Lydia Serrano

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...

Remote sensing of nitrogen and lignin in Mediterranean vegetation from AVIRIS data: Decomposing biochemical from structural signals

Remote sensing estimates of vegetation nitrogen (N) and lignin concentration are central to assess ecosystem processes such as growth and decomposition. Although remote sensing techniques have been proven useful to assess N and lignin contents in continuous green canopies, more studies are needed to address their capabilities, particularly in low and sparsely vegetated ecosystems. We investigated the possibility of estimating canopy N and lignin concentrations in chaparral vegetation using Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) reflectance acquired over an area around Point Dume in the Santa Monica Mountains (Los Angeles, CA, USA). Two approaches were tested: multiple stepwise regression based on first difference reflectance (FDR) and reflectance (R) indices. Multiple stepwise regressions (of three or fewer wavelengths) accounted for a large variance in canopy biochemical concentration (r 2 � 0.9, P<0.01). Log transformed R indices [log (1/R)] formulated on the basis of previously known N and lignin absorption wavelengths also showed significant correlations (P<0.01) with canopy biochemical concentration (r 2 ranging from 0.39 to 0.48). In addition, the contribution of structural and biochemical signals and background effects on the performance of these indices was evaluated. These indices accounted for a increased variance when adding information on canopy structural attributes (e.g., relative contribution of each species and biomass amount) to foliar biochemical concentration. The relative contributions of foliar biochemical concentration and canopy structure (biomass amount) on the spectral signal were further evaluated by analyzing the residuals from linear regressions: foliar N concentration accounted for 42% of the variance for a normalized difference index based on the 1510-nm N absorption feature, while the foliar lignin concentration accounted for 44% of the variance for a normalized difference index based on the 1754 nm lignin absorption feature. These percentages increased to 58% when stands with senescing vegetation were disregarded. We propose the two indices, Normalized Difference Nitrogen Index (NDNI = [log (1/R1510) � log (1/R1680)]/[log (1/R1510)+log (1/R1680)]) and Normalized Difference Lignin Index (NDLI = [log (1/R1754) � log (1/R1680)]/ [log (1/R1754)+log (1/R1680)]) as indices to assess N and lignin in native shrub vegetation. D 2002 Elsevier Science Inc. All rights reserved.

Deriving Water Content of Chaparral Vegetation from AVIRIS Data

Spectral imaging data acquired with Advanced Visible Infrared Imaging Spectrometer over Point Dume (Los Angeles County, CA, USA) were used to assess the ability of hyperspectral reflectance data to estimate canopy Relative Water Content (RWC) at the landscape level. The study was performed on 23 vegetation stands comprised of three characteristic chaparral plant communities, with contrasting phenological stages and canopy cover. Several estimates of water content based on the near-infrared (NIR; reflectance indices and water thickness derived from reflectance and radiance data) and shortwave infrared (SWIR) water absorption bands were compared to measurements of vegetation structure and water content made on the ground. The Water Index (WI) and Normalized Difference Water Index (NDWI), reflectance indices formulated from the NIR water absorption bands, were the best indicators of canopy RWC estimated from combining leaf relative water content with measures of canopy structure. A stepwise multiple regression revealed that canopy structure explained 36% and 41% of the variation in WI and NDWI, respectively. The explained variance in WI and NDWI increased to 44% and 48% when leaf relative water content was included in the model. By contrast, the inclusion of leaf relative water content did not contribute significantly to the explained variance in indices formulated using SWIR water absorption bands and in those based on water thickness. The relationship between WI and the canopy RWC significantly improved when only data from plots with green vegetation cover >70% were considered (r2=0.88, p<0.001). All the indices studied had an important structural component (as indicated by the strong correlation with NDVI), yet only the indices WI and NDWI additionally responded to water content. These results indicate that the WI and NDWI are sensitive to variations in canopy relative water content at the landscape scale.

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...

Nitrate leaching and strawberry production under drip irrigation management

Abstract The aim of the present study is the understanding of N leaching and uptake in an experimental crop. The experiment was carried out in one of the European aquifers most polluted by agricultural practices (Maresme area, Barcelona, Spain) and performed on a widespread crop in the area. The study aimed to determine the effect of continuous fertigation regimes through drip irrigation on N uptake and leaching as well as on the yield of strawberries. Irrigation regimes were imposed by watering when suction was −0.01 MPa (wet) and −0.07 MPa (dry) in the root zone. The nutrient solution was the same for both treatments. Foliage and fruit N concentration did not differ between the treatments, but N uptake was higher in the wet treatment; as a result, plant production and biomass increased. Nitrate nitrogen (NO3-N) leachates under the root zone were 1535 and 471 kg N ha−1, respectively; N uptake was 12% and 23% of the total applied. The wet irrigation regime resulted in significantly increased yields. Experimental conditions revealed a slow transit time through the vadose zone. Management practices should be improved to account for crop needs and thus, improve N uptake efficiency and reduce N leaching.

