Rain Exclusion Shelters Research Articles

Responses of soil CO2 efflux and microbial activity to water deficit under conventional and adaptation technology

Frequent periods of low rainfall accompanying climate change are expected to affect many aspects of agricultural production and greenhouse gas emissions. In this work, we conducted field experiments on soil CO2 efflux and soil microbial activity using two precipitation treatments and two crop management technologies. While the control treatment was exposed to natural precipitation, the reduced water availability was induced during the early stages of crop growth using rain-exclusion shelters within the water deficit treatment. We aimed to test the difference in the response to water deficit between two management technologies: conventional tillage (CT - ploughed without cover crops) and adaptation technology (AT - no-till with species rich cover crops). After a month of simulated water deficit, the soil CO2 efflux under a regime of adaptation technology was essentially the same as that under the conventional technology, and the decrease in soil CO2 efflux ranged between 22 % and 55 %. The water deficit had a technology-specific effect on microbes. While no effect on the microbial carbon under CT was observed, the water deficit decreased Cmic under AT. In contrast, the opposite pattern was observed for DHA. When a normal soil water regime was restored, CO2 efflux recovered rapidly while recovery of microbial biomass and dehydrogenase activity was more variable. Generally, the soil microbial biomass and activity recovered better under AT. The results show that while the reduction in Rs during water deficit and its recovery after the restoration of the ambient precipitation regime were not affected by the technology, the response of microbial biomass and activity to water deficit depended on the technology with better recovery under AT.

Field application of silicon alleviates drought stress and improves water use efficiency in wheat.

Detrimental impacts of drought on crop yield have tripled in the last 50 years with climate models predicting that the frequency of such droughts will intensify in the future. Silicon (Si) accumulation, especially in Poaceae crops such as wheat (Triticum aestivum L.), may alleviate the adverse impacts of drought. We have very limited information, however, about whether Si supplementation could alleviate the impacts of drought under field conditions and no studies have specifically manipulated rainfall. Using field-based rain exclusion shelters, we determined whether Si supplementation (equivalent to 39, 78 and 117kg ha-1) affected T. aestivum growth, elemental chemistry [Si, carbon (C) and nitrogen (N)], physiology (rates of photosynthesis, transpiration, stomatal conductance, and water use efficiency) and yield (grain production) under ambient and drought (50% of ambient) rainfall scenarios. Averaged across Si treatments, drought reduced shoot mass by 21% and grain production by 18%. Si supplementation increased shoot mass by up to 43% and 73% in ambient and drought water treatments, respectively, and restored grain production in droughted plants to levels comparable with plants supplied with ambient rainfall. Si supplementation increased leaf-level water use efficiency by 32-74%, depending on Si supplementation rates. Water supply and Si supplementation did not alter concentrations of C and N, but Si supplementation increased shoot C content by 39% and 83% under ambient and drought conditions, respectively. This equates to an increase from 6.4 to 8.9 tonnes C ha-1 and from 4.03 to 7.35 tonnes C ha-1 under ambient and drought conditions, respectively. We conclude that Si supplementation ameliorated the negative impacts of drought on T. aestivum growth and grain yield, potentially through its beneficial impacts on water use efficiency. Moreover, the beneficial impacts of Si on plant growth and C storage may render Si supplementation a useful tool for both drought mitigation and C sequestration.

Can trees buffer the impact of climate change on pasture production and digestibility of Mediterranean dehesas?

