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Agronomic crops, including grains, forages, and fiber, are central to California agriculture, yet face many challenges. To ensure resilience, a statewide survey was conducted by the University of California Cooperative Extension service (UCCE) to identify high-priority needs and inform extension programming based on grower, consultant, and allied industry input. The goal was to compare the importance of different topics with the level of satisfaction regarding UCCE’s delivery of information on these topics. Survey respondents identified integrated pest management, nutrient and irrigation management, and variety testing as high-priority needs, with overall high satisfaction regarding UCCE’s program delivery on these topics. Topics needing more focus (high priority but below-average level of satisfaction) included testing new products, soil health management, and water conservation and storage. Areas of low priority and low satisfaction included niche marketing, emerging crops, organic production, harvest/post-harvest technology, salinity management, compost and manure management, and greenhouse gas emission reductions. To address stakeholder challenges, results from this study suggest that research and extension efforts should prioritize issues directly impacting on-farm crop production. At the same time, areas of low interest reflect a need for more support to engage farmers on these topics, particularly those concerning state environmental regulations and challenges to local and global food production and security.

Climate change is globally affecting rainfall patterns, necessitating the improvement of drought tolerance in crops. Sorghum bicolor is a relatively drought-tolerant cereal. Functional stay-green sorghum genotypes can maintain green leaf area and efficient grain filling during terminal post-flowering water deprivation, a period of ~10 weeks. To obtain molecular insights into these characteristics, two drought-tolerant genotypes, BTx642 and RTx430, were grown in replicated control and terminal post-flowering drought field plots in Californias Central Valley. Photosynthetic, photoprotective, and water dynamics traits were quantified and correlated with metabolomic data collected from leaves, stems, and roots at multiple timepoints during control and drought conditions. Physiological and metabolomic data were then compared to longitudinal RNA sequencing data collected from these two genotypes. The unique metabolic and transcriptomic response to post-flowering drought in sorghum supports a role for the metabolite galactinol in controlling photosynthetic activity through regulating stomatal closure in post-flowering drought. Additionally, in the functional stay-green genotype BTx642, photoprotective responses were specifically induced in post-flowering drought, supporting a role for photoprotection in the molecular response associated with the functional stay-green trait. From these insights, new pathways are identified that can be targeted to maximize yields under growth conditions with limited water.

Plant response to drought stress involves fungi and bacteria that live on and in plants and in the rhizosphere, yet the stability of these myco- and micro-biomes remains poorly understood. We investigate the resistance and resilience of fungi and bacteria to drought in an agricultural system using both community composition and microbial associations. Here we show that tests of the fundamental hypotheses that fungi, as compared to bacteria, are (i) more resistant to drought stress but (ii) less resilient when rewetting relieves the stress, found robust support at the level of community composition. Results were more complex using all-correlations and co-occurrence networks. In general, drought disrupts microbial networks based on significant positive correlations among bacteria, among fungi, and between bacteria and fungi. Surprisingly, co-occurrence networks among functional guilds of rhizosphere fungi and leaf bacteria were strengthened by drought, and the same was seen for networks involving arbuscular mycorrhizal fungi in the rhizosphere. We also found support for the stress gradient hypothesis because drought increased the relative frequency of positive correlations.

