability of roots to supply water relative to shoot transpiration demand is proposed as a major means for woody perennial plants to tolerate drought, and is often expressed as changes in the ratios of leaf to root area (AL:AR). Seasonal root proliferation in a directed manner could increase the water supply function of roots independent of total root area (AR) and represents a mechanism whereby water supply to demand could be increased. To address this issue, seasonal root proliferation, stomatal conductance (gs) and whole root system hydraulic conductance (kr) were investigated for a drought-tolerant grape root system (Vitis berlandieri×V. rupestris cv. 1103P) and a non-drought-tolerant root system (Vitis riparia×V. rupestris cv. 101-14Mgt), upon which had been grafted the same drought-sensitive clone of Vitis vinifera cv. Merlot. Leaf water potentials (ψL) for Merlot grafted onto the 1103P root system (–0.91±0.02 MPa) were +0.15 MPa higher than Merlot on 101-14Mgt (–1.06±0.03 MPa) during spring, but dropped by approximately –0.4 MPa from spring to autumn, and were significantly lower by –0.15 MPa (–1.43±0.02 MPa) than for Merlot on 101-14Mgt (at –1.28±0.02 MPa). Surprisingly, gs of Merlot on the drought-tolerant root system (1103P) was less down-regulated and canopies maintained evaporative fluxes ranging from 35–20 mmol vine−1 s−1 during the diurnal peak from spring to autumn, respectively, three times greater than those measured for Merlot on the drought-sensitive rootstock 101-14Mgt. The drought-tolerant root system grew more roots at depth during the warm summer dry period, and the whole root system conductance (kr) increased from 0.004 to 0.009 kg MPa−1 s−1 during that same time period. The changes in kr could not be explained by xylem anatomy or conductivity changes of individual root segments. Thus, the manner in which drought tolerance was conveyed to the drought-sensitive clone appeared to arise from deep root proliferation during the hottest and driest part of the season, rather than through changes in xylem structure, xylem density or stomatal regulation. This information can be useful to growers on a site-specific basis in selecting rootstocks for grape clonal material (scions) grafted to them.

Some physiological and morphological characteristics of citrus plants for drought resistance

Abstract Tolerance and avoidance mechanisms to drought stress were studied in 6-month-old plants of Newhall orange ( Citrus sinensis (L.) Osbeck) and Ellendale tangor (orange × mandarin hybrid) ( Citrus sinensis (L) Osbeck × Citrus reticulata Blanco) during a drought/rewatering cycle under controlled conditions. Drought stress did not promote osmotic adjustment, while elastic adjustment (tissue elasticity increase) was noted in stressed orange and tangor plants. Both citrus plants showed a parallel decrease in leaf conductance ( g 1 ) and leaf water potential ( Ψ 1 ) under water stress. Tangor plants had a more efficient water conservative strategy than orange, based on the characteristics of canopy architecture (lower canopy area and a more closed canopy with leaves nearly vertically oriented) together with a significant decrease in cuticular transpiration rates (TRc) under stress.

Remotely measured canopy temperature of greenhouse strawberries as indicator of water status and yield under mild and very mild water stress conditions

Abstract Strawberry plants were submitted to mild and very mild water stress regimes in a tunnel (simple semicircular greenhouse) by planting them in two plots fertirrigated when the soil matric potential reached −0.07 MPa and −0.01 MPa, respectively. The plants were monitored for water stress by measuring the foliage temperature with a hand-held infrared thermometer. Parallel to this, weather variables, the difference between leaf and air temperature, the derived crop water stress index (CWSI), the soil matric potential, the leaf water potential, the photosynthetic gas exchange rates, the transpiration rates, photosynthetic pigments, sugars, starch, canopy structure and accumulated yield were measured. The wet treatment (WT) presented a higher yield and higher leaf area index (LAI). In WT, leaves were disposed mostly in a monolayer oriented to the south, whereas in dry treatment (DT) leaves were distributed in a multilayer pattern and oriented to the north. During the hotter part of cloudless days and before irrigation took place, the canopy temperature of WT was about 3°C less than that of DT. Accumulated stress degree days (SDD) were then higher in DT. WT presented lower average CWSI values, between 0.045 and 0.54, while those of DT were between 0.32 and 0.70. It was concluded that leaf temperature, its difference with air temperature (δ T) and the derived indices, such as SDD and CWSI, are useful for the assessment of even these mild and very mild water stresses in strawberries under protected conditions. Regression analysis showed that under the very mild water stress conditions tested in WT, the contribution of air vapor pressure deficit to variation in leaf water potential was significant. This did not happen under the mild water stress of DT.