Sustainability and functioning of silvopastoral ecosystems are being threatened by the forecasted warmer and drier environments in the Mediterranean region. Scattered trees of these ecosystems could potentially mitigate the impact of climate change on herbaceous plant community but this issue has not yet tested experimentally. We carried out a field manipulative experiment of increased temperature (+2–3 °C) using Open Top Chambers and rainfall reduction (30%) through rain-exclusion shelters to evaluate how net primary productivity and digestibility respond to climate change over three consecutive years, and to test whether scattered trees could buffer the effects of higher aridity in Mediterranean dehesas. First, we observed that herbaceous communities located beneath tree canopy were less productive (351 g/m2) than in open grassland (493 g/m2) but had a higher digestibility (44% and 41%, respectively), likely promoted by tree shade and the higher soil fertility of this habitat. Second, both habitats responded similarly to climate change in terms of net primary productivity, with a 33% increase under warming and a 13% decrease under reduced rainfall. In contrast, biomass digestibility decreased under increased temperatures (−7.5%), since warming enhanced the fiber and lignin content and decreased the crude protein content of aerial biomass. This warming-induced effect on biomass digestibility only occurred in open grasslands, suggesting a buffering role of trees in mitigating the impact of climate change. Third, warming did not only affect these ecosystem processes in a direct way but also indirectly via changes in plant functional composition. Our findings suggest that climate change will alter both the quantity and quality of pasture production, with expected warmer conditions increasing net primary productivity but at the expense of reducing digestibility. This negative effect of warming on digestibility might be mitigated by scattered trees, highlighting the importance of implementing strategies and suitable management to control tree density in these ecosystems.

Estimating the capacity of Chamaecrista fasciculata for adaptation to change in precipitation.

Adaptation through natural selection may be the only means by which small and fragmented plant populations will persist through present day environmental change. A population's additive genetic variance for fitness (VA (W)) represents its immediate capacity to adapt to the environment in which it exists. We evaluated this property for a population of the annual legume Chamaecrista fasciculata through a quantitative genetic experiment in the tallgrass prairie region of the Midwestern United States, where changing climate is predicted to include more variability in rainfall. To reduce incident rainfall, relative to controls receiving ambient rain, we deployed rain exclusion shelters. We found significant VA (W) in both treatments. We also detected a significant genotype-by-treatment interaction for fitness, which suggests that the genetic basis of the response to natural selection will differ depending on precipitation. For the trait-specific leaf area, we detected maladaptive phenotypic plasticity and an interaction between genotype and environment. Selection for thicker leaves was detected with increased precipitation. These results indicate capacity of this population of C. fasciculata to adapt in situ to environmental change.

Registration of GP‐VT NC 01 peanut germplasm

AbstractGP‐VT NC 01 (Reg. no. GP‐238, PI 691613) is a virginia‐type peanut (Arachis hypogaea L. subsp. hypogaea var. hypogaea) with good agronomic characteristics and improved drought tolerance. GP‐VT NC 01 was tested under the experimental designation N05006. Replicated yield tests started in the F6 generation in North Carolina in 2005. From 2008 through 2010, GP‐VT NC 01 was tested in replicated trials in the Peanut Variety and Quality Evaluation, multistate project S1038, in Virginia, North Carolina, and South Carolina. In 2013 and 2014, GP‐VT NC 01 was tested in the field at the Tidewater Agricultural Research and Experiment Station in Suffolk, VA, under rain exclusion shelters with controlled water regimes ranging from optimum soil moisture to severe drought. In these tests, GP‐VT NC 01 was compared with the current cultivars grown in Virginia and the Carolinas—‘Bailey’, ‘Sugg’, ‘Sullivan’, and ‘Wynne’—and a drought‐tolerant species derived germplasm line (GP‐NC WS17). Under “normal” weather, in rainfed production, GP‐VT NC 01 had similar yield to Bailey and Sugg and significantly better yield than Sullivan or Wynne. Extra‐large kernels content was least, although not significantly smaller than Sullivan and Wynne. No significant differences in sound mature kernels were noted among the cultivars tested. However, during dry and hot years, and under drought‐controlled conditions, the yield of GP‐VT NC 01 was significantly higher than that of the other cultivars tested. Thus, the release of GP‐VT NC 01 provides producers with a profitable option that will perform well under drought stress.

