Stephen Machado

Combined effects of drought and high temperature on water relations of wheat and sorghum

Drought and high temperature are major factors limiting crop production. The two stresses occur together in many regions, but they usually are investigated separately. This study tested the hypothesis that high temperature interacts with drought to affect water relations, and the effect is greater in heat-sensitive wheat (Triticum aestivum L.) than in sorghum (Sorghum bicolor L. Moench). Wheat and sorghum were grown in soil that was well watered or not watered in controlled chambers at 15/10, 25/20, 35/30 and 40/35 °C day/night. Soil water content (SWC), leaf relative water content (RWC), leaf water potential (Ψ), leaf osmotic potential (π), leaf turgor potential (P) and osmotic adjustment (OA) were determined at 2-d intervals. All values held nearly constant at all temperatures when soil was well watered but were affected strongly by high temperature when water was withheld. The combined stresses reduced SWC, RWC, Psi and π, and unevenly raised P over time, particularly in sorghum. Sorghum also exhibited marked OA at high temperature, which was usually lethal to wheat. High temperature appeared to interact with drought to affect water relations by altering SWC and not by influencing OA. The results demonstrated that crops maintain nearly stable water relations regardless of temperature when moisture is ample, but high temperature strongly affects water relations when water is limiting. Increasing the thermotolerance of wheat might improve its potential to acclimate to both high temperature and drought.

Hydraulic lift in drought-tolerant and -susceptible maize hybrids.

Hydraulic lift was investigated in a greenhouse study involving two drought-tolerant maize (Zea mays L.) hybrids (TAES176 and P3223) and a drought-susceptible hybrid (P3225) during the flowering stage. Root systems were grown in two soil compartments – a drier upper soil and a wetter deep soil. The plants were shaded for 3 h during the daytime. Soil volumetric water content (Øv) in the upper pots was measured with time domain reflectometry (TDR) before and after shading. An increase in Øv in the upper pot was detected with TDR in the drought-tolerant hybrids following 3 h of shading, but not in the drought-susceptible hybrid. Furthermore, water exuded from roots in the top soil layers was greater in the more drought-tolerant TAES176 than in P3223 (489 vs. 288 g per pot in 3 h, P<0.005). The sizable amount of water from hydraulic lift allowed TAES176 to reach a peak transpiration rate 27–42% higher than the drought-susceptible hybrid P3225 on the days when the evaporative demand was high. To our knowledge, this is the first experiment that reveals a significant surge of transpiration due to hydraulic lift following midday shading. Hydraulic lift also prevented soil moisture depletion in the upper pots with TAES176, but not with P3223 or P3225. Root characteristics may be responsible for differences in hydraulic lift of the three maize hybrids. There were 2.3–3.3-fold more primary roots in the deep moist soil in P3223 and TAES176 than in P3225 that may enable these hybrids to absorb and transport water at faster rates. Therefore, more water can be exuded into the upper drier soil when transpiration is suppressed by shading. Larger primary roots (20–28% larger diameter) and a higher root volume in the upper soil in TAES176 and P3223 than in P3225 may contribute to higher root hydraulic conductance and greater water efflux from the roots. The negligible hydraulic lift in P3225 may also relate to higher night-time transpiration of the hybrid. This report has documented, for the first time, the existence of genetic variations in hydraulic lift among maize hybrids and links between hydraulic lift and drought tolerance within maize plants. It appears that one of drought tolerance mechanisms in maize may lie in the extent of hydraulic lift.

Pratylenchus neglectus Reduces Yield of Winter Wheat in Dryland Cropping Systems

Wheat (Triticum aestivum) in low-precipitation regions of eastern Oregon and Washington is grown mostly as rainfed biennial winter wheat (10-month growing season) planted into cultivated fallow (14-month crop-free period). There are increasing trends for cultivated fallow to be replaced by chemical fallow and for spring cereals to be planted annually without tillage. Most fields are infested by the root-lesion nematodes Pratylenchus neglectus or P. thornei. A replicated multiyear experiment was conducted to compare cropping systems on soil infested by P. neglectus. Populations became greater with increasing frequency of the host crops mustard, pea, and wheat. Annual winter wheat had the highest P. neglectus populations, the lowest capacity to extract soil water, and a lower grain yield compared with wheat grown biennially or rotated with other crops. Populations of P. neglectus did not differ for cultivated versus chemical fallow. Lowest populations occurred in annual spring barley. Winter wheat yield was inversely correlated with the population of P. neglectus. Measures to monitor and to reduce the population of P. neglectus in Pacific Northwest wheat fields are recommended.