Tissue-water relations of two co-occurring evergreen Mediterranean species in response to seasonal and experimental drought conditions

Tissue-water relations were used to characterize the responses of two Mediterranean co-occurring woody species (Quercus ilex L. and Phillyrea latifolia L.) to seasonal and experimental drought conditions. Soil water availability was reduced ∼15% by partially excluding rain throughfall and lateral flow (water runoff). Seasonal and experimental drought elicited physiological and morphological adaptations other than osmotic adjustment: both species showed large increases in cell-wall elasticity and decreased saturated-to-dry-mass ratio. Increased elasticity (lower elastic modulus) resulted in concurrent decreases in relative water content at turgor loss. In addition, P. latifolia showed significant increases in apoplastic water fraction. Decreased saturated-to-dry-mass ratio and increased apoplastic water fraction were accompanied by an increased range of turgor maintenance, which indicates that leaf sclerophyllous traits might be advantageous in drier scenarios. In contrast, the degree of sclerophylly (as assessed by the leaf mass-to-area ratio) was not related to tissue elasticity. An ∼15% reduction in soil water availability resulted in significant reductions in diameter growth when compared to control plants in both species. Moreover, although P. latifolia underwent larger changes in tissue water-related traits than Q. ilex in response to decreasing water availability, growth was more sensitive to water stress in P. latifolia than in Q. ilex. Differences in diameter growth between species might be partially linked to the effects of cell-wall elasticity and turgor pressure on growth, since Q. ilex showed higher tissue elasticity and higher intrinsic tolerance to water deficit (as indicated by lower relative water content at turgor loss) than P. latifolia.

AM inoculation as a biological tool to improve plant revegetation of a disturbed soil with Rosmarinus officinalis under semi-arid conditions

Abstract Rosmarinus officinalis L., is an evergreen sclerophyllous mediterranean shrub that can be used for revegetation purposes under semi-arid conditions. Plants, some of which had been inoculated with the arbuscular mycorrhizal (AM) fungus Glomus intraradices Schenck & Smith, were grown on eroded topsoil collected from a sideslope of a recently constructed motorway. The effects of the symbiosis on plant growth and soil coverage and the development of the number of AM propagules in the soil were studied to assess the usefulness of AM inoculation as a cultural practice to ensure successful establishment of plants in eroded soils. Results showed that AM inoculation could have a positive effect on the revegetation process.

Allelopathic potential of the neotropical dry-forest tree Acacia pennatula Benth.: inhibition of seedling establishment exceeds facilitation under tree canopies