Silicon uptake by a pasture grass experiencing simulated grazing is greatest under elevated precipitation

BackgroundGrasses are hyper-accumulators of silicon (Si) and often up-regulate Si following herbivory. Positive correlations exist between Si and plant water content, yet the extent to which Si uptake responses can be mediated by changes in soil water availability has rarely been studied and never, to our knowledge, under field conditions. We used field-based rain-exclusion shelters to investigate how simulated grazing (shoot clipping) and altered rainfall patterns (drought and elevated precipitation, representing 50% and 150% of ambient precipitation levels, respectively) affected initial patterns of root- and shoot-Si uptake in a native Australian grass (Microlaena stipoides) in Si-supplemented and untreated soils.ResultsSi supplementation increased soil water retention under ambient and elevated precipitation but not under drought, although this had little effect on Si uptake and growth (tiller numbers or root biomass) of M. stipoides. Changes in rainfall patterns and clipping had strong individual effects on plant growth and Si uptake and storage, whereby clipping increased Si uptake by M. stipoides under all rainfall treatments but to the greatest extent under elevated precipitation. Moreover, above-ground–below-ground Si distribution only changed following elevated precipitation by decreasing the ratio of root:shoot Si concentrations.ConclusionsResults highlight the importance of soil water availability for Si uptake and suggest a role for both active and passive Si transport mechanisms. Such manipulative field studies may provide a more realistic insight into how grasses initially respond to herbivory in terms of Si-based defence under different environmental conditions.

Post-Fire Seedling Recruitment and Morpho-Ecophysiological Responses to Induced Drought and Salvage Logging in Pinus halepensis Mill. Stands

Salvage logging is the commonest post-fire emergency action, but has unclear ecological effects. In the Mediterranean Basin, drought periods and fire regimes are changing and forest management should be adapted. In summer 2009, a mid-high severity fire burned 968 ha of Aleppo pine (Pinus halepensis Mill.) forest in southeast Spain, which was submitted to salvage logging six months later. In spring 2010, plots were set in untreated and logged areas to monitor the recruitment and survival of the main tree species and three companion species: Stipa tenacissima L. (resprouter), Cistus clusii Dunal and Rosmarinus officinalis L. (obligate seeders). We evaluated responses to different scenarios in relation to intensification of summer droughts and forest management to obtain differences in water stress, growth, and gas exchange to cope with summer drought. Drought was induced by using rain-exclusion shelters and recorded ecophysiological characteristics were obtained with a portable gas exchange system. The main tree species recruitment was poor, but companion species showed a high survival ratio. Lower water stress was found for obligate seeder seedlings, which was higher in logged areas with induced drought. The initial post-fire stage was similar for the studied areas. However, after two drought periods (2010 and 2011), significant differences were found in the morphological and ecophysiological responses. In the unmanaged area, the biggest size of individuals due to the most marked increases in height and coverage were observed mainly in resprouter S. tenacissima. In the area submitted to salvage logging, the growth ratios in plots with induced drought were lower, mainly for seeders. Greater productivity was related to higher transpiration, stomatal conductance, and net photosynthetic ratio, but lower water use efficiency was found in obligate seeders with no drought induction, and S. tenacissima obtained higher values in untreated areas. Our results confirm that both forest management and intensification of summer droughts influenced the resilience and productivity of the ecosystems in the short term. Adaptive forest management after fire can imply successful survival and recovery of plant communities in the mid to long term. This study provide a scientific basis to develop tools for the post-fire restoration of serotinous pine forests occurring in low-altitudinal areas of the Mediterranean Basin, prone to summer droughts and fire events.