Does intercropping have a role in modern agriculture

Intercropping—growing two or more crops at the same time on a single field—is an ancient practice still used in much of the developing world. For example, in Africa, corn (Zea mays L.), sorghum (Sorghum bicolor (L.) Moench), or millet (Panicum and Pennisetum spp.) are grown with pumpkin (Cucurbita spp.) cowpeas (Vigna unguiculata (L.) Walp), pigeon peas (Cajanus cajan (L.) Millsp.), or beans (Phaseolus spp.). Cocoa (Theobroma cacao L.) is grown with yams (Dioscorea spp.) or cassava (Manihot esculenta Crantz). In the tropical Americas, maize (corn) is grown with beans and squash (Cucurbita spp.). In both Africa and Latin America, beans or peas (Pisum sativum L.) climb tall cornstalks while pumpkins or squash cover the ground below. In these countries, many farmers have limited access to agricultural chemicals and equipment so prevalent in the developed world. Besides, intercropping is much less risky in that if one crop fails another or the others may still be harvested. Before the 1940s in the United States and Europe, growing more than one crop in the same field was common practice (Kass 1978; Andersen 2005), again because there was less risk. But with mechanization and the availability of relatively cheap synthetic fertilizers and pesticides, monocropping—i.e.,…

Spatial and Temporal Variability of Sorghum Grain Yield: Influence of Soil, Water, Pests, and Diseases Relationships

This study was conducted to determine relationships between biotic and abiotic factors and to generate information needed to improve the management of site-specific farming (SSF). The effects of water (80% evapotranspiration (ET) and 50% ET), hybrid (drought-tolerant and -susceptible), elevation, soil texture, soil NO3--N, soil pH, and greenbugs (Schizaphis graminum) (Gb) on sorghum grain yield were investigated at Halfway, TX on geo-referenced locations on a 30-m grid in 1997, 1998, and 1999. Grain yields were influenced by interrelationships among many factors. Grain yields were consistently high under 80% ET treatment and in the upper slopes where the clay and silt fractions of the soil were high. Soil NO3--N, rainfall, hybrid, and Gb effects on grain yields were seasonally unstable. Soil NO3--N increased grain yield when water was abundant and depressed grain yields when water was limiting. Plant density effects on grain yield were confounded with hybrid responses to drought and Gb infestation. Managing seasonally unstable factors is a major challenge for farmers and better ways to monitor crop growth and diagnose causes of poor plant growth are needed. To improve the management of SSF, effects of the relationships between biotic and abiotic factors on crop yield must be integrated and evaluated as a system. Based on our study, information on seasonally stable factors like elevation and soil texture is useful in identifying management zones for water and fertilizer application. Water and fertilizers management should be complemented by in-season management of seasonally unstable factors like soil NO3--N, rainfall, hybrid, and Gb effects on grain yield.

Spatial and Temporal Variability of Corn Grain Yield: Site-Specific Relationships of Biotic and Abiotic Factors

Inadequate information on factors affecting crop yield variability has contributed to the slow adoption of site-specific farming (SSF). This study was conducted to determine the effects of biotic and abiotic factors on the spatial and temporal variability of irrigated corn grain yields and to derive information useful for SSF. The effects of water (80% evapotranspiration (ET) and 50% ET), hybrid (drought-tolerant and -susceptible), elevation, soil index (SI)(texture), soil NO3–N, arthropods, and diseases on corn grain yield were investigated at Halfway, TX on geo-referenced locations. Grain yields were influenced by interrelationships among biotic and abiotic factors. Grain yields were consistently high under high water treatment, at higher elevations, and on soils with high SI (high clay and silt). Soil NO3–N increased grain yields when water was adequate. Management zones for variable rate fertilizer and water application should, therefore, be based on information on elevation, SI, and soil NO3–N. The effects of arthropods, diseases, and crop stress (due to drought and N) on corn grain yield were unpredictable. Spider mite (Oligonychus pratensis) and common smut (Ustilago zeae) damage occurred under hot and dry conditions in 1998. Spider mite infestations were high in areas with high soil NO3–N. Moderate air temperatures and high relative humidity in 1999 favored southwestern corn borer (Diatraea grandiosella) and common rust (Puccinia maydis) incidences. Knowledge of conditions that favor arthropods and diseases outbreak and crop stress can improve the efficiency of scouting and in-season management of SSF. Management of SSF can be improved when effects of biotic and abiotic factors on grain yield are integrated and evaluated as a system.

Speciation and Long- and Short-term Molecular-level Dynamics of Soil Organic Sulfur Studied by X-ray Absorption Near-Edge Structure Spectroscopy

We investigated speciation, oxidative state changes, and long- and short-term molecular-level dynamics of organic S after 365 d of aerobic incubation with and without the addition of sugarcane residue using XANES spectroscopy. Soil samples were collected from the upper 15 cm of undisturbed grasslands since 1880, from undisturbed grasslands since 1931, and from cultivated fields since 1880 in the western United States. We found three distinct groups of organosulfur compounds in these grassland-derived soils: (i) strongly reduced (S to S) organic S that encompasses thiols, monosulfides, disulfides, polysulfides, and thiophenes; (ii) organic S in intermediate oxidation (S to S) states, which include sulfoxides and sulfonates; and (iii) strongly oxidized (S) organic S, which comprises ester-SO-S. The first two groups represent S directly linked to C and accounted for 80% of the total organic S detected by XANES from the undisturbed soils. Aerobic incubation without the addition of sugarcane residue led to a 21% decline in organanosulfur compounds directly linked to C and to up to an 82% increase inorganic S directly bonded to O. Among the C-bonded S compounds, low-valence thiols, sulfides, thiophenic S, and intermediate-valence sulfoxide S seem to be highly susceptible to microbial attack and may represent the most reactive components of organic S pool in these grassland soils. Sulfonate S exhibited a much lower short-term reactivity. The incorporation of sugarcane residue resulted in an increase in organosulfur compounds directly bonded to C at the early stage of incubation. However, similar to soils incubated without residue addition, the proportion of organic S directly linked to C continued to decline with increasing duration of aerobic incubation, whereas the proportion of organic S directly bonded to O showed a steady rise.