Arbuscular mycorrhizal fungi (AMF), the mutualistic symbionts with mostcrops, constitute a research system of human-associated fungi whoserelative simplicity and synchrony are conducive to experimental ecology.However, little is known about the shifts in adaptive strategies ofsorghum associated AMFs where strong AMF succession replaces initiallyruderal species with competitive ones and where the strongest plantresponse to drought is to manage these AMF. First, we hypothesize that,when irrigation is stopped to mimic drought, competitive AMF speciesshould be replaced by AMF species tolerant to drought stress. We then,for the first time, correlate AMF abundance and host plant transcriptionto test two novel hypotheses about the mechanisms behind the shift fromruderal to competitive AMF. Surprisingly, despite imposing droughtstress, we found no stress tolerant AMF. Remarkably, we found strong anddifferential correlation between the successional shift from ruderal tocompetitive AMF and sorghum genes whose products (i) produce and releasestrigolactone signals, (ii) perceivemycorrhizal-lipochitinoligosaccharide (Myc-LCO) signals, (iii) provideplant lipid and sugar to AMF and, (iv) import minerals and waterprovided by AMF. These novel insights into host gene expression andsuccession of AMF show adaptive strategies evolved by AMF and theirhosts and provide a rationale for selecting AMF to reduce inputs andmaximize yield in commercial agriculture. Future research opportunitiesinclude testing the specifics and generality of our hypotheses byemploying genetically modified host plants, and exploring additionalgenes underlying the adaptive strategies in natural succession.

Recent studies have demonstrated that drought leads to dramatic, highly conserved shifts in the root microbiome. At present, the molecular mechanisms underlying these responses remain largely uncharacterized. Here we employ genome-resolved metagenomics and comparative genomics to demonstrate that carbohydrate and secondary metabolite transport functionalities are overrepresented within drought-enriched taxa. These data also reveal that bacterial iron transport and metabolism functionality is highly correlated with drought enrichment. Using time-series root RNA-Seq data, we demonstrate that iron homeostasis within the root is impacted by drought stress, and that loss of a plant phytosiderophore iron transporter impacts microbial community composition, leading to significant increases in the drought-enriched lineage, Actinobacteria. Finally, we show that exogenous application of iron disrupts the drought-induced enrichment of Actinobacteria, as well as their improvement in host phenotype during drought stress. Collectively, our findings implicate iron metabolism in the root microbiome's response to drought and may inform efforts to improve plant drought tolerance to increase food security.

Histones belong to a family of highly conserved proteins in eukaryotes. They pack DNA into nucleosomes as functional units of chromatin. Post-translational modifications (PTMs) of histones, which are highly dynamic and can be added or removed by enzymes, play critical roles in regulating gene expression. In plants, epigenetic factors, including histone PTMs, are related to their adaptive responses to the environment. Understanding the molecular mechanisms of epigenetic control can bring unprecedented opportunities for innovative bioengineering solutions. Herein, we describe a protocol to isolate the nuclei and purify histones from sorghum leaf tissue. The extracted histones can be analyzed in their intact forms by top-down mass spectrometry (MS) coupled with online reversed-phase (RP) liquid chromatography (LC). Combinations and stoichiometry of multiple PTMs on the same histone proteoform can be readily identified. In addition, histone tail clipping can be detected using the top-down LC-MS workflow, thus, yielding the global PTM profile of core histones (H4, H2A, H2B, H3). We have applied this protocol previously to profile histone PTMs from sorghum leaf tissue collected from a large-scale field study, aimed at identifying epigenetic markers of drought resistance. The protocol could potentially be adapted and optimized for chromatin immunoprecipitation-sequencing (ChIP-seq), or for studying histone PTMs in similar plants.