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.

Contribution of physiological and morphological adjustments to drought resistance in two Mediterranean tree species

Plant water potential (ψ), its components, and gas exchange data of two Mediterranean co-occurring woody species (Quercus ilex L. and Phillyrea latifolia L.) were measured in response to seasonal changes in water availability over two consecutive years. The relative contribution of physiological and morphological adjustments to drought resistance was assessed through Principal Component Analyses. There were large adjustments in stomatal conductance (∼36 % of accounted variance). Net photosynthetic rate and water use efficiency were closely tuned to water availability and accounted for ∼17 % of variance. The slope of the water potential vs. relative water content (dψ/dRWC0) below zero pressure potential increased as a result of seasonal and ontogenic increases in apoplastic water fraction and accounted for ∼20 % variance. This tolerance mechanism was accompanied by an increased range of positive pressure potential, suggesting a functional role of sclerophylly in these Mediterranean evergreens. Similarly, changes in the slope of dψ/dRWC in the range of positive pressure potential (∼13 % of accounted variance) were associated to variations in cell wall elasticity and resulted in lower RWC at zero pressure potential. When considering the species studied separately, the results indicated the primary role of stomatal regulation in the drought resistance of Qilex, while increased apoplastic water fraction had a major contribution in the drought resistance of P. latifolia.

ESTIMATION OF CANOPY PHOTOSYNTHETIC AND NONPHOTOSYNTHETIC COMPONENTS FROM SPECTRAL TRANSMITTANCE

Spectral transmittance signatures (expressed as absorbances) were studied as a potential indicator of photosynthetic and nonphotosynthetic contributions to the canopy-absorbed photosynthetically active radiation (PAR). An analytical approach was made under laboratory conditions using synthetic canopies in an integrating sphere. This approach provided the basis for identifying spectral (absorbance-based) features and indices to estimate green (photosynthetic) and nongreen (structural and dead materials) contributions to canopy absorbance. A strong relationship was found between the amplitude of the first derivative of the absorbance (ARE) and green area, while the integrated absorbance in the PAR region (APAR) mainly responded to variations in total area. The ratio ARE/APAR was closely correlated to the fraction of photosynthetic area to total area (i.e., the canopy green fraction). Similarly, the ratio and normalized difference of the absorbances at 680 and 900 nm (ASR and ANDVI) closely tracked variations in the canopy green fraction. Subsequently, these indices were tested in field plots with contrasting structural characteristics. Under field conditions, ARE was a good indicator of green biomass. The indices ASR and ANDVI were also reliable indicators of green biomass but were affected by changes in sampling conditions. As in the lab study, ARE/APAR was a good indicator of canopy green fraction. Thus, ground-based measurements of canopy spectral transmittance provided a tool for determining the photosynthetic contribution to canopy-absorbed PAR by correcting for nonphotosynthetic canopy components. Moreover, ARE showed a strong correlation with conventional vegetation indices derived from spectral reflectance measurements. This technique could be a useful tool for plant ecophysiology studies and a field-validation method for remote-sensing studies.

Effects of leaf structure on reflectance estimates of chlorophyll content

Assessment of chlorophyll content is a valuable tool for agricultural and non‐managed ecosystem studies, since it provides information on key vegetation properties that are, in turn, linked to net primary production. The effects of varying leaf structure (leaf thickness [LT], leaf mass area [LMA] and leaf mass density [LMD]) on reflectance‐based chlorophyll indices were assessed using regression and correlation analyses for seven Mediterranean species. The chlorophyll indices used were: (1) corrected for differences in internal scattering, (2) corrected for differences in surface scattering and (3) based on first reflectance first derivatives. Within species, chlorophyll indices showed similar correlation with chlorophyll content (r 2 values larger than 0.80, p<0.001) while, across species, indices corrected for surface scattering and first reflectance derivative indices were more closely related to chlorophyll content (r 2 = 0.78 and r 2 = 0.75, respectively, p<0.001) than reflectance simple ratio indices (r 2 = 0.70, p<0.001). Nonetheless, species with thicker leaves showed lower index values at a similar chlorophyll content than species with thinner leaves. In species with thicker leaves, the increases in chlorophyll content were associated with increases in LMD rather than to changes in LT and were accompanied by significant reductions in NIR radiation scattering at 800 nm. The contribution of LT and LMD to changes in LMA, and their effects on NIR scattering, might promote deviation from the relationship between reflectance based chlorophyll indices and chlorophyll content.