Edmundo Acevedo

TILLAGE EFFECT ON SOIL ORGANIC MATTER, MYCORRHIZAL HYPHAE AND AGGREGATES IN A MEDITERRANEAN AGROECOSYSTEM

Arbuscular mycorrhizal fungi (AMF) and their product glomalin (GRSP) play a decisive role in the soil aggregation, affecting the carbon (C) dynamics in agroecosystems. Tillage affects the AMF activity and GRSP content, influencing the stability and the soil C forms as well. The aim of this study was to compare the effect of no tillage (NT) and conventional tillage (CT) on: i) arbuscular mycorrhizal hyphal length and GRSP content; ii) the nature of soil organic matter by means of physical fractionation (free particulate organic matter (fPOM); occluded particulate organic matter (oPOM) and mineral-associated soil organic matter (Mineral)), as well as chemical fractionation (fulvic acid, humic acid and humin), and iii) the relationships between AMF parameters, soil carbon and water stable aggregates (WSA) in a Mollisol of Central Chile managed for 6 years under NT and CT using a wheat-corn rotation. Higher values in the AMF hyphal length, GRSP and WSA in NT compared with CT were observed. Significant relationships were found between GRSP and WSA (r = 0.66, p < 0.01) and total mycelium and GRSP (r = 0.58, p< 0.05). The total carbon increased 44% under NT compared with CT. The chemical fractionation showed percentage greater than 95% for humim in both treatments. Physical fractionation indicates that the higher part of the SOC (89.4 - 95.1%) was associated with the mineral fraction.

Modeling of par interception and productivity by Opuntia ficus-indica

A model is described that permits accurate calculation of radiation interception and shadows cast by a set of planar surfaces whose Cartesian coordinates are known. The model is applied to the calculation of interception of photosynthetically active radiation (PAR) by the cladodes (flattened stems) of the widely cultivated cactus, Opuntia ficus-indica. Taking into account both direct and diffuse PAR, a PAR index is determined for both sides of the 27 cladodes on two plants. This index has a maximum value of 1.00 when net CO2 uptake by the cladodes is not limited by PAR. The product with similar indices for water status and temperature indicates the fraction of maximum net CO2 uptake expected under particular environmental conditions, the environmental productivity index (EPI). EPI is used to predict the net assimilation rate (NAR) of O. ficus-indica under field conditions in central Chile, where its NAR has been measured. The predicted NAR closely agreed with seasonal trends and average annual values, but underestimated measured NAR in the winter, presumably due to over-representation of the low NAR caused by isotropic diffuse radiation at this cloudy time of the year. Applying the PAR model to different plant spacings, the annual productivity predicted for a stem area index (stem area per ground area) of 1.4 was 1.4 kg m−2 y−1, which closely matched field measurements; maximal productivity nearly threefold higher was predicted at a stem area index of 7. The PAR model was also used to analyze a hypothetical plant with all cladodes in a plane so that the influence of cladode orientation on the PAR index at various times of the year for both clear and cloudy days could be determined. Besides indicating PAR distribution within the canopy of the platyopuntias, the PAR model can be used for other three-dimensional distributions of photosynthetic surfaces, especially those that have regularly repeating units in space, such as orchards and vineyards.

Modelling the spectral response of the desert tree Prosopis tamarugo to water stress

In this paper, we carried out a laboratory experiment to study changes in canopy reflectance of Tamarugo plants under controlled water stress. Tamarugo (Prosopis tamarugo Phil.) is an endemic and endangered tree species adapted to the hyper-arid conditions of the Atacama Desert, Northern Chile. Observed variation in reflectance during the day (due to leaf movements) as well as changes over the experimental period (due to water stress) were successfully modelled by using the Soil-Leaf-Canopy (SLC) radiative transfer model. Empirical canopy reflectance changes were mostly explained by the parameters leaf area index (LAI), leaf inclination distribution function (LIDF) and equivalent water thickness (EWT) as shown by the SLC simulations. Diurnal leaf movements observed in Tamarugo plants (as adaptation to decrease direct solar irradiation at the hottest time of the day) had an important effect on canopy reflectance and were explained by the LIDF parameter. The results suggest that remote sensing based assessment of this desert tree should consider LAI and canopy water content (CWC) as water stress indicators. Consequently, we tested fifteen different vegetation indices and spectral absorption features proposed in literature for detecting changes of LAI and CWC, considering the effect of LIDF variations. A sensitivity analysis was carried out using SLC simulations with a broad range of LAI, LIDF and EWT values. The Water Index was the most sensitive remote sensing feature for estimating CWC for values less than 0.036 g/cm2, while the area under the curve for the spectral range 910–1070 nm was most sensitive for values higher than 0.036 g/cm2. The red-edge chlorophyll index (CIred-edge) performed the best for estimating LAI. Diurnal leaf movements had an effect on all remote sensing features tested, particularly on those for detecting changes in CWC.