Renewable fuels are needed to replace fossil fuels in the immediate future. Lignocellulosic bioenergy crops provide a renewable alternative that sequesters atmospheric carbon. To prevent displacement of food crops, it would be advantageous to grow biofuel crops on marginal lands. These lands will likely face more frequent and extreme drought conditions than conventional agricultural land, so it is crucial to see how proposed bioenergy crops fare under these conditions and how that may affect lignocellulosic biomass composition and saccharification properties. We found that while drought impacts the plant cell wall of Sorghum bicolor differently according to tissue and timing of drought induction, drought-induced cell wall compositional modifications are relatively minor and produce no negative effect on biomass conversion. This contrasts with the cell wall-related transcriptome, which had a varied range of highly variable genes (HVGs) within four cell wall-related GO categories, depending on the tissues surveyed and time of drought induction. Further, many HVGs had expression changes in which putative impacts were not seen in the physical cell wall or which were in opposition to their putative impacts. Interestingly, most pre-flowering drought-induced cell wall changes occurred in the leaf, with matrix and lignin compositional changes that did not persist after recovery from drought. Most measurable physical post-flowering cell wall changes occurred in the root, affecting mainly polysaccharide composition and cross-linking. This study couples transcriptomics to cell wall chemical analyses of a C4 grass experiencing progressive and differing drought stresses in the field. As such, we can analyze the cell wall-specific response to agriculturally relevant drought stresses on the transcriptomic level and see whether those changes translate to compositional or biomass conversion differences. Our results bolster the conclusion that drought stress does not substantially affect the cell wall composition of specific aerial and subterranean biomass nor impede enzymatic hydrolysis of leaf biomass, a positive result for biorefinery processes. Coupled with previously reported results on the root microbiome and rhizosphere and whole transcriptome analyses of this study, we can formulate and test hypotheses on individual gene candidates' function in mediating drought stress in the grass cell wall, as demonstrated in sorghum.

Community assembly of crop-associated fungi is thought to be strongly influenced by deterministic selection exerted by the plant host, rather than stochastic processes. Here we use a simple, sorghum system with abundant sampling to show that stochastic forces (drift or stochastic dispersal) act on fungal community assembly in leaves and roots early in host development and when sorghum is drought stressed, conditions when mycobiomes are small. Unexpectedly, we find no signal for stochasticity when drought stress is relieved, likely due to renewed selection by the host. In our experimental system, the host compartment exerts the strongest effects on mycobiome assembly, followed by the timing of plant development and lastly by plant genotype. Using a dissimilarity-overlap approach, we find a universality in the forces of community assembly of the mycobiomes of the different sorghum compartments and in functional guilds of fungi.

Drought is the most important environmental stress limiting crop yields. The C4 cereal sorghum [Sorghum bicolor (L.) Moench] is a critical food, forage, and emerging bioenergy crop that is notably drought-tolerant. We conducted a large-scale field experiment, imposing preflowering and postflowering drought stress on 2 genotypes of sorghum across a tightly resolved time series, from plant emergence to postanthesis, resulting in a dataset of nearly 400 transcriptomes. We observed a fast and global transcriptomic response in leaf and root tissues with clear temporal patterns, including modulation of well-known drought pathways. We also identified genotypic differences in core photosynthesis and reactive oxygen species scavenging pathways, highlighting possible mechanisms of drought tolerance and of the delayed senescence, characteristic of the stay-green phenotype. Finally, we discovered a large-scale depletion in the expression of genes critical to arbuscular mycorrhizal (AM) symbiosis, with a corresponding drop in AM fungal mass in the plants' roots.

The ecology of fungi lags behind that of plants and animals because most fungi are microscopic and hidden in their substrates. Here, we address the basic ecological process of fungal succession in nature using the microscopic, arbuscular mycorrhizal fungi (AMF) that form essential mutualisms with 70-90% of plants. We find a signal for temporal change in AMF community similarity that is 40-fold stronger than seen in the most recent studies, likely due to weekly samplings of roots, rhizosphere and soil throughout the 17 weeks from seedling to fruit maturity and the use of the fungal DNA barcode to recognize species in a simple, agricultural environment. We demonstrate the patterns of nestedness and turnover and the microbial equivalents of the processes of immigration and extinction, that is, appearance and disappearance. We also provide the first evidence that AMF species co-exist rather than simply co-occur by demonstrating negative, density-dependent population growth for multiple species. Our study shows the advantages of using fungi to test basic ecological hypotheses (e.g., nestedness v. turnover, immigration v. extinction, and coexistence theory) over periods as short as one season.