Suitability of Canopy Temperature Depression, Specific Leaf Area, and SPAD Chlorophyll Reading for Genotypic Comparison of Peanut Grown in a Sub-humid Environment

ABSTRACT Peanut (Arachis hypogaea L.) is mostly grown under rainfed production with water deficit being the major limiting factor. Several physiological characteristics have been proposed as surrogates for yield and genotypic selection under water deficit in arid climates, but their suitability for selection under sub-humid rainfed production where water deficit can also occur is not clear. Canopy temperature depression (CTD), specific leaf area (SLA), and SPAD chlorophyll reading of eight virginia-type peanut genotypes were evaluated at three growth stages in field trials involving rainfed and irrigated plants in sub-humid environments in northeastern Virginia-Carolina (VC) region in 2011 and 2012. Significant (p≤0.05) variation in pod yield and all physiological characteristics was observed in response to water regime in both years. Rainfed plants had warmer (CTD 2.2 vs. 3.1 °C) and greener canopies in 2011 but cooler (CTD 3.9 vs. 2.2 °C) and less green canopies in 2012 than the irrigated plants. Compared to irrigated plants, rainfed plants had slightly increased SLA in 2011 (135 vs. 131 cm2 g−1), but decreased SLA in 2012 (133 vs. 144 cm2 g−1). Differences (p≤0.05) among genotypes were observed for pod yield, SLA, and SPAD chlorophyll reading, but not for CTD. Among the physiological characteristics, only SPAD chlorophyll readings were significantly correlated to pod yield in all water regimes and growth stages in 2012, but not in 2011. Based on these results, CTD, SLA, and SPAD chlorophyll reading appear to be unsuited for genotypic selection for yield and water-deficit tolerance for peanut grown in sub-humid environment of the Virginia-Carolina region in part due to unpredictable rainfall amount and distribution. For reproducible field evaluations, additional methods will have to be used such as use of rain exclusion shelters.

Comparison of the responses of two Indian mustard (Brassica juncea L.) genotypes to post-flowering soil water deficit with the response of canola (B. napus L.) cv. Monty

The responses to water stress during the post-flowering period of two mustard breeding lines (887.1.6.1 and Muscon) and a commercial canola cv. Monty were tested in the field at Merredin in the low-rainfall Mediterranean-type environment of Western Australia. Three water-stress treatments were imposed using supplemental irrigation and a rain-exclusion shelter. Increasing water stress in the post-flowering period significantly reduced dry matter production and seed yields. Harvest index was slightly increased by mild stress, but reduced back to control levels by severe stress. Pods/plant, seeds/pod, and 1000-seed weight were all reduced by water stress. Dry matter production was higher in mustard than in canola, due to its greater water use and radiation interception. Water-use efficiency (WUE) for dry matter production and radiation-use efficiency (RUE) were higher in mustard than in canola. WUE for dry matter production and RUE were insensitive to the levels of water stress in mustard in this experiment, but declined significantly in canola. The greater water use in mustard and insensitivity of WUE for dry matter production and RUE to water stress were attributed to significantly higher levels of osmotic adjustment in mustard, although osmotic adjustment was also observed in canola. Despite this, canola seed yield was not significantly lower than the seed yield of the better mustard genotype, although stress caused a significantly greater percentage yield reduction in canola. This is because canola had a higher harvest index, which also meant it had higher WUE than mustard for grain production under mild stress. Mustard’s poorer harvest index was due to more of the dry matter being invested in stem and, in the case of cv. Muscon, to a short reproductive duration and a low proportion of pod weight allocated to seed. Canola had significantly higher seed oil concentration than mustard, which meant that it produced higher total oil yield despite sometimes producing lower seed yield. However, its oil concentration was reduced more by stress than mustard’s, so under the most severe stress conditions, both mustard genotypes produced higher total oil yield. Mustard has potential as an oil-producing crop in the low-rainfall Mediterranean-type environments of Western Australia, but improved genotypes, greater harvest index, and greater seed yield are required.