Herbicidal Activity of Glucosinolate Degradation Products in Fermented Meadowfoam (Limnanthes alba) Seed Meal

Meadowfoam ( Limnanthes alba ) is an oilseed crop grown in western Oregon. After extraction of the oil from the seeds, the remaining seed meal contains 2-4% of the glucosinolate glucolimnanthin. This study investigated the effect of fermentation of seed meal on its chemical composition and the effect of the altered composition on downy brome ( Bromus tectorum ) coleoptile emergence. Incubation of enzyme-inactive seed meal with enzyme-active seeds (1% by weight) resulted in complete degradation of glucolimnanthin and formation of 3-methoxybenzyl isothiocyanate in 28% yield. Fermentation in the presence of an aqueous solution of FeSO(4) (10 mM) resulted in the formation of 3-methoxyphenylacetonitrile and 2-(3-methoxyphenyl)ethanethioamide, a novel natural product. The formation of the isothiocyanate, the nitrile, and the thioamide, as a total, correlated with an increase of herbicidal potency of the seed meal (r(2) = 0.96). The results of this study open new possibilities for the refinement of glucosinolate-containing seed meals for use as bioherbicides.

Effects of Crop Rotations and Tillage on Pratylenchus spp. in the Semiarid Pacific Northwest United States

There is interest in converting rainfed cropping systems in the Pacific Northwest from a 2-year rotation of winter wheat and cultivated fallow to direct-seed (no-till) systems that include chemical fallow, spring cereals, and food legume and brassica crops. Little information is available regarding effects of these changes on plant-parasitic nematodes. Eight cropping systems in a low-precipitation region (<330 mm) were compared over 9 years. Each phase of each rotation occurred each year. The density of Pratylenchus spp. was greater in cultivated than chemical fallow, became greater with increasing frequency of host crops, and was inversely associated with precipitation (R2 = 0.92, α < 0.01). Densities after harvesting mustard, spring wheat, winter wheat, and winter pea were greater (α < 0.01) than after harvesting spring barley or spring pea. Camelina also produced low densities. Winter wheat led to a greater density of Pratylenchus neglectus and spring wheat led to a greater density of P. thornei. Density of Pratylenchus spp. was correlated (R2 = 0.88, α < 0.01) but generally higher when detected by real-time polymerase chain reaction on DNA extracts from soil than when detected by a traditional method. Selection of different Pratylenchus spp. by different wheat cultivars or growth habit must be addressed to minimize the level of nematode risk to future plantings of intolerant crops.

Anthropogenic and climate influences on biogeochemical dynamics and molecular-level speciation of soil sulfur.

The soil environment is a primary component of the global biogeochemical sulfur (S) cycle, acting as a source and sink of various S species and mediating oxidation state changes. However, ecological significance of the various S forms and the impacts of human intervention and climate on the amount and structural composition of these compounds are still poorly understood. We investigated the long-term influences of anthropogenically mediated transitions from natural to managed ecosystems on molecular-level speciation, biogeochemical dynamics, and the apparent temperature sensitivity of S moieties in temperate, subtropical, and tropical environments with mean annual temperature (MAT) ranging from 5 degrees C to 21 degrees C, using elemental analysis and X-ray absorption near-edge structure (XANES) spectroscopy. Land-use and land-cover changes led to the depletion of total soil S in all three ecoregions over a period of up to 103 years. The largest decline occurred from tropical forest agroecosystems (67% Kakamega and 76% Nandi, Kenya), compared to losses from temperate (36% at Lethbridge, Canada, and 40% at Pendleton, USA) and subtropical (48% at South Africa) grassland agroecosystems. The total S losses correlated significantly with MAT. Anthropogenic interventions profoundly altered the molecular-level composition and resulted in an apparent shift in oxidation states of organic S from native ecosystems composed primarily of S moieties in intermediate and highly reduced oxidation states toward managed agroecosystems dominated by organic S rich in strongly oxidized functionalities. The most prominent change occurred in thiols and sulfides, the proportion of which decreased by 46% (Lethbridge) and 57% (Pendleton) in temperate agroecosystems, by 46% in subtropical agroecosystems, and by 79% (Nandi) and 81% (Kakamega) in tropical agroecosystems. The proportion of organic S directly linked to O increased by 81%, 168%, 40%, 92%, and 85%, respectively. Among the various organic S functionalities, thiols and sulfides seem to have higher apparent temperature sensitivity, and thus these organic S moieties may become prone to losses due to land-use changes, even from the cooler regions of the world if MAT of these regions rise in the future.