BackgroundSorghum bicolor is the fifth most commonly grown cereal worldwide and is remarkable for its drought and abiotic stress tolerance. For these reasons and the large size of biomass varieties, it has been proposed as a bioenergy crop. However, little is known about the genes underlying sorghum's abiotic stress tolerance and biomass yield.ResultsTo uncover the genetic basis of drought tolerance in sorghum at a genome-wide level, we undertook a high-density phenomics genome wide association study (GWAS) in which 648 diverse sorghum lines were phenotyped at two locations in California once per week by drone over the course of a growing season. Biomass, height, and leaf area were measured by drone for individual field plots, subjected to two drought treatments and a well-watered control. The resulting dataset of ~ 171,000 phenotypic data-points was analyzed along with 183,989 genotype by sequence markers to reveal 213 high-quality, replicated, and conserved GWAS associations.ConclusionsThe genomic intervals defined by the associations include many strong candidate genes, including those encoding heat shock proteins, antifreeze proteins, and other domains recognized as important to plant stress responses. The markers identified by our study can be used for marker assisted selection for drought tolerance and biomass. In addition, our results are a significant step toward identifying specific sorghum genes controlling drought tolerance and biomass yield.

Drought stress is a major obstacle to crop productivity, and the severity and frequency of drought are expected to increase in the coming century. Certain root-associated bacteria have been shown to mitigate the negative effects of drought stress on plant growth, and manipulation of the crop microbiome is an emerging strategy for overcoming drought stress in agricultural systems, yet the effect of drought on the development of the root microbiome is poorly understood. Through 16S rRNA amplicon and metatranscriptome sequencing, as well as root metabolomics, we demonstrate that drought delays the development of the early sorghum root microbiome and causes increased abundance and activity of monoderm bacteria, which lack an outer cell membrane and contain thick cell walls. Our data suggest that altered plant metabolism and increased activity of bacterial ATP-binding cassette (ABC) transporter genes are correlated with these shifts in community composition. Finally, inoculation experiments with monoderm isolates indicate that increased colonization of the root during drought can positively impact plant growth. Collectively, these results demonstrate the role that drought plays in restructuring the root microbiome and highlight the importance of temporal sampling when studying plant-associated microbiomes.

Cotton production in the San Joaquin Valley has traditionally relied heavily on tillage for its presumed benefits to plant establishment, yields and insect management. Research in the 1960s and 1970s demonstrated the potential of precision or zone tillage, which foreshadowed the introduction of a variety of minimum tillage implements in the early 1990s. During a 3-year comparison study from 2001 to 2003, cotton yields in strip tillage plots matched or exceeded yields of standard tillage plots in all 3 years. In a 12-year study from 1999 to 2011, tillage costs were lowered an average of $70 per acre in 2011 dollars using no-tillage compared to standard tillage while achieving statistically comparable yields, provided that adequate crop stands were achieved. If bottom-line profitability can be maintained, conservation tillage may become increasingly attractive to cotton producers in the San Joaquin Valley.

Ethanol use in California is expected to rise to 1.62 billion gallons per year in 2012, more than 90% of which will be trucked or shipped into the state. Switchgrass, a nonnative grass common in other states, has been identified as a possible high-yielding biomass crop for the production of cellulosic ethanol. The productivity of the two main ecotypes of switchgrass, lowland and upland, was evaluated under irrigated conditions across four diverse California ecozones — from Tulelake in the cool north to warm Imperial Valley in the south. In the first full year of production, the lowland varieties yielded up to 17 tons per acre of biomass, roughly double the biomass yields of California rice or maize. The yield response to nitrogen fertilization was statistically insignificant in the first year of production, except for in the Central Valley plots that were harvested twice a year. The biomass yields in our study indicate that switchgrass is a promising biofuel crop for California.

Despite nearly 30 years of research supporting the need for subsurface drainage-water disposal facilities, the lack of these facilities continues to plague agriculture on the San Joaquin Valley’s west side. One option for coping with the resulting soil salinity and shallow water-table problems is to convert from furrow or sprinkle irrigation to drip irrigation. Commercial field studies showed that subsurface drip systems can be highly profitable for growing processing tomatoes in the San Joaquin Valley, provided that the leaching fraction can achieve adequate salinity control in the root zone. Computer simulations of water and salt movement showed localized leaching fractions of about 25% under subsurface drip irrigation, when water applications equaled the potential crop evapotranspiration. This research suggests that subsurface drip irrigation can be successfully used in commercial fields without increasing root-zone soil salinity, potentially eliminating the need for subsurface drainage-water disposal facilities.

