Assessing corn recovery from early season nutrient stress under different soil moisture regimes.

Corn (Zea mays) biomass accumulation and nutrient uptake by the six-leaf collar (V6) growth stage are low, and therefore, synchronizing nutrient supply with crop demand could potentially minimize nutrient loss and improve nutrient use efficiency. Knowledge of corn's response to nutrient stress in the early growth stages could inform such nutrient management. Field studies were conducted to assess corn recovery from when no fertilizer application is made until the V6 growth stage, and thereafter, applying fertilizer rates as those in non-stressed conditions. The early season nutrient stress and non-stress conditions received the same amount of nutrients. As the availability of nutrients for plant uptake is largely dependent on soil moisture, corn recovery from the early season nutrient stress was assessed under different soil moisture regimes induced via irrigation scheduling at 50% and 80% field capacity under overhead and subsurface drip irrigation (SSDI) systems. Peanut (Arachis hypogaea) was the previous crop under all conditions, and the fields were under cereal rye (Secale cereale) cover crop prior to planting corn. At the V6 growth stage, the nutrient concentrations of the early season-stressed crops, except for copper, were above the minimum threshold of sufficiency ranges reported for corn. However, the crops showed poor growth, with biomass accumulation being reduced by over 50% compared to non-stressed crops. Also, the uptake of all nutrients was significantly lower under the early season nutrient stress conditions. The recovery of corn from the early season nutrient stress was low. Compared to non-stress conditions, the early season nutrient stress caused 1.58 Mg ha-1 to 3.4 Mg ha-1 yield reduction. The percent yield reduction under the SSDI system was 37.6-38.2% and that under the overhead irrigation system was 11.7-13%. The high yield reduction from the early season nutrient stress under the SSDI system was because of water stress conditions in the topsoil soil layer. The findings of the study suggest ample nutrient supply in the early season growth stage is critical for corn production, and thus, further studies are recommended to determine the optimum nutrient supply for corn at the initial growth stages.

A Laboratory Evaluation of Chemigation to Manage Stink Bugs (Hemiptera: Pentatomidae).

Application of insecticides for stink bug management through overhead irrigation, also called chemigation, could reduce application costs, soil compaction, and applicator exposure, while enabling growers to treat multiple fields simultaneously. The objective of these laboratory experiments was to compare knockdown, survival, and efficacy of insecticides when appropriately diluted for ground sprayer and chemigation applications. Treatments included water, bifenthrin [0.11 kg (AI)/ha] and dicrotophos [0.56 kg (AI)/ha] diluted for a ground sprayer (93.5 liters/ha), bifenthrin and dicrotophos diluted for chemigation (25,396 liters/ha), and bifenthrin and dicrotophos plus adjuvants diluted for ground sprayer or chemigation. Two- to 14-day-old adults of Nezara viridula (L.), Euschistus servus (Say), and Halyomorpha halys (Stål) were briefly submerged in appropriately diluted insecticides and then introduced into a disposable petri dish with or without food. Dishes were placed in a growth chamber provisioned with digital video cameras to monitor knockdown and feeding after insecticide exposure. Knockdown was visually assessed at 24 h after treatment followed by mortality and recovery from knockdown at 48 h after treatment. All stink bugs were knocked down within 1 h and never recovered when exposed at ground sprayer dilutions. However, many bugs survived chemigation dilutions. Less than half of the stink bugs were knocked down when exposed to dicrotophos (with or without adjuvants) and survival ranged from 17 to 77%, compared to 7-90% survival when exposed to bifenthrin at chemigation dilutions. These results strongly suggest that chemigation applications for stink bug management need to be closely examined.

Leaf ontogeny strongly influences photosynthetic tolerance to drought and high temperature in Gossypium hirsutum.

Temperature and drought are major abiotic limitations to crop productivity worldwide. While abiotic stress physiology research has focused primarily on fully expanded leaves, no studies have investigated photosynthetic tolerance to concurrent drought and high temperature during leaf ontogeny. To address this, Gossypium hirsutum plants were exposed to five irrigation treatments, and two different leaf stages were sampled on three dates during an abnormally dry summer. Early in the growing season, ontogenic PSII heat tolerance differences were observed. Photosystem II was more thermotolerant in young leaves than mature leaves. Later in the growing season, no decline in young leaf net photosynthesis (PN) was observed as leaf temperature increased from 31 to 37°C, as average midday leaf water potential (ΨMD) declined from -1.25 to -2.03MPa. In contrast, mature leaf PN declined 66% under the same conditions. Stomatal conductance (gs) accounted for 84-98% of variability in leaf temperature, and gs was strongly associated with ΨMD in mature leaves but not in young leaves. We conclude that young leaves are more photosynthetically tolerant to heat and drought than mature leaves. Elucidating the mechanisms causing these ontogenic differences will likely help mitigate the negative impacts of abiotic stress in the future.

Predawn respiration rates during flowering are highly predictive of yield response in Gossypium hirsutum when yield variability is water-induced.

