The limits of the binary power law describing spatial variability for incidence data

Taylor's power law (TPL), an empirical law relating the observed variance to mean density (or abundance), has found wide applicability for characterizing heterogeneity in many disciplines. However, when the density variable has an upper bound, the TPL does not hold and the binary power law (BPL) needs to be used instead. The BPL has been shown to describe the heterogeneity of numerous plant disease epidemic systems. In this study, a generic stochastic simulator was used to study the extent to which the BPL can satisfactorily describe incidence data. Results showed that the symmetrical BPL does hold whenever there is a positive correlation among neighbours on the probability of a plant becoming infected, or where disease development is not influenced by the neighbours. These results held for a wide range of neighbourhood sizes, strengths of neighbourhood influence, and size of the sampling quadrats. However, the symmetrical BPL did not hold when there is a negative influence among neighbours. The more general asymmetrical BPL (ABPL) fitted the data with positive or negative neighbourhood influence, but because a negative neighbourhood effect is generally unlikely for plant epidemics, the symmetrical BPL is preferred over the ABPL because of its parsimony. The magnitude of the estimated BPL parameters increased with increasing neighbourhood influence and sampling‐quadrat size. However, except when the power parameter equals 1, inferring specific underlying mechanisms generating the data or comparing BPL estimates from different studies is difficult, because of the large effect of sampling on the BPL parameter estimates.     

Epiphytic colonization of Ustilaginoidea virens on biotic and abiotic surfaces implies the widespread presence of primary inoculum for rice false smut disease

Ustilaginoidea virens (Uv), the causative agent of rice false smut disease, infects developing rice spikelets at the booting stage, and transforms individual grains of the panicle into smut balls. Epidemics of the disease occur when the rice booting and heading stages coincide with rainy days. Using a green fluorescent protein (GFP)‐labelled Uv isolate that can form false smut balls on rice panicles, it was found that under high humidity and free water conditions the Uv isolate could colonize leaves of plants belonging to various families including the Poaceae (Oryza sativa, Echinochloa crusgalli, Digitaria sanguinalis and Leptochloa chinensis), the Brassicaceae (Arabidopsis thaliana) and the Solanaceae (Nicotiana benthamiana) without symptoms. Over several days, some conidia could germinate on the leaves of these plants and in water on the surface of Parafilm and cellophane, form hyphae and differentiate conidiophores to generate a large number of secondary conidia, while other conidia were able to directly produce secondary conidia. Conversely, in the absence of water some conidia could either bud to form new conidia or were converted into chlamydospores. These data indicate that Uv is one of a few fungal pathogens reported to have epiphytic characteristics. The rapid generation of a large number of spores on biotic and abiotic surfaces greatly increases the inoculum that can infect rice spikelets, resulting in the occurrence of rice false smut disease epidemics. These findings are important in the development of disease control strategies.     

A model calculation approach towards the optimization of a standard scheme of seed‐parent line development in hybrid rye breeding

The importance of hybrid rye (Secale cereale L.) breeding has steadily increased over the last decades. This paper presents results of model calculations aiming to optimize the number of candidates, testers to assess combining ability and test locations at each selection stage of a standard scheme of seed‐parent line development. Two variants of the scheme differing in the number of stages and the cycle length are investigated. Optimization criterion is the expected selection gain per year under the restriction of a fixed budget. Prediction of selection gain rests on quantitative–genetic and economic parameters estimated from breeders’ data. Optimization covers different genetical and economical situations. Results show that the optimum number of testers to assess combining ability depends on the relative amount of dominance variance. The efficiency of a breeding scheme strongly increases with decreasing cycle length. A larger budget should mainly be used to increase the number of candidates at all selection stages. Recommendations for practical breeding schemes are given. We conclude that model calculations are a valuable tool for the optimization of breeding schemes.     