Twelve, 19.5 acre test plots arranged in a randomized complete block design are set within two 155 acre fields to provide a realistic production environment. These blocks of welldrained Panoche clay loam were formerly irrigated with California Aqueduct water and sprinklers for the last 30 years. Overall the field electrical conductivity (EC) ranged from 0.5 to 4.5, averaging 1.57 dS/m to a 3-foot depth. Saturation extract boron was 0.6 ppm. The area is underlain by a semi-saline aquifer that has been made worse over the decades by contamination from oilfield leachate water. Several production wells were drilled in fall 2003 to begin using this water. A drip tape irrigation system was set up to allow the planting of 6 rows of cotton every 22 feet the first year of the project (2004) followed with the planting of 1 year old pistachio seedling rootstocks March 2005 in 22 foot rows interplanted with 4, 38 inch rows of pima cotton. Salinity of the shallow groundwater for the test fields has varied from 4 to 7.5 dS/m with 8 to 11 ppm B. Three treatments were imposed: AQUEDUCT/CONTROL: EC ~ 0.4 dS/m (Aqueduct water only), BLEND: EC ~ 2.5 dS/m (50/50 mix) and WELL: EC ~ 5 dS/m.Chloride and boron contents in late season cotton petioles and pistachio leaf tissues were significantly higher in the WELL treatment compared to the Control. Saturation extract EC and B in the top three feet of rootzone were significantly increased in the BLEND and WELL treatments over the Control at the end of the season, with most salts increasing significantly to 3 feet with a significant increase in Cl to 5 feet.Cotton yield and tree development: Pima cotton lint yields were nearly 4 bale/acre in 2004, but crashed to about 2 bale/acre in 2005 due to very cool spring conditions that made for poor stand establishment. Cotton yields and plant height were unaffected by salinity. Spring 2006 provided excellent conditions for cotton growth, but excessive salts accumulated in the top 4 inches of the Well treatment beds reduced cotton emergence by 14% (statistically insignificant) and lint yield from the saline Well treatment was reduced by 275 lb/ac compared to the Aqueduct water. However, the Well treatment yield was still excellent at 3.12 bale/ac.Increase in pistachio rootstock diameter and general tree development was unaffected by salinity for both rootstocks for the first three years, but PG1 rootstocks showed a significant 7% decrease at the end of 4th leaf (Figure 1). However, Photoshop® pixel counts of green foliage down the row show an equal amount of green foliage for all treatments as of 10/18/08.Salinity and sustainability: At the end of 2006, after three seasons of cotton irrigation this program applied about 6,600, 32,500 and 54,000 lb/ac of salt in the Aqueduct, Blend and Well treatments, respectively. Rootzone salinity to 5 feet in the wettest part of the profile (between the two hoses) measured by Spring and Fall soil samples has remained surprisingly stable at about an ECe of 2.5 dS/m for the Aqueduct and about 5 to 6 dS/m for the Well treatment. However, in-season ECe in the top two feet is much higher as water and salts sub up from the buried drip tape. Without 6 to 10 inches of effective rainfall or fresh water winter irrigation for efficient leaching this system may not be sustainable. Due to the decrease in cotton yield in 2006, combined with a 50% increase in the Well water EC over the last four years we reduced the salinity of the Well treatment (by blending with Aqueduct water) down to 4.5 dS/m. This is about the salinity of the Well at the start of the test in 2004. After one year of cotton and four years of drip tape irrigation in the pistachios a total of 73,823 lb salt/wetted acre (about a 9.5 foot wide zone of subbing) was applied in the active rootzone of the Well treatment compared to 8,050 lb salt/wetted acre for Aqueduct water.At an average pima price of $1.08/lb, an economic analysis of cotton production and yields for the year prior to and first two years after planting pistachios shows a net return of $2,120 for Aqueduct water @ $120/ac-ft and $2,249 for Well water @ $45/ac-ft for this system.