PESTICIDE STORAGE AND RELEASE IN UNSATURATED SOIL IN ILLINOIS, USA1*

The chemical fate and movement of pesticides may be subject to transient storage in unsaturated soils during periods of light rainfall, and subsequent release into shallow groundwater by increased rainfall. The objective of this study was to conduct field-scale experiments to determine the relative importance of transient storage and subsequent release of agrichemicals from the vadose zone into potential aquifers. Two field-scale experiments were conducted under a rain exclusion shelter. In the 1× experiment, atrazine and chlorpyrifos were applied at application-rate equivalents (1.6 kg ha−1 and 1.3 kg ha−1, respectively). In the 4× experiment, atrazine was applied in an amount that was four times greater than that usually applied to fields (6.7 kg ha−1). Water was either applied to simulate rain or withheld to simulate dry periods. In the 1× experiment, atrazine was detected in the water samples whereas chlorpyrifos was not detected in the majority of the samples. The dry period imposed on the treatment plot did not appear to result in storage of the chemicals, whereas the wet period resulted in greater leaching of atrazine, although the concentrations remained less than the Maximum Contaminant Level of 3 μg L−1. Both chemicals were detected in soil samples collected from a 20- to 30-cm depth, but it appeared that both chemicals dissipated before the field experiment was concluded. It appeared that the one-time application of atrazine and chlorpyrifos at the label rates did not result in a sufficient mass to be stored and flushed in significant concentrations to the saturated zone. When atrazine was applied at 4× and a longer drought period was imposed on the treatment plot, the resulting concentrations of dissolved atrazine were still less than 3 μg L−1. Atrazine was detected in only the near-surface (0 to 15 cm) soil samples and the herbicide dissipated before the onset of the dry period in the treatment plot. The results of this field study demonstrated that atrazine and chlorpyrifos were not sufficiently persistent to be stored and then released in significantly large concentrations to the saturated zone. The dissipation half-life of atrazine in the 4× application was about 44 days. This study, in addition to others, suggested that atrazine may be less persistent in surface soil than has been generally reported. * Publication authorized by the Chief of the Illinois State Geological Survey.

Consistent differences among wheat cultivars in osmotic adjustment and their relationship to plant production

Osmotic adjustment (OA) is generally considered an important component of drought resistance. Several reports by J.M. Morgan [Morgan, J.M., 1983. Osmoregulation as a selection criterion for drought tolerance in wheat. Aust. J. Agric. Res. 34, 607–614; 1992. Osmotic components and properties associated with genotypic differences in osmoregulation in wheat. Aust. J. Plant Physiol. 19, 67–76; 1995. Growth and yield of wheat lines with differing osmoregulative capacity at high soil water deficit in seasons of varying evaporative demand. Field Crops Res. 40, 143–152; Morgan, J.M., Condon, A.G., 1986. Water-use, grain yield and osmoregulation in wheat. Aust. J. Plant Physiol. 13, 523–532] from Australia concluded that consistent genetic differences in OA existed among wheat cultivars and that high OA cultivars tended to yield better than low OA cultivars under drought stress. Our study was performed to assess his results with his and other genetic materials. Two of Morgan’s spring wheat lines with high OA (‘H.Osm-134’) and low OA (‘L.Osm-136’) capacity in addition to eight other diverse spring wheat cultivars were tested for OA and plant production when grown in small plots under a rain exclusion shelter at Bet Dagan, Israel in 1996. OA of five of these cultivars (including Morgan’s lines) was also measured in two independent greenhouse tests in 1997 (Israel) and 1998 (Texas). The five cultivars differed significantly and ranked consistently for OA in all tests. No significant cultivar by test interaction for OA was revealed. OA was well correlated across cultivars between tests. The significantly higher OA capacity of H.Osm-134 as compared with L.Osm-136 was repeated in all tests. OA of all ten cultivars was positively correlated with biomass ( r = 0.73; p = 0.02) and yield ( r = 0.55; p = 0.09) under pre-flowering drought stress in the rain exclusion shelter. H.Osm-134 line performed significantly ( p ≤ 0.05) better than L.Osm-136 line for both biomass and yield under drought stress. We therefore support Morgan’s results and conclude that consistent differences in OA exist among wheat cultivars and that these differences can be associated with plant production under pre-flowering drought stress.

