In controlled greenhouse conditions, we grew tomato plants from seeds for two weeks and then thinned the plants to one individual per pot where pots either contained soil treated with 0.4 wt % hydrogel mixed in the top half of the soil, near the plant root zone, or were untreated controls. We continued watering the plants regularly for two weeks to ensure healthy plant growth before subjecting to various watering treatments. Individual pots were subjected to one of three conditions: watering every two days to simulate normal growth conditions ; subjecting to drought for nine days followed by a recovery for seven days ; or drought for nine days followed by a recovery for seven days, followed by another drought for ten days where “-G” denotes a treatment with gel amendment. Here, drought conditions were simulated by discontinuing watering while normal watering conditions and recovery were applied by watering every other day. We included treatments with drought recovery and a second drought to evaluate the effect of the hydrogels on the tomato plants’ ability to recover and then be re-subjected to drought. Re-watering has demonstrated improved growth, stomata re-opening,macetas para viveros and photosynthetic recovery in plants.The recovery time of crop function after drought heavily determines the drought’s impact on plant health and wetter conditions typically expedite recovery.
Understanding how hydrogels affect recovery is therefore vital to assessing their potential as conditioners. Throughout the experiment, various measurements were taken to monitor the health and productivity of the plants. Chlorophyll content , leaf water potential , and stomatal conductance measurements were collected at various time points throughout the experiment. Soil plant analysis development chlorophyll measurements are highly correlated with chlorophyll concentration and used as an indicator of maximal photosynthetic capacity .In drought conditions, plants often experience reductions in photosynthesis and therefore changes in chlorophyll content.We evaluated SPAD for the tomato plants on day 0, 5, 9, and 16 of the experiment . Here, we generally observed no significant difference in chlorophyll content between the various conditions whether the plants were watered or not, or whether they were planted in soil with hydrogel or not. As such, the hydrogel’s presence is not detrimental or advantageous for leaf chlorophyll synthesis. The stomatal conductance of the tomato plant leaves were paired with Ψleaf measurements. The stomata are microvalves on leaf surfaces that facilitate the gas exchange of carbon dioxide and water between plants and the atmosphere.Stomatal opening enables leaf photosynthesis, plant growth, and water use while stomatal closure is necessary for plant survival during drought. This regulation is crucial for plants as a majority of loss from transpiration is through stomatal opening.
As most leaf water exchange occurs through stomatal pores, drought stress causes leaf stomatal closure and therefore reduced stomatal conductance.Stomatal conductance was significantly reduced for treatments that underwent drought and was then recovered after watering. Additionally, for plants that underwent one drought cycle , we observed a lower stomatal conductance in plants that were not treated compared to those treated with 0.4 wt % commercial hydrogel.The stomatal conductance of the plants that underwent two droughts were low and indistinguishable whether gel was present or not. This may be attributed to permanent damage of the stomatal pores; stomatal pores can reopen once rehydrated only if there is not irreversible cellular damage.Lastly, we compared the final dry biomass of all conditions as water stress can impose negative effects on crop yield.RGR was also calculated as the growth rate of each tomato plant relative to its size at a previously determined time point. Specifically, RGR determines the exponential relationship of the mass of plants at two time points relatives to the length of time between measurements. RGR is related to plant health as increased masses of roots, stems, and leaves, and thus higher yield, are indicative of a healthier plant. For all watering conditions, we observed significant differences in RGR of tomato stems , leaves , and roots . Additionally, in regular watering conditions , tomato plants grown in soil treated with hydrogel demonstrated a higher RGR of reproductive leaves compared to the control .
However, for conditions OD and TD, where drought were applied, reproductive leaves were not observed on any plants that were subjected to two droughts. RGR was also improved by the addition of Terra-sorb hydrogel for all treatments . A visual representative of OD and OD-G before harvesting can be seen in Figure 3.11. We also observed root penetration of hydrogels which has previously suggested increased plant available water in studies that applied hydrogels to lettuce and barley seedlings14 and tree cuttings.Using the similar experimental conditions described for Terra-sorb, we applied trehalose hydrogel, also at a 0.4 wt% concentration . The following changes were made from the previous experiment: the droughts both lasted for 11 days instead of 9 days and SPAD , Ψleaf, and gs measurements were collected at slightly different time points; all were collected on days 0, 11, 18, and 28. Overall, there were no statistical differences in the SPAD , Ψleaf , gs , or growth measurements , between the plants with or without gel under the same treatment. Plants that underwent two droughts are shown in Figure 3.21. The only statistically differences were between treatments which were watered and those subject to drought. It should be noted that the gels did not negatively affect plant growth and, as demonstrated in previous studies with other gels,46 may be effective at higher concentrations or in different environmental conditions, like a sandier soil. We believe the trehalose hydrogels did not work well when applied at 0.4 wt % because it is not as water absorbent as Terra-sorb. The hydrophobic styrenyl backbone of the hydrogel prevents high swelling ratios, so it is not effective as a soil conditioner in the treatments described in this report. However, the gels generally did not negatively influence plant health, and so could potentially be applied for another agricultural application.As the world population rapidly grows, global food demand is increasing at an unprecedented rate.At the same time, less land resources are available for food production due to urbanization, competition from non-food industries, and climate change-induced desertification of arable land.It follows that 90 % of growth in crop production is projected to come from enhancing harvest yields rather than through increased land use.By controlling harmful insect, weed, or fungal populations, pesticides are a critical technology for agriculture. However, the utilization efficiency of pesticides is low due to undesirable properties like high volatility, mobility through soil, and vulnerability to degradation. This ultimately leads to nonspecific over-application where up to 75 % of pesticides never reach their desired target.
