Detrimental effect of rust infection on the water relations of bean

Bean plants (Phaseolus vulgaris L.) infected with the rust Uromyces phaseoli became unusually susceptible to drought as sporulation occurred. Under the conditions used (1,300 ft-c, 27 C, and 55% relative humidity) such plants wilted at soil water potentials greater than −1 bar, whereas healthy plants did not wilt until the soil water potential fell below −3.4 bars. Determinations of leaf water and osmotic potentials showed that an alteration in leaf osmotic potential was not responsible for the wilting of diseased plants. When diffusive resistance was measured as a function of decreasing leaf water content, the resistance of healthy leaves increased to 50 sec cm⁻¹ by the time relative water content decreased to 70%, whereas the resistance of diseased leaves remained less than 8 sec cm⁻¹ down to 50% relative water content. Apparently, water vapor loss through cuticle damaged by the sporulation process, together with the reduction in root to shoot ratio which occurs in diseased plants, upset the water economy of the diseased plant under mild drought conditions.     

Identification of Phytophthora citrophthora with Cloned DNA Probes

Two different DNA fragments, one of 2.9 kilobases and the other of 5.1 kilobases, were cloned from Phytophthora citrophthora and showed no homology with DNA from plants and other related fungi. These DNA probes hybridized with DNA from 12 different P. citrophthora isolates obtained from a variety of hosts but did not hybridize with DNA from 6 P. citrophthora isolates obtained from cacao. Southern blot analysis revealed that the probes contained repetitive DNA, and restriction fragment length polymorphisms were identified among several P. citrophthora isolates. Of the isolates tested, two major groups were observed whose genetic similarity correlated with geographical distribution. One of the DNA probes was used to detect P. citrophthora growing from infected citrus roots incubated on semiselective medium. P. citrophthora was not detected by a hybridization assay of total DNA extracted directly from infected roots.     

Effects of mycorrhizal infection on drought tolerance and recovery in safflower and wheat

The influence of arbuscular mycorrhizal fungi on drought tolerance and recovery was studied in safflower (Carthamus tinctorius L.) and wheat (Triticum aestivum L.). Plants were grown with and without the mycorrhizal fungus, Glomus etunicatum Becker & Gerd., in nutrient-amended soil under environmentally-controlled conditions to yield mycorrhizal and nonmycorrhizal with similar leaf areas, root length densities, dry weights, and adequate tissue phosphorus. When drought stress was induced, mycorrhizal infection did not affect changes in leaf water, osmotic or pressure potentials, or osmotic potentials of leaf tissue rehydrated to full turgor in either safflower or wheat. Furthermore, in safflower, infection had little effect on drought tolerance as indicated by the level of leaf necrosis. Mycorrhizal wheat plants, however, had less necrotic leaf tissue than uninfected plants at moderate levels of drought stress (but not at severe levels) probably due to enhanced phosphorus nutrition. To determine the effects of infection on drought recovery, plants were rewatered at a range of soil water potentials from –1 to –4 MPa. We found that although safflower tended to recover more slowly from drought after rewatering than wheat, mycorrhizal infection did not directly affect drought recovery in either plant species. Daily water use after rewatering was reduced and was correlated to the extent that leaves were damaged by drought stress in both plant species, but was not directly influenced by the mycorrhizal status of the plants.     

Inoculation and habitat amelioration efforts in biological soil crust recovery vary by desert and soil texture

As dryland degradation continues, it is increasingly important to understand how to effectively restore biocrust communities. Potential techniques include the addition of biocrust inoculum to accelerate biocrust recovery. Enhanced erosion typical of degraded environments creates a challenge for these approaches, due to loss by wind or water and burial by saltating particles. To retain and protect added inoculum, the inclusion of habitat‐amelioration techniques can improve recovery rates. This study tested three different types of inoculum (field‐collected, greenhouse‐cultivated, and laboratory‐cultivated biocrust) coupled with two treatments to augment soil stability and ameliorate habitat limitations: soil surface polyacrylamide additions and installation of straw barriers. This was done across two deserts (Great Basin and Chihuahuan) and separated into generally coarse‐ or finer‐textured soils in each desert, with results monitored for 3 years (2015, 2016, 2017). While the inoculum type, coupled with habitat ameliorations, occasionally enhanced biocrust growth across years and treatments, in other cases, it made no difference compared to natural recovery rates. Rather, the desert location and soil texture groupings were the most prominent factors in determining recovery trajectories. Recovery proportions were similar in the finer‐textured sites in both the Great Basin and the Chihuahuan deserts, while the coarser‐textured site in the Great Basin did show some recovery over time and the Chihuahuan coarser‐textured site did not. This study demonstrates the importance of understanding site potential and identifying key limitations to biocrust recovery for successful restoration projects.     