An Analysis of Urban Size and Territorial Location Effects on Employment Probabilities: The Spanish Case

The probability of being employed varies depending on several factors. Many of these are related to personal characteristics such as educational level, age, gender, or number and age of children. Nevertheless, other factors may be relevant, in particular the geographical environment. The purpose of this paper is to analyse the relevance of urban size and the position of each territory (in terms of its distance from large metropolises) for the probability of being employed in the Spanish economy. Following the set of economic regions suggested by Polése, Shearmur and Rubiera (2007 ), we try to explain the spatial patterns of employment distribution. Our results show some relevant differences between these alternative economic areas. We find that municipalities with similar sizes and located at a similar distance from a metropolis but belonging to different Autonomous Communities or provinces share similar employability patterns.     

Water utilization patterns around isolated Acacia karroo trees in the False Thornveld of the eastern Cape

The effect of various vegetation treatments on two soil moisture regimes (i.e. the proportion of the experimental period where the soil had sufficient water for: (a) growth, and (b) to keep the plants turgid in the vicinity of experimentally isolated A. karroo trees) was monitored over a two‐year period. Removal of all vegetation had the greatest effect on soil moisture, increasing the moisture regime by around 200%. Grass removal had the next most significant effect, increasing moisture regimes within 9 m of the tree by around 100%. Removal of the tree had the smallest significant effect, increasing the moisture regime by less than 20%. There was no significant difference in the moisture regime surrounding trees with heights ranging between 1,4 m and 2,5 m, or where various combinations of tree and/or grass defoliations were implemented. We conclude that water supply to the trees is enhanced when soil water extraction is reduced (e.g. during winter or when the sward is harmed) and argue that this may be a mechanism of accelerating bush encroachment in semi‐arid savannas.     

Characterizing Submergence Survival Strategy in Rice Via Chlorophyll Fluorescence

Rice plants cope with flash floods using either an ‘escape strategy’ involving rapid shoot elongation or a ‘quiescence strategy’ involving survival underwater with minimal activity. To clarify the differences in the response of leaf photosynthesis properties to conditions during and after submergence, two rice cultivars were compared: a non‐shoot‐elongating cultivar IR 67520‐B‐14‐1‐3‐2‐2 (IR67520) and a shoot‐elongating cultivar IR72442‐6B‐3‐2‐1‐1 (IR72442). Twenty‐three‐day‐old seedlings were submerged in 80‐cm‐deep water for 14 days. During submergence, the chlorophyll contents of the upper fully expanded leaf (5th leaf) and newly developed leaf later (6th leaf) and the maximal quantum yield of photosystem II (F_v/F_m) of the fifth leaf decreased earlier in IR72442 than in IR67520. In the submerged sixth leaf, F_v/F_m was higher in IR72442 than in IR67520 at early measurement. Although F_v/F_m of the sixth leaf in submerged IR67520 increased substantially from 2 days post‐submergence, IR72442 decreased because of leaf chlorosis. Therefore, a non‐shoot‐elongating cultivar coped with submergence by inhibiting photodamage and maintaining high chlorophyll content in the leaves. The shoot‐elongating cultivar was able to maintain the photosynthetic capacity of the newly developed leaf during submergence by prompt reduction of chlorophyll and chlorophyll fluorescence in the leaf that developed before submergence.     

‘Water war’ in the Mekong Basin?

The Mekong River system provides a crucial source of natural resources for riparian nations. However, the increasingly rapid pace of hydro‐development in the Mekong Basin is threatening the integrity of the river system, posing a real concern for Lower Basin states, which are particularly dependent on the basin. This scenario has led to warnings of armed conflict, or even ‘water war’, between riparian states. Certainly, the expanding scale of hydro‐development can be expected to continue increasing interstate tensions in the Mekong region; but are these tensions really likely to escalate to armed conflict? This paper explores this question by drawing on the water and conflict theory of Aaron Wolf. Ultimately, this paper concludes that interstate tensions over Mekong hydro‐development are unlikely to generate armed conflict. This is in part due to the strategic impracticality of such a conflict as well as the presence of a river basin management institution. Most compellingly, though, armed conflict is unlikely because the economic imperative shared by Mekong states is better served by cooperation – or at least non‐interference – than conflict, over regional hydro‐development. In closing, the paper urges that the study of water and conflict in the Mekong Basin be refocused at the intrastate level.