Estimation of pasture drought severity using canopy red-to-far-red radiance

Reliable methods for quantifying the impact of drought on pasture leaf canopies in humid, temperate regions are not available. This study was conducted to determine the utility of the red-to-far-red ratio of canopy radiance for estimating the evapotranspiration (ET) rate of water-limited pasture during drought. The hypothesis that the relationship between the ET and the red-to-far-red ratio of radiance is not linear was evaluated. The ET and spectral radiance were measured for orchardgrass ( Dactylis glomerata L.) and tall fescue ( Festuca arundinacea Schreb.) growing in large monolith weighing lysimeters at Kearneysville, WV. Drought was imposed using rain-exclusion shelters during periods of predicted rainfall. During the study period, the ET decreased to 0.14 and 0.23 of the potential rates for orchardgrass and tall fescue respectively. The process by which the canopies of the two grass species senesced in response to severe drought differed. Despite this difference, a single non-linear relationship effectively estimated the ET for both grass species from the red-to-far-red ratio of canopy radiance under midday sunlight. This relationship may be a useful tool for developing improved pasture management strategies and for hydrologic assessment in regions dominated by pasture.

Effect of Water and Nutrient Stresses on Apple Rootstock Growth, Respiration, and Capacitance

We are evaluating techniques for measuring intact apple rootstock (Malus domestica cv. M.9 and MM.111) responses to low, medium, and high soil-water potential, and low, medium, and high concentrations of N, K, and Ca, in sterile sand culture. Root respiration and functional surface area were estimated with an IRGA chamber and electric capacitance meter, respectively. Root length and surface area were determined by digital image analysis of extracted root systems. Low N supply reduced root respiration, while low K levels increased respiration relative to well-nourished controls. Calcium effects were inconsistent among the rootstocks. Total root length and respiration rates of MM.111 were higher than M.9, but M.9 had higher root:shoot ratios. Root capacitance was correlated with total root length (P < 0.001); and M.9 root systems had greater capacitance than MM.111. In a related field experiment, root growth and respiration of 4-year-old `Mutsu' apple trees on M.9 rootstock were measured in soil under low and moderate drought stress established by rain exclusion shelters, using capacitance and IRGA meters, and a minirhizotron video camera inserted into Plexiglas tubes transecting the rhizosphere. Root growth rates peaked in July (coinciding with maximal shoot growth), then declined gradually during late summer; but variability among trees was greater than among water stress treatments. Root/soil respiration maxima of 4.5 μmol CO2/m2 per s occurred in mid June, late July (when new root counts peaked), and the end of August (when root turnover was maximal).

Root Distribution and Soil Moisture Depletion Pattern of a Drought‐Resistant Soybean Plant Introduction

AbstractIn the southeastern USA, soybean [Glycine max (L.) Merr.] yield is often limited by water availability. Breeders have explored the use of exotic germplasm as a means of developing a root system capable of exploiting soil water reserves more fully. In this study, the drought‐resistant PI 416937 was compared with ‘Forrest’ for soil moisture utilization and root distribution on a Dothan loamy sand (fine‐loamy, siliceous, thermic Plinthic Paleudults). Water treatments (well‐watered and water‐deficient) were established during R4 to late R6 with rain exclusion shelters and differential irrigation. Soil moisture utilization in row and interrow areas was monitored via neutron attenuation and tensiometry. A direct measurement of root distribution was obtained on a second site by a trench profile technique. Results suggest that PI 416937's advantage may reside in its ability to exploit upper soil horizons (above 68 cm) with a network of fibrous roots. Localized measurements of soil moisture tension indicated that PI 416937's rate of soil desiccation was slower than Forrest's, but it appeared to exploit a larger total soil volume. Trench profile results indicated that the lateral spread of PI 416937's root system was greater than that of Forrest. The usefulness of PI 416937 may lie in the opportunity it provides soybean breeders for adding diversity to the root morphology of present soybean cultivars.