The influence of the mycorrhiza Glomus etunicatum on drought acclimation in safflower and wheat

A study was done to determine the effects of vesicular‐arbuscular mycorrhizal (VAM) colonization on drought acclimation of host plants. Safflower ( Carthamus tinctorius L. cv. S555) and wheat ( Triticum aestivum L. cv. Anza) were grown under environmentally controlled conditions with or without the VAM fungus, Glomus etunicatum Becker and Gerd., and were either acclimated (by pre‐exposing plants to a 10–11 day drought period) or unacclimated to drought. Plants from all treatments were then exposed to drought for 9 days, and plant water status and root water uptake were measured. To minimize interactions between drought and P uptake, growth periods were adjusted so that acclimated and unacclimated plants were similar in size when measurements were made. When wheat was acclimated to drought, osmotic adjustment occurred (leaf solute potentials of leaf tissue rehydrated to full turgor were approximately 0.5 MPa lower in acclimated than unacclimated plants); in safflower, osmotic adjustment was minimal when plants were acclimated. Consequently, acclimated wheat plants were able to tolerate drought better than unacclimated plants, and maintained higher leaf water potentials and relative water contents as soil water was depleted. For both safflower and wheat, acclimated plants had higher water use efficiency, and therefore produced more biomass when water availability was limited, than unacclimated plants. However, mycorrhizal colonization did not affect osmotic adjustment, plant water status, water use efficiency or water uptake in either plant species, and therefore had no effect on drought acclimation or resistance.     

Transient population dynamics impede restoration and may promote ecosystem transformation after disturbance

The apparent failure of ecosystems to recover from increasingly widespread disturbance is a global concern. Despite growing focus on factors inhibiting resilience and restoration, we still know very little about how demographic and population processes influence recovery. Using inverse and forward demographic modelling of 531 post‐fire sagebrush populations across the western US, we show that demographic processes during recovery from seeds do not initially lead to population growth but rather to years of population decline, low density, and risk of extirpation after disturbance and restoration, even at sites with potential to support long‐term, stable populations. Changes in population structure, and resulting transient population dynamics, lead to a > 50% decline in population growth rate after disturbance and significant reductions in population density. Our results indicate that demographic processes influence the recovery of ecosystems from disturbance and that demographic analyses can be used by resource managers to anticipate ecological transformation risk.     

Adapting management to a changing world: Warm temperatures,dry soil,and interannual variability limit restoration success of a dominant woody shrub in temperate drylands

Restoration and rehabilitation of native vegetation in dryland ecosystems, which encompass over 40% of terrestrial ecosystems, is a common challenge that continues to grow as wildfire and biological invasions transform dryland plant communities. The difficulty in part stems from low and variable precipitation, combined with limited understanding about how weather conditions influence restoration outcomes, and increasing recognition that one‐time seeding approaches can fail if they do not occur during appropriate plant establishment conditions. The sagebrush biome, which once covered over 620,000 km² of western North America, is a prime example of a pressing dryland restoration challenge for which restoration success has been variable. We analyzed field data on Artemisia tridentata (big sagebrush) restoration collected at 771 plots in 177 wildfire sites across its western range, and used process‐based ecohydrological modeling to identify factors leading to its establishment. Our results indicate big sagebrush occurrence is most strongly associated with relatively cool temperatures and wet soils in the first spring after seeding. In particular, the amount of winter snowpack, but not total precipitation, helped explain the availability of spring soil moisture and restoration success. We also find considerable interannual variability in the probability of sagebrush establishment. Adaptive management strategies that target seeding during cool, wet years or mitigate effects of variability through repeated seeding may improve the likelihood of successful restoration in dryland ecosystems. Given consistent projections of increasing temperatures, declining snowpack, and increasing weather variability throughout midlatitude drylands, weather‐centric adaptive management approaches to restoration will be increasingly important for dryland restoration success.