Oxygen diffusion in soils: Understanding the factors and processes needed for modeling

Oxygen is an important element for plant growth. Reducing its concentration in the soil affects plant physiological processes such as nutrient and water uptake as well as respiration, the redox potential of soil elements and the activity of microorganisms. The main mechanism of oxygen transport in the soil is by diffusion, a dynamic process greatly influenced by soil physical properties such as texture and structure, conditioning, pore size distribution, tortuosity and connectivity. Organic matter is a modifying agent of the soil’s chemical and physical properties, affecting its structure and the porous matrix, which are determinants of oxygen transport. This study reviews the theory of soil gas diffusion and the effect of soil organic matter on the soil’s physical properties and transport of gases. It also reviews gas diffusion models, particularly those including the effect of soil organic matter.

Assessing Water Stress of Desert Tamarugo Trees Using in situ Data and Very High Spatial Resolution Remote Sensing

The hyper-arid Atacama Desert is one of the most extreme environments for life and only few species have evolved to survive its aridness. One such species is the tree Prosopis tamarugo Phil. Because Tamarugo completely depends on groundwater, it is being threatened by the high water demand from the Chilean mining industry and the human consumption. In this paper, we identified the most important biophysical variables to assess the water status of Tamarugo trees and tested the potential of WorldView2 satellite images to retrieve these variables. We propose green canopy fraction (GCF) and green drip line leaf area index (DLLAIgreen) as best variables and a value of 0.25 GCF as a critical threshold for Tamarugo survival. Using the WorldView2 spectral bands and an object-based image analysis, we showed that the NDVI and the Red-edge Chlorophyll Index (CIRed-edge) have good potential to retrieve GCF and DLLAIgreen. The NDVI performed best for DLLAIgreen (RMSE = 0.4) while the CIRed-edge was best for GCF (RMSE = 0.1). However, both indices were affected by Tamarugo leaf movements (leaves avoid facing direct solar radiation at the hottest time of the day). Thus, monitoring systems based on these indices should consider the time of the day and the season of the year at which the satellite images are acquired.

CULTIVO DE TOMATE (LYCOPERSICON ESCULENTUM MILL.) HIDROPÓNICO CON AGUA DESALINIZADA Y DESBORIFICADA EN EL VALLE DE LLUTA

Se realizo un cultivo hidroponico de tomate (Lycopersicon esculentum Mill.) en invernadero en el Campus Azapa de la Universidad de Tarapaca, con el objetivo de estudiar el efecto del ambiente controlado y la calidad del agua de riego sobre el rendimiento total y la calidad de los frutos de tomate. El ensayo conto con cuatro tratamientos: T1 solucion nutritiva preparada con agua del rio Lluta (CE 3,96 dS m-1; 7,1 ppm B); T2 solucion nutritiva preparada con agua tratada (desalinizada y desborificada) + boro (CE 3,1 dS m-1; 7,1 ppm B); T3 solucion nutritiva preparada con agua tratada + sulfato de magnesio (CE 3,96 dS m-1; 0,8 ppm B), y T4 solucion nutritiva preparada con agua tratada (CE 3,1 dS m-1; 0,8 ppm B). El diseno fue de bloques completos al azar con cuatro repeticiones. Las plantas se cosecharon hasta el quinto racimo (120 dias desde el trasplante), obteniendose los siguientes rendimientos: T1 = 40 t ha-1, T2 = 39 t ha-1, T3 = 64 t ha-1 y T4 = 61 t ha-1. Se observo que el exceso de boro de los tratamientos T1 y T2 produjo una disminucion importante en el rendimiento total de tomate en un 40% y en el rendimiento comercial de un 55%. La totalidad de los tratamientos tuvo un rendimiento mayor al mejor rendimiento informado para los cultivos de tomate al aire libre en el valle de Lluta (30 Mg ha-1). Se concluyo que el cultivo hidroponico de tomate en invernadero en el valle de Lluta es una alternativa para aumentar los rendimientos de este cultivo, y que la principal limitante de este valle para el cultivo de tomate es el alto contenido de boro en sus aguas de riego.