Identification of Peanut Genotypes with Improved Drought Avoidance Traits1

Abstract Peanuts (Arachis hypogaea L.) are often subjected to drought during some period in the growing season. A large root system may improve the plant's ability to continue growth during a drought. During greenhouse and field screening trials for resistance to the peanut root-knot nematode [Meloidogyne arenaria (Neal) Chitwood, race 1], 16 peanut genotypes were observed to have very large root systems. Using these 16 genotypes plus cultivars Florunner, Southern Runner, and germplasm line Tifton 8 as checks, several studies were conducted to evaluate these genotypes for drought avoidance characteristics. In the first study, root and shoot development were observed at 15-d intervals on plants grown from seed in sand-filled pots. In a second 2-yr study, selections were grown in the field under portable rain-exclusion shelters that created controlled periods of stress. In addition, the genotypes were also planted and observed in unsheltered naturally drought stressed field plots. In the sand-filled pot study, plant inventory (PI) numbers 315628, 268885, 318740, 269106, and 314893 developed the largest root systems. In the field drought stress studies, low visual stress ratings were recorded for Southern Runner, Tifton 8, PI 295722, and PI 315628. Low canopy temperatures characterized PI 315628, Tifton 8, and PIs 295722, 259637, and 268885. When averaged over three tests, sheltered (1991 and 1992) and unsheltered (1991), Tifton 8, PI 318740, Florunner, PI 315622, and PI 315628 produced the highest yields. Two of these higher yielding genotypes (Tifton 8 and PI 315628) had low stress and temperature ratings and PI 315628 also had the largest root system measured in this study.

090 THE EFFECTS OF WATER-STRESS, ROOTSTOCK AND CROP LOAD ON VINE AND ROOT GROWTH IN SEYVAL GRAPEVINES.

This experiment was conducted to evaluate interrelationships between differing crop loads and water stress on carbohydrate partitioning for above and below-ground tissues of Seyval grapevines grafted on three different rootstocks. Fine root development and depth were determined by quantifying root images from video recordings taken to depths of 80 cm at approximately 2 week intervals throughout the growing season. Two-year own-rooted Seyval grapevines, and Seyval grafted to 5BB and to Seyval were grown under a rain exclusion shelter and provided with 10 or 2.5 liters of water/plant/week. Treatments were cropping level, either 0 or 6 clusters per vine. Shoot length, number of mature nodes, leaf area and dry leaf weight of vines under high cropping level were reduced compared to vines growing under the low cropping level; so was root number and depth of penetration. Conditions of low soil-moisture resulted in carbohydrate partitioning in favor of the clusters at the expense of the roots, and carbohydrate partitioning in favor of the roots at the expense of the above-ground vegetation.

EARLY FALL COLD HARDINESS AND NONSTRUCTRAL CARBOHYDRATE LEVELS IN YOUNG, DROUGHT-STRESSED APPLE AND CRABAPPLE ON DIFFERENT ROOTSTOCKS

One-year-old `Imperial Gala' on Mark, M.9 EMLA, and MM.1ll; and `Indian Summer' on MM.lll and MM.106 rootstocks were planted in a rain exclusion shelter in May 1991. All trees were irrigated. Half the trees were drought stressed and received no water for two, 30-day drought cycles. Four trees from each scion ×rootstock×irrigation combination were excavated in mid-October. Nonstructural carbohydrate reserves of stems and roots were determined. Cold hardiness, determined by visual examination of tissue after controlled freezing, was influenced by rootstock, drought, and stem age. Concentrations of several carbohydrates were correlated with cold hardiness. Regression models of carbohydrate concentrations on cold hardiness were significant. Removal of root tissue, which was cold sensitive and had high carbohydrate levels, altered the regression equations. Rootstock significantly influenced root concentrations of sorbitol, sucrose, and starch. Root sorbitol increased following drought stress. Mark and MM.106 roots had the largest increases in sorbitol. Irrigated Mark roots had very low levels of sorbitol.