The unused pesticides cause undesirable effects, including environmental pollution, toxicity to non-target organisms, and inducing resistant pests. Propesticides are relatively inactive derivatives of pesticides that are converted to more active parent compounds upon a transformation event, such as hydrolysis or enzymolysis.Through tuning the physiochemical properties and pharmacokinetic behavior of active ingredients, propesticides have demonstrated improved targeting capabilities with reduced off-site movement and less premature degradation.Despite the additional synthetic steps to create conjugates,pot growing supplies they are considered more green holistically since lower application rates are required to reach a similar level of efficacy as the unmodified compound, thus reducing environmental impact. As a result, propesticides are widely used and account for a significant portion of pesticide sales. In particular, proherbicides made up 37 % of total herbicide sales in 2015.Cyclic b-triketone herbicides are 4-hydroxyphenylpyruvate dioxygenase inhibitors and applied globally to protect a variety of crops, including two staple crops, maize and rice.In 2017, they made up 4.4 %, or $1.3 billion, of the global herbicide market.Currently, there is one b-triketone proherbicide on the market, benzobicyclon, which is approved for use in rice paddy fields and transforms into its active hydrolysate form upon hydrolysis.b-Triketone proherbicides that can transform in other relevant conditions, like maize cultivation, have potential to improve their sustainable use in agriculture. Furthermore, there have been recent developments of transgenic crops that are resistant to HPPD inhibiting herbicides, and complementary herbicide formulation technologies will further increase their agrochemical utilization efficiency. Mesotrione -2-nitrobenzoyl -1,3-cyclohexanedione is a top-selling, highly selective b-triketone herbicide commonly used to protect maize from broad leaf weeds and annual grass weeds,16 but is susceptible to degradation and dissipation in soil.One study demonstrated that mesotrione had a half-life of two to 18 days in soil ranging from pH 4.2 to 8.3.Additionally, mesotrione and its degradation products have shown significant toxic effects in microorganisms,aquatic species,and off-target crops.Despite the popularity of and need for improved application of mesotrione, the synthesis and evaluation of proherbicide mesotrione conjugates have not been reported.
We hypothesized that by creating reversible modifications to mesotrione, its properties could be improved without sacrificing the herbicidal activity of the parent compound. Specifically, the addition of hydrophobic moieties could reduce soil mobility as lipophilic compounds have high affinity for soil particles.Herein, we report the synthesis of mesotrione thioether, ester, and enamine conjugates, detailing the unique reactivity of the b-triketone moiety. The hydrolytically-reversible proherbicides were then utilized for the controlled delivery of mesotrione in aqueous conditions and through soil. Finally, the efficacy of the ester conjugate against a common broad leaf weed, Chenopodium album, was evaluated. To create an effective proherbicide, the enolizable b-triketone moiety of mesotrione was chosen as a target location for conjugation. The anionic form of mesotrione is more water soluble, which could lead to environmental problems through groundwater leaching.We postulated that modifying the enol with hydrophobic moieties would reduce mesotrione’s water solubility and soil leaching potential. Moreover, other b-triketones are used for a variety of applications including additional herbicides, human therapeutics, recyclable plastics,and solid phase peptide synthesis.Therefore, understanding conjugation strategies to mesotrione’s b-triketone functional group could possibly be transferrable to other important compounds. The key structural components of mesotrione that determine the reactivity, stability, and properties of its conjugates are outlined in Scheme 4.1. The b-triketone moiety will readily tautomerize into its b-keto-enol form due to its low pKa of 3.12.The resulting enol/enolate of mesotrione provides a site for conjugation and is also the group that coordinates with HPPD active sites for inhibition.It is well-known that more acidic cyclic b-triketones are more active against weeds, so these herbicides commonly have electron-withdrawing benzoyl substituents to further decrease their pKa.Mesotrione is especially acidic due to the nitro- and methylsulfonyl- groups in the 2- and 4- positions of the benzene, respectively.With the aim of making the conjugate reversible in agriculturally-relevant conditions, we focused on creating linkages that were susceptible to hydrolytic degradation . First, a phenyl thioether conjugate was created using a method similar to the synthesis of benzobicyclon, which is known to degrade in aqueous solutions.A diketovinyl chloride derivative was first synthesized through Vilsmeier reagent, oxalyl chloride and a catalytic amount of DMF. Chlorinated mesotrione was then directly added to an alkaline solution of phenyl thioether to form the final product . An alkyl thioether derivative was synthesized using a similar method to form an ethyl thioether analogue of mesotrione as well. Although it is possible for multiple isomers to form, via endocyclic- or exocyclic- substitution, H-NMR and 13C-NMR spectroscopy of the conjugates confirmed that the cyclohexane 4- and 6- methylene groups have distinct shifts. This indicates that the endocyclic substitution likely formed as the exocyclic substitution would have rendered the methylene groups equivalent.Next, we created a phenyl ester derivative using benzoyl chloride and the isolated mesotrione-TEA salt . Initially, we attempted to synthesize the ester from a solution of mesotrione in base. However, after screening base and electrophile equivalents, we observed the disubstituted mesotrione product formed favorably . We hypothesize that the pKa of the proton at position 4 or 6 of the cyclohexane ring decreases significantly upon forming a monosubstituted ester, thus favoring the disubstituted product when reacted with base. As such, we pre-isolated a mesotrione enolate salt by reacting with an excess of TEA and recrystallizing the product. The salt was then used to prepare the monosubstituted phenyl ester derivative of mesotrione. We then aimed to produce alkyl ester derivatives from acetyl chloride and hexanoyl chloride, but they were unstable to purification attempts.