Anatomía del tejido fotosintético de diez taxa de Opuntia establecidos en el secano árido mediterráneo de Chile

Se evaluo el efecto de la exposicion de cladodios a la radiacion en la anatomia del tejido fotosintetico de 10 taxa de Opuntia, (plantas con metabolismo acido crasulaceo, CAM) establecidos en al secano arido de la IV Region de Chile. A nivel del tejido epidermico se evaluo el grosor de la cuticula, la densidad de estomas y las dimensiones de celulas oclusivas. En el tejido fotosintetico, se evaluo las dimensiones celulares con el objeto de estimar la superficie de paredes celulares expuestas al intercambio gaseoso por unidad de tejido fotosintetico, relacion conocida como Ames/A. Los resultados senalan grandes dimensiones celulares y baja densidad estomatica, lo que determina un numero reducido de estomas por unidad de superficie en relacion a especies de otras vias metabolicas (C3 y C4). La comparacion entre taxones, muestra diferencias significativas en la relacion de areas Ames/A, parametro estrechamente relacionado a la capacidad fotosintetica. Estas caracteristicas anatomicas y sus modificaciones en combinacion con el metabolismo CAM, contribuyen a su adaptacion a condiciones de pluviometria limitada

Conservation tillage and water availability for wheat in the dryland of central Chile

The dryland areas of Central Chile are associated to Mediterranean climate. Under these conditions, water availability during crop growth is a crucial factor for crop productivity. Conservation tillage systems play an important role in this area, increasing soil water availability; however, crop yield responses can be variable. Soil management should be aimed at reducing water loss and promoting water use by crops. The aim of this review is to analyze and discuss the factors affecting water availability in the Mediterranean drylands of Central Chile, as well as to study the effect of tillage systems on the water use efficiency of wheat.

Water Relations and Foliar Isotopic Composition of Prosopis tamarugo Phil., an Endemic Tree of the Atacama Desert Growing at Three Levels of Water Table Depth.

Prosopis tamarugo Phil. is a strict phreatophyte tree species endemic to the “Pampa del Tamarugal”, Atacama Desert. The extraction of water for various uses has increased the depth of the water table in the Pampa aquifers threatening its conservation. This study aimed to determine the effect of the groundwater table depth on the water relations of P. tamarugo and to present thresholds of groundwater depth (GWD) that can be used in the groundwater management of the P. tamarugo ecosystem. Three levels of GWD, 11.2 ± 0.3 m, 10.3 ± 0.3 m, and 7.1 ± 0.1 m, (the last GWD being our reference) were selected and groups of four individuals per GWD were studied in the months of January and July of the years 2011 through 2014. When the water table depth exceeded 10 m, P. tamarugo had lower pre-dawn and mid-day water potential but no differences were observed in minimum leaf stomatal resistance when compared to the condition of 7.1 m GWD; the leaf tissue increased its δ13C and δ18O composition. Furthermore, a smaller green canopy fraction of the trees and increased foliage loss in winter with increasing water table depth was observed. The differences observed in the physiological behavior of P. tamarugo trees, attributable to the ground water depth; show that increasing the depth of the water table from 7 to 11 m significantly affects the water status of P. tamarugo. The results indicate that P. tamarugo has an anisohydric stomatal behavior and that given a reduction in water supply it regulates the water demand via foliage loss. The growth and leaf physiological activities are highly sensitive to GWD. The foliage loss appears to prevent the trees from reaching water potentials leading to complete loss of hydraulic functionality by cavitation. The balance achieved between water supply and demand was reflected in the low variation of the water potential and of the variables related to gas exchange over time for a given GWD. This acclimation capacity of P. tamarugo after experiencing increases in GWD has great value for the implementation of conservation strategies. The thresholds presented in this paper should prove useful for conservation purposes of this unique species.

Traits Explaining Durum Wheat (Triticum turgidum L. spp. Durum) Yield in Dry Chilean Mediterranean Environments

Yield under water stress (YS) is used as the main criterion in the selection of wheat varieties for dry Mediterranean environments. It has been proposed that selection of genotypes using YS assisted by morphological and physiological traits associated with YS is more efficient in selecting high yielding genotypes for dry environments. A study was carried out at the Antumapu Experiment Station of the University of Chile, located in Santiago, Chile (33° 40′S and 70° 38′ W). The objective was to evaluate the extent to which morpho physiological traits could explain YS. For this purpose, grain yield and yield components of 185 durum wheat genotypes from ICARDA (International Center for Agricultural Research in the Dry Areas) and INIA (Chilean National Institute for Agricultural Research) were evaluated along with seed size and weight, days to heading (DH), glaucousness (GLAU), plant height (PH) and 13C discrimination (Δ). The design was an α-lattice with two replications, the genotypes were grown in two different water conditions (high and low irrigation) during two seasons (2011-2012/2012-2013). Grain weight (GW) was the only yield component with high H associated with YS, but it was not associated with yield under high irrigation (YI). The combination of YI with DH+GLAU+PH+Δ+GW obtained in LI environments explained a greater fraction of YS (38%) across years; these traits had lower genotype x environment interaction than YS, they also explained a higher proportion of yield under drought than YI. None of the traits studied could replace YS in selections for grain yield. It is concluded that these traits could aid in the selection of durum wheat subject to water stress, particularly in early generations.