Respiratory carbon evolution by leaves under abiotic stress is implicated as a major limitation to crop productivity; however, respiration rates of fully expanded leaves are positively associated with plant growth rates. Given the substantial sensitivity of plant growth to drought, it was hypothesized that predawn respiration rates (RPD) would be (1) more sensitive to drought than photosynthetic processes and (2) highly predictive of water-induced yield variability in Gossypium hirsutum. Two studies (at Tifton and Camilla Georgia) addressed these hypotheses. At Tifton, drought was imposed beginning at the onset of flowering (first flower) and continuing for three weeks (peak bloom) followed by a recovery period, and predawn water potential (ΨPD), RPD, net photosynthesis (AN) and maximum quantum yield of photosystem II (Fv/Fm) were measured throughout the study period. At Camilla, plants were exposed to five different irrigation regimes throughout the growing season, and average ΨPD and RPD were determined between first flower and peak bloom for all treatments. For both sites, fiber yield was assessed at crop maturity. The relationships between ΨPD, RPD and yield were assessed via non-linear regression. It was concluded for field-grown G. hirsutum that (1) RPD is exceptionally sensitive to progressive drought (more so than AN or Fv/Fm) and (2) average RPD from first flower to peak bloom is highly predictive of water-induced yield variability.

Water deficit in field-grown Gossypium hirsutum primarily limits net photosynthesis by decreasing stomatal conductance, increasing photorespiration, and increasing the ratio of dark respiration to gross photosynthesis.

Much effort has been expended to improve irrigation efficiency and drought tolerance of agronomic crops; however, a clear understanding of the physiological mechanisms that interact to decrease source strength and drive yield loss has not been attained. To elucidate the underlying mechanisms contributing to inhibition of net carbon assimilation under drought stress, three cultivars of Gossypium hirsutum were grown in the field under contrasting irrigation regimes during the 2012 and 2013 growing season near Camilla, Georgia, USA. Physiological measurements were conducted on three sample dates during each growing season (providing a broad range of plant water status) and included, predawn and midday leaf water potential (ΨPD and ΨMD), gross and net photosynthesis, dark respiration, photorespiration, and chlorophyll a fluorescence. End-of-season lint yield was also determined. ΨPD ranged from -0.31 to -0.95MPa, and ΨMD ranged from -1.02 to -2.67MPa, depending upon irrigation regime and sample date. G. hirsutum responded to water deficit by decreasing stomatal conductance, increasing photorespiration, and increasing the ratio of dark respiration to gross photosynthesis, thereby limiting PN and decreasing lint yield (lint yield declines observed during the 2012 growing season only). Conversely, even extreme water deficit, causing a 54% decline in PN, did not negatively affect actual quantum yield, maximum quantum yield, or photosynthetic electron transport. It is concluded that PN is primarily limited in drought-stressed G. hirsutum by decreased stomatal conductance, along with increases in respiratory and photorespiratory carbon losses, not inhibition or down-regulation of electron transport through photosystem II. It is further concluded that ΨPD is a reliable indicator of drought stress and the need for irrigation in field-grown cotton.

Variable rate application of nematicides on cotton fields: a promising site-specific management strategy.

Field tests were conducted to determine if differences in response to nematicide application (i.e., root-knot nematode (RKN) populations, cotton yield, and profitability) occurred among RKN management zones (MZ). The MZ were delineated using fuzzy clustering of five terrain (TR) and edaphic (ED) field features related to soil texture: apparent soil electrical conductivity shallow (ECa-shallow) and deep (ECa-deep), elevation (EL), slope (SL), and changes in bare soil reflectance. Zones with lowest mean values of ECa- shallow, ECa- deep, NDVI, and SL were designated as at greater risk for high RKN levels. Nematicide-treated plots (4 rows wide and 30 m long) were established in a randomized complete block design within each zone, but the number of replications in each zone varied from four to six depending on the size of the zone.The nematicides aldicarb (Temik 15 G) and 1,3-dichloropropene (1,3-D,Telone II) were applied at two rates (0.51 and 1.0 kg a.i./ha for aldicarb, and 33.1 and 66.2 kg a.i./ha for 1,3-D) to RKN MZ in commercial fields between 2007 and 2009. A consolidated analysis over the entire season showed that regardless of the zone, there were not differences between aldicarb rates and 1,3-D rates. The result across zones showed that 1,3-D provided better RKN control than did aldicarb in zones with low ECa values (high RKN risk zones exhibiting more coarse-textured sandy soils). In contrast, in low risk zones with relatively higher ECa values (heavier textured soil), the effects of 1,3-D and aldicarb were equal and application of any of the treatments provided sufficient control. In low RKN risk zones, a farmer would often have lost money if a high rate of 1,3-D was applied. This study showed that the effect of nematicide type and rate on RKN control and cotton yield varied across management zones (MZ) with the most expensive treatment likely to provide economic benefit only in zones with coarser soil texture. This study demonstrates the value of site specific application of nematicides based on management zones, although this approach might not be economically beneficial in fields with little variability in soil texture.