California Pima cotton production has and been particularly hard hit resulting in acreage declines caused by water supply reductions in the western San Joaquin Valley. Water management methods that improve water use efficiency while maintaining high agronomic yield are of value to the cotton producers that face water supply limitations. We tested structured and predictive approaches to water management and collected field water management information that evaluated our ability to match varying water supply amounts in a way that maximized production and limited economic losses due to reductions in crop quality. Evaluations at four separate sites conducted during the 2005 and 2006 cropping seasons confirmed Pima cotton’s ability to respond favorably to irrigation guidelines recently developed by the UCCE and to modest deficit irrigation approaches that have been developed for Acala type Upland cottons. Variations in soil water storage at each site were large and played a critical role in how Pima cotton responded to in-season water deficits.

The shifts in adaptive strategies revealed by ecological succession and the mechanisms that facilitate these shifts are fundamental to ecology. These adaptive strategies could be particularly important in communities of arbuscular mycorrhizal fungi (AMF) mutualistic with sorghum, where strong AMF succession replaces initially ruderal species with competitive ones and where the strongest plant response to drought is to manage these AMF. Although most studies of agriculturally important fungi focus on parasites, the mutualistic symbionts, AMF, constitute a research system of human‐associated fungi whose relative simplicity and synchrony are conducive to experimental ecology. First, we hypothesize that, when irrigation is stopped to mimic drought, competitive AMF species should be replaced by AMF species tolerant to drought stress. We then, for the first time, correlate AMF abundance and host plant transcription to test two novel hypotheses about the mechanisms behind the shift from ruderal to competitive AMF. Surprisingly, despite imposing drought stress, we found no stress‐tolerant AMF, probably due to our agricultural system having been irrigated for nearly six decades. Remarkably, we found strong and differential correlation between the successional shift from ruderal to competitive AMF and sorghum genes whose products (i) produce and release strigolactone signals, (ii) perceive mycorrhizal‐lipochitinoligosaccharide (Myc‐LCO) signals, (iii) provide plant lipid and sugar to AMF, and (iv) import minerals and water provided by AMF. These novel insights frame new hypotheses about AMF adaptive evolution and suggest a rationale for selecting AMF to reduce inputs and maximize yields in commercial agriculture. [ABSTRACT FROM AUTHOR]

With Fusarium wilt (Fusarium oxysporum f. sp. vasinfectum W.C. Snyder & H.N. Hansen) race 4 (FOV4) formally identified in proximity to the High Plains of west Texas—the largest upland cotton (Gossypium hirsutum L.) producing region in the United States—the need to develop cultivars resistant to FOV4 has become urgent. Currently, there are no commercial upland cultivars available claiming FOV4 resistance. Fusarium wilt race 4, a soil‐borne fungus, has affected the cotton crop in the San Joaquin Valley of California for two decades. The primary purpose for the release of the upland PSSJ‐FRU01–PSSJ‐FRU17 (Reg. no. GP‐110–GP‐1126, PI 699966–PI 699982) germplasm lines is to provide cotton breeders with urgently needed sources for FOV4 resistance in upland cotton. The lines were derived from six different cross‐combinations using 10 parental lines with different genetic backgrounds (e.g., 'DES 920' [PI 536522], 'NM12Y1004' [NuMex COT 15 GLS, PI 678371], 'MARS ROSE CLUSTER' [PI 528483], 'AUBURN M' [PI 529214], and 'SA‐3208' [LIAO MIAN 7 HAO]). Several cycles of selection were applied to these developed lines based on an asymptomatic single plant selection after greenhouse FOV4 inoculations or infested field evaluations from F1 to F4. In 2019–2021 evaluations, germplasm lines showed enhanced resistance to FOV4 with significantly lower percentage mortality and vascular root staining as compared to resistant controls ('Pima‐S6' and 'PHY 881 RF') and improved fiber strength and some long fibers. These released lines will help to reduce the vulnerability of upland cotton to this fungal pathogen and advance efforts to broaden the resistance genetic base which is critical for the upland cotton industry. Core Ideas: Germplasm lines provide urgently needed sources for improving Fusarium wilt race 4 (FOV4) resistance in upland cotton.Germplasm lines increase the upland cotton safety net by reducing vulnerability against FOV4 fungal pathogen.With different genetic background, parents were selected to increase genetic diversity in developed germplasm lines.Lines possess improved resistance to FOV4 when compared to commercial cultivars.This is the first public set of upland germplasm lines with improved FOV4 resistance developed from parents with diverse genetic backgrounds. [ABSTRACT FROM AUTHOR]