PHYSIOLOGICAL RESPONSES OF YOUNG APPLE TREES ON 3 ROOTSTOCKS TO DROUGHT STRESS

`Imperial Gala' on M.9 EMLA, MM.111 and Mark rootstocks were planted in a rain exclusion shelter. Two drought stress periods lasting approximately 1 month each were imposed during 1991. Water was supplied at 2 liters per day per tree before and after each drought cycle while water was withheld from half of the trees during the drought stresses. Maximal and variable chlorophyll fluorescence and fluorescence quenching were significantly reduced by the drought stress with M.111 generally affected first and with the largest difference between drought and control followed by Mark and then M.9. Leaf and stomatal conductance, assimilation and transpiration usually occurred first and were lowest for M.9 followed by Mark and then M.111 during the first stress cycle while Mark responded more rapidly and to a greater extent than M.9 and M.111 during the second stress. Water potential was lower for the stressed trees during both stress periods but osmotic and turgor potentials were reduced only during the first stress period. Changes in water relations were noticed first and to a greater extent for Mark followed by M.9 with M.111 exhibiting the least sensitivity and differences.

EFFECTS OF CROP LOAD AND LOW-SOIL MOISTURE ON PHYSIOLOGY AND ROOT DYNAMICS IN SEYVAL GRAPEVINES.

An experiment was conducted to evaluate interrelationships between differing crop loads and water stress on physiology and root dynamics of 3 year old Seyval grapevines grafted to 5-BB, Seyval and Seyval own-rooted stock grown under a rain exclusion shelter. Treatments were: 1) cropping level, either 0 (defruited) or 6 clusters/vine (heavily cropped) and 2) irrigation level, either 2.5 (stress) or 10 liters (control) of water/plant/week. Vines had significantly different root dynamics in regards to crop load, water status and rootstock. Water stressed vines had significantly fewer and smaller leaves (area cm 2 lighter trunk weights (g) and shorter shoot length compared to control vines. Heavily cropped vines had significantly fewer mature nodes, shorter shoot growth and higher bud mortality (winter injury) compared to defruited vines.

Microbial Populations in an Agronomically Managed Mollisol Treated with Simulated Acid Rain

AbstractA fertile well‐buffered mollisol (Flanagan silt loam, fine montmorillonitic mesic Aquic Argiudoll) under cultivation with corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] was subjected to simulated rain of pH 5.6, 4.2, and 3.0, while moisture‐activated rain exclusion shelters provided protection from ambient rain. Soil was sampled to a depth of 3 cm on four dates throughout the 1985 growing season. The following microorganisms were enumerated by plate counts or most probable number: general heterotrophic bacteria, general soil fungi, free‐living N‐fixing bacteria, fluorescent pseudomonads, autotrophic ammonium‐oxidizing, nitrite‐oxidizing, and thiosulfate‐oxidizing bacteria. The ANOVA was used to determine the combined and individual effects of rain treatments, crop field, and sampling date. Crop field and sampling date affected microbial numbers more than rain treatments. Overall, rain treatment effects were limited to nitrite‐oxidizing bacteria; lower numbers occurred in the corn field in subplots treated with rain of pH 4.2 and 3.0, and in the soybean field treated with rain of pH 3.0. The trend was strongest in June and July. In the corn field in subplots treated with rain of pH 3.0, numbers of thiosulfate‐oxidizing bacteria were higher and numbers of general heterotrophic bacteria were lower; however, these trends were comparatively weak. Rain treatments caused essentially no decrease in soil pH, suggesting that acid rain constituents affect certain microbial populations without causing overt changes in pH. Because they appear to be unusually sensitive, nitrite‐oxidizing bacteria could be used as experimental indicators of changes in soil microbial communities subjected to acid rain.