Water Stress Enhances the Progression of Branch Dieback and Almond Decline under Field Conditions.

Branch dieback and tree decline have been described as a common complex disease worldwide in woody crops, with Botryosphaeriaceae and Diaporthaceae being considered the most frequent fungi associated with the disease symptoms. Their behaviour is still uncertain, since they are considered endophytes becoming pathogenic in weakened hosts when stress conditions, such as water deficiency occur. Therefore, the main goal of this study was to determine if water stress enhances general decline on weakened almond trees subjected to different irrigation treatments under natural field conditions. In parallel, the occurrence of fungal species associated with almond decline was also determined in relation to disease progression by fungal isolation, and morphological and molecular based-methods. The symptoms of branch dieback and general decline were observed over time, mainly in the experimental plots subjected to high water deficiency. Botryosphaeriaceae were the most consistently isolated fungi, and Botryosphaeria dothidea was the most frequent. Collophorina hispanica was the second most frequent species and Diaporthe and Cytospora species were isolated in a low frequency. Most of them were recovered from both asymptomatic and symptomatic trees, with their consistency of isolation increasing with the disease severity. This work reveals the need to elucidate the role of biotic and abiotic factors which increase the rate of infection of fungal trunk pathogens, in order to generate important knowledge on their life cycle.

OliveCan: A Process-Based Model of Development, Growth and Yield of Olive Orchards.

Several simulation models of the olive crop have been formulated so far, but none of them is capable of analyzing the impact of environmental conditions and management practices on water relations, growth and productivity under both well-irrigated and water-limiting irrigation strategies. This paper presents and tests OliveCan, a process-oriented model conceived for those purposes. In short, OliveCan is composed of three main model components simulating the principal elements of the water and carbon balances of olive orchards and the impacts of some management operations. To assess its predictive power, OliveCan was tested against independent data collected in two 3-year field experiments conducted in Córdoba, Spain, each of them applying different irrigation treatments. An acceptable level of agreement was found between measured and simulated values of seasonal evapotranspiration (ET, range 393 to 1016 mm year-1; RMSE of 89 mm year-1), daily transpiration (Ep, range 0.14-3.63 mm d-1; RMSE of 0.32 mm d-1) and oil yield (Yoil, range 13-357 g m-2; RMSE of 63 g m-2). Finally, knowledge gaps identified during the formulation of the model and further testing needs are discussed, highlighting that there is additional room for improving its robustness. It is concluded that OliveCan has a strong potential as a simulation platform for a variety of research applications.

Stomatal oscillations in olive trees: analysis and methodological implications.

Stomatal oscillations have long been disregarded in the literature despite the fact that the phenomenon has been described for a variety of plant species. This study aims to characterize the occurrence of oscillations in olive trees (Olea europaea L.) under different growing conditions and its methodological implications. Three experiments with young potted olives and one with large field-grown trees were performed. Sap flow measurements were always used to monitor the occurrence of oscillations, with additional determinations of trunk diameter variations and leaf-level stomatal conductance, photosynthesis and water potential also conducted in some cases. Strong oscillations with periods of 30-60 min were generally observed for young trees, while large field trees rarely showed significant oscillations. Severe water stress led to the disappearance of oscillations, but moderate water deficits occasionally promoted them. Simultaneous oscillations were also found for leaf stomatal conductance, leaf photosynthesis and trunk diameter, with the former presenting the highest amplitudes. The strong oscillations found in young potted olive trees preclude the use of infrequent measurements of stomatal conductance and related variables to characterize differences between trees of different cultivars or subjected to different experimental treatments. Under these circumstances, our results suggest that reliable estimates could be obtained using measurement intervals below 15 min.

Effect of soil temperature on root resistance: implications for different trees under Mediterranean conditions.

The effect of temperature on radial root hydraulic specific resistance (Rp) is a known phenomenon; however, the impact ofRpvariations expected from soil temperature changes over the tree root system is unknown. The present article analyses the relations hip ofRpwith temperature in olive 'Picual' and a hybrid rootstock, GF677, at five different temperatures, showing that a variation of 3- and 4.5-folds exists for olive 'Picual' and GF677 in the range from 10 to 20 °C. The functions obtained were scaled up to show the theoretical changes of total radial root system resistance in a common tree orchard in a Mediterranean climate at a daily and seasonal scale, using recorded soil temperature values: a difference between summer and winter of 3.5-fold for olive 'Picual' and 9-fold for GF677 was observed. Nevertheless,Rpchanges are not only related to temperature, as cavitation or circadian rhythms in aquaporin expression may also play a role. The results obtained from an experiment with the two cultivars submitted to constant pressure and temperature during several hours exhibited a variation inRp, but this was of lower magnitude than that observed due to temperature changes. Finally, a comparison ofRpat 25 °C between GF677 and GN15 (another rootstock obtained from the same parental as GF677) showed significant differences. According to our results, diurnal and seasonal changes inRpdue to temperature variations are of significant importance, and it would therefore be advisable to assess them explicitly into soil-plant-atmosphere continuum models.

Balancing crop yield and water productivity tradeoffs in herbaceous and woody crops.

The links between water and crop yield are well known. In agricultural systems, maximum yield and maximum water productivity (WP; yield divided by water use) are not always compatible goals. In water-limited situations, optimal solutions must be reached by finding a compromise between the levels of crop production and WP. The tradeoffs between production and WP are reviewed here and the dominant effects of the environment on WP are examined. Genetic improvement for WP generally has yield tradeoffs, whereas management measures devised to improve WP also enhance yield. It is shown that partial closure of the stomata in response to environmental stimuli has a variable impact on canopy transpiration, depending on the degree of coupling between the canopy and the atmosphere. In contrast to the behaviour of the major herbaceous crops, WP increases in some woody crops in response to water stress, suggesting that biomass and transpiration are not linearly related, and that deficit irrigation should be successful in these species. Avoiding high evaporative demand periods (e.g. through tolerance to low temperatures) is an important option that aims to increase production and WP. A case study is presented for improving sunflower (Helianthus annuus L.) yield and WP in temperate environments.