Secondary succession after land abandonment in tropical dry forests has been suggested to be favoured by the facilitation effects for seedling establishment exerted by pioneer trees isolated in these savannah-like landscapes. However, it has also been noticed that these pioneer species may sometimes have an encroaching effect and arrest succession for several decades. We investigated in this study whether allelopathy can play a role in limiting seedling establishment of co-occurring tree species under the canopy of Acacia pennatula by means of lab bioassays and field experiments in north-west Nicaragua. Leaf extracts of A. pennatula did not affect seed germination but reduced the general growth and especially the development of the root compartment in seedlings, shifting their biomass allocation model to a reduced root/shoot ratio. Survival of planted seedlings under the canopy of A. pennatula was about 20–30% lower than outside, and this reduction was particularly pronounced as the dry season progressed, despite the milder conditions (e.g. higher soil moisture) being experienced in the inner positions under the canopy. Altogether, our results suggest that, rather than facilitating, A. pennatula may inhibit the establishment of seedlings under its canopy probably by means of an allelopathic interference in the development of the root system with critical negative consequences for young seedlings in terms of overcoming the dry season. This article warns about overemphasizing the nucleation effect that remnant and isolated trees may have to facilitate secondary succession in these highly disturbed savannah-like tropical dry forests.

Effects of a nursery CO2 enriched atmosphere on the germination and seedling morphology of two Mediterranean oaks with contrasting leaf habit

The use of an enriched CO2 atmosphere in tree nurseries has been envisaged as a promising technique to increase productivity and to obtain seedlings with a higher root/shoot ratio, an essential trait to respond to water stress in Mediterranean-type ecosystems. In that framework, we have analyzed the effects of three levels of atmospheric CO2 concentration (350, 500 and 700 ppm) on the germination rate, growth and morphology of seedlings of two Mediterranean oaks used in reforestation programs: the evergreen Quercus ilex L. and the deciduous Quercus cerrioides Wilk. et Costa. CO2 enrichment increased the germination rate of Q. cerrioides (from 70 ± 7 to 81 ± 3%) while it decreased that of Q. ilex (from 71 ± 10 to 41 ± 12%). Seedlings of both species increased approximately 60% their total biomass in response to CO2 enrichment but at two different CO2 concentrations: 500 ppm for Q. cerrioides and 700 ppm for Q. ilex. This increase in seedlings biomass was entirely due to an augmentation of root biomass. Considering germination and biomass partitioning, an enriched CO2 atmosphere might not be appropriate for growing Mediterranean evergreen oaks, such as Q. ilex, since it reduces acorn germination and the only gains in root biomass occur at a high concentration (700 ppm). On the other hand, a moderate CO2 enrichment (500 ppm) appears as a promising nursery technique to stimulate the germination, growth and root/shoot ratio of deciduous oaks, such as Q. cerrioides.

Effects of long‐term exposure to elevated CO2 conditions in slow‐growing plants using a 12C‐enriched CO2‐labelling technique

Despite their relevancy, long-term studies analyzing elevated CO(2) effect in plant production and carbon (C) management on slow-growing plants are scarce. A special chamber was designed to perform whole-plant above-ground gas-exchange measurements in two slow-growing plants (Chamaerops humilis and Cycas revoluta) exposed to ambient (ca. 400 micromol mol(-1)) and elevated (ca. 800 micromol mol(-1)) CO(2) conditions over a long-term period (20 months). The ambient isotopic (13)C/(12)C composition (delta(13)C) of plants exposed to elevated CO(2) conditions was modified (from ca. -12.8 per thousand to ca. -19.2 per thousand) in order to study carbon allocation in leaf, shoot and root tissues. Elevated CO(2) increased plant growth by ca. 45% and 60% in Chamaerops and Cycas, respectively. The whole-plant above-ground gas-exchange determinations revealed that, in the case of Chamaerops, elevated CO(2) decreased the photosynthetic activity (determined on leaf area basis) as a consequence of the limited ability to increase C sink strength. On the other hand, the larger C sink strength (reflected by their larger CO(2) stimulatory effect on dry mass) in Cycas plants exposed to elevated CO(2) enabled the enhancement of their photosynthetic capacity. The delta(13)C values determined in the different plant tissues (leaf, shoot and root) suggest that Cycas plants grown under elevated CO(2) had a larger ability to export the excess leaf C, probably to the main root. The results obtained highlighted the different C management strategies of both plants and offered relevant information about the potential response of two slow-growing plants under global climate change conditions.