Little is known about the availability of residual fertilizer-N and cotton (Gossypium spp.) residue-N to subsequent crops. This study determined the effects of two soil types and N fertilization rates on the recovery of residual [sup.15]N-fertilizer and [sup.15]N from labeled cotton residue in the soil and plants of subsequent cotton crops. Microplots in the San Joaquin Valley, California, on a Panoche clay loam [fine-loamy, mixed (calcareous), thermic Typic Terriorthents] and a Wasco sandy loam (coarse-loamy, mixed, nonacid, thermic Typic Torriorthents) were used to trace the fate of [sup.15]N from labeled abovcground cotton residue separately from the [sup.15]N in roots and soil. Total [sup.15]N -fertilizer recovery in the second year after application averaged 5.8% for Acala (G. hirsutum L.) and 2.9% for Pima (G. barbadense L.) cotton. In the third year after application, total [sup.15]N-fertilizer recovered by Acala averaged 2.0% on the clay loam soil and 3.3% on the sandy loam. Most of this recovered [sup.15]N-fertilizer was cycled through soil pools and roots and only small amounts originated from labeled aboveground residue. Virtually all of the [sup.15]N applied in the form of labeled aboveground residue and recovered in the soil was found in the top 0.3 m. The [sup.15]N-fertilizer which cycled through belowground pools was found mainly in the top 0.6 m of the soil. Recovery of residual [sup.15]N-fertilizer appears to contribute little to total cotton N uptake in the second and third crop after application.

A better understanding of plant nitrogen (N) dynamics in modern varieties of Acala (Gossypium hirsutum L.) and Pima (G. barbadense L.) cotton grown with prevailing management practices may help in the design of N management strategies that optimize N utilization and lint yields. The response of Acala cotton to different N fertility levels was evaluated on a Panoche clay loam (fine-loamy, mixed (calcareous), thermic Typic Torriorthents) and on a Wasco sandy loam (coarse-loamy, mixed, nonacid, thermic Typic Torriorthents). Nitrogen treatments of 56, 112, 168, and 224 kg N [ha.sup.-1] and check plots were established in 1998, 1999, and 2000. Plant shoots were collected and fractionated into different components at several growth stages for determination of N concentration. Both Acala and Pima tissue N concentrations were strongly affected by developmental stage and N treatment. Acala tissue N concentrations differed significantly between soil types. Maximum N uptake rates of both Pima and Acala occurred between early square and early bloom. Physiological N use efficiency averaged 11.4 kg lint [kg.sup.-1] plant N for Acala grown on the Panoche clay loam and 9.4 kg [kg.sup.-1] on the Wasco sandy loam, and 13.2 kg [kg.sup.-1] for Pima grown on Panoche day loam. Leaf N concentration as an in-season indicator of N status and estimates of seasonal soil N mineralization potential may be useful in optimizing cotton N management., NITROGEN MANAGEMENT is an important factor in optimizing cotton lint yield and quality. Cotton is produced on a wide range of soil types, in contrasting climatic conditions, and with varied [...]