Reflections on food security under water scarcity.

Forecasts on population growth and economic development indicate that there will be substantial increases in food demand for the forthcoming decades. We focus here on the water requirements of food production, on the issue of whether there would be enough water to produce sufficient food in the future, and we offer options to face this challenge based on recent trends observed in some agricultural systems. Given the competition for water faced by the agricultural sector, and the uncertainties associated with climate change, improving the efficiency of water use in both rain-fed and irrigated systems is the main avenue to face the challenge. In rain-fed agriculture, managing the risk associated with rainfall variability is a promising option to increase productivity. In irrigated systems, a case study on the improvements in water productivity in Andalusia, Spain, is used to illustrate some of the opportunities to make progress. Progress in reducing irrigation water use in recent decades has been substantial, but decreasing the consumptive use of crops is a much more difficult challenge. The need for more research and technology transfer on improving water-limited crop production is highlighted, and emphasis is placed on interdisciplinary approaches to gain the insight needed to achieve new breakthroughs that would help in tackling this complex problem.

Changes in phenolic compounds in garlic (Allium sativum L.) owing to the cultivar and location of growth.

The contents of total phenolic compounds, flavonoids, and phenolic acids were determined in selected garlic cultivars grown at four locations. The total phenolic content varied from 3.4 mg gallic acid equivalents (GAE)/g of dry matter (dm) to 10.8 mg GAE/g of dm with a mean value of 6.5 mg GAE/g of dm. The myricetin, quercetin, kaempferol, and apigenin flavonoids were not detected in any of the samples. Caffeic acid and ferulic acid were the major phenolic acids found with mean values of 2.9 mg/kg of dm and 2.6 mg/kg of dm, respectively. The mean contents of vanillic, p-hydroxybenzoic, and p-coumaric acids were comparable (0.4-0.8 mg/kg of dm), and the level of sinapic acid was negligible (< 0.1 mg/kg of dm). There was a significant effect of location but an insignificant effect of genotype on contents of caffeic, vanillic, p-hydroxybenzoic, and p-coumaric acids. However, genotype but not location affected the contents of total phenolics and ferulic acid. On average, the white garlic cultivars and Chinese garlic cultivars contained higher contents of total phenolics and ferulic acid than the purple garlic cultivars. However, the differences in the total phenolic content between the purple and white garlic cultivars were not significant.

Effect of genetic characteristics and environmental factors on organosulfur compounds in garlic ( Allium sativum L.) grown in Andalusia, Spain.

The content of organosulfur compounds was determined in selected garlic cultivars grown at four locations in Andalusia, Spain. The organosulfur compounds studied were three γ-glutamyl peptides, namely, γ-l-glutamyl-S-(2-propenyl)-l-cysteine (GSAC), γ-l-glutamyl-S-(trans-1-propenyl)-l-cysteine (GSPC), and γ-l-glutamyl-S-methyl-l-cysteine (GSMC), and four cysteine sulfoxides (alliin, isoalliin, methiin, and cycloalliin). There was a significant effect of the location, cultivar, and garlic ecotype on individual organosulfur compound contents. Purple-type cultivars showed on average the highest contents of GSMC, GSAC, alliin, and methiin but the lowest isoalliin content. The impact of genotype was relatively high for GSAC, whereas this factor hardly contributed to the total variability in alliin and isoalliin content. Planting date had a significant effect on the content of alliin and isoalliin. Discriminant analysis evidenced the ability of organosulfur compounds to distinguish among garlic bulbs from different locations or ecotypes with 81 or 86% accuracy, respectively.

A model of daily mean canopy conductance for calculating transpiration of olive canopies.

We tested the hypothesis that the transpiration (λEp) of high-coupled canopies, such as olive groves, may be calculated on a daily basis with sufficient precision by the Penman-Monteith 'big leaf' equation, by a model of bulk daily canopy conductance (gc) capable of scaling for canopy dimension. Given the limited data required, such a model could replace the standard approach (ET0 × Kc) for calculating olive water requirements, enhancing the precision of estimates. We developed a specific model of daily gc for unstressed olive canopies that was calibrated by transpiration measurements obtained by water balance from a 2-year experiment in a mature orchard with λEp ranging from 0.6 (February 1993) to 11.5 (July 1994) MJ m-2 day-1 and where leaf area index (L) changed from 1.25 to 2.5. The model uses the intercepted fraction of daily PAR and a linear function of average daytime temperature. The model was validated with λEp data collected by eddy covariance in a 3-year experiment conducted in a growing orchard that differed in L and cultivar from the one used in the calibration. The gc model, when used in the Penman-Monteith equation, gave very good daily λEp predictions for all seasons during 3 years, ranging from 0.5 (November 1998) to 5.5 (June 2000) MJ m-2 day-1, indicating that the goals of dealing with the dependence of olive gc on L and of simulating the seasonal variations in gc were achieved. A comparison with the Jarvis gc model, calibrated with 2 months of measured gc hourly data, showed that the gc model developed here performed better than the Jarvis model for the 3-year dataset. The exception to this was the period in which the Jarvis model was calibrated. This indicates that (1) the Jarvis model did not account for the seasonal variations in gc of the olive trees; and (2) the spatial and temporal scale assumptions required in the calibration of gc generate seasonal errors in the simulated bulk daily λEp for this crop. The applicability of this bulk gc model is restricted to well watered olive canopies and to the one-layer approach of calculating λEp but it could be adapted to rain-fed canopies in the future.