Current Program Research Projects
Foliar Applications of Weak Acids and Iron to Prevent Iron Deficiency in Subtropical and Tropical Fruit Crops
Ecophysiology Lab in Collaboration with Dr. Yuncong Li’s Plant and Soil Nutrition Lab
Subtropical and tropical fruit crops grown in calcareous soils often exhibit symptoms of iron deficiency such as interveinal chlorosis of leaves. A standard method to prevent iron chlorosis in calcareous soil is by adding chelated iron to the soil. Chelated iron is extremely expensive. Reducing the internal pH of chlorotic leaves with foliar applications of dilute acids with or without Fe2SO4 has resulted in a “re-greening” of leaves of some plant species by increasing the reduction of Fe3+ (the form generally most abundant in the soil) to Fe2+ (the form metabolized by the plant). Dilute acids and Fe2SO4 are substantially cheaper than chelated iron. Thus, the potential exists for the use of foliar applied weak acids as low-cost alternatives to expensive chelated iron for preventing iron deficiency in subtropical and tropical fruit crops. This project is aimed at evaluating foliar applications of weak acids with and without Fe2SO4 as low cost alternatives to applying chelated iron to the soil to prevent iron deficiency in subtropical and tropical fruit crops grown in calcareous soils. Greenhouse and orchard experiments have been conducted with avocado, carambola and lychee. Current studies are focusing on pond apple (Annona glabra).
Mango Internal Discoloration
Internal discoloration (“cutting black”) of mango fruit is considered to be a preharvest disorder that develops its symptoms during ripening. The disorder has been a recurring problem with mangos from Ecuador and Peru exported to the U.S. The specific environmental conditions or cultural practices that lead to internal discoloration have not been determined. Previous research with similar mango fruit disorders indicate that cutting black (corte negro) may be the result a low calcium (Ca) to nitrogen (N) ratio in the fruit during fruit development coupled with typical postharvest refrigeration temperatures, which are not cold enough to cause fruit damage unless the Ca/N ratio in the fruit is too low.
The overall objective of this project is to determine the cause of the internal discoloration (cutting black) in mango fruit grown in Ecuador and Peru and how to prevent its occurrence.
A survey of mango growers in Ecuador and Peru was conducted to identify when and where internal discoloration has occurred: the seasons, and times within seasons; the locations and varieties for past outbreaks.
On-farm trials are being conducted in Ecuador and Peru to try to induce/prevent the disorder. Treatments have been established whereby different concentrations Ca and N fertilizers are applied to trees to alter the Ca/N ratio in the fruit during early fruit development. Nutrient analyses of all major and minor elements in the soil, leaves and fruit (at several stages of fruit development) are being assessed.
Fruit in each pre-harvest fertilization treatment are subjected to the following postharvest treatments: 1) storage at ambient (room) temperature, no hot water treatment, 2) cold storage (8-10oC), no hot water treatment, 3) storage at ambient (room) temperature, hot water treatment, 4) cold storage (8-10oC), no hot water treatment. The incidence of internal discoloration in each postharvest treatment is being related to fruit nutrition, including Ca/N ratios and orchard fertilization practices to try to relate the internal fruit discoloration to fertilization practices and develop fertilizer recommendations for preventing the disorder.
Separation Plant Transpired Water from Streamflow and Groundwater Using Stable Isotopes of Water
Ecophysiology Lab in Collaboration with Dr. Leo Sternberg, Department of Biology, University of Miami
Stable oxygen (δ18O) and hydrogen (δ2H) isotope composition of precipitation, soil and plants have been studied over the years to understand the mechanism of soil water movement and the depth of plant water uptake in the soil water profile Recent studies have suggested that in soil during the wet season, tightly bound water does not mix with mobile water but is retained in the soil until the dry season when it is taken up by plants via the force of transpiration. To test this, we sampled plant stem water and soil δ18O and δ2H as a proxy for wet season mobile water and dry season bound water in two types of soils to determine if mixing occurs between mobile and tightly bound soil water. Plastic pots were filled with clay or very gravelly loam soil and a Persea americana tree was planted in each pot. Soil in each pot was first saturated with tap water to fully label the bound water with the isotopic identity of tap water and then fully saturated with either tap water (T) or isotopically-enriched pool water (P) and covered with white polyethylene to prevent evaporation. After saturating the soil, δ18O and δ2H of water draining from each pot were similar to those of water added to each pot for both the T and P treatments.
For each treatment, δ18O and δ2H in plant stem water were sampled 2-3 days after soil was initially saturated (simulated wet season; soil tension < 0.10 kPa) representing the mobile water and again 7-9 days after soil was saturated representing the bound water (simulated dry season; soil tension > 80.0 kPa). During the “dry season”, there was a significant difference between T and P treatments for δ18O and δ2H in plants, indicating bound water accessed by plants in the P did not retain the tap water label and mixing occurred between mobile and bound water in the soil. Comparing P-T in the wet season with P-T in the dry season indicated that as much as (~95%) of water freely exchanged between the mobile and bound components of the soil. Thus, the isotopic signature of bound water taken up by the plant in the dry season was not significantly different than that of the mobile water taken by the plant in the wet season. This is contrary to recent studies suggesting that no mixing occurs.
Use of Plant Extracted Essential Oils for Controlling Postharvest Decays in Papaya and Avocado Fruit
Ali Sarkoosh, Ph.D., Visiting Scientist from the Department of Primary Industries, Northern Territory, Australia. Ecophysiology Lab in Collaboration with Dr. Aaron Palmateer’s Plant Pathology Lab
One of the main devastating diseases in subtropical and tropical fruit, especially papaya and avocado, is anthracnose caused by the fungus Colletotrichum spp. Anthracnose infects fruit in the field while they are immature. Symptoms appear on fruit after ripening, causing damage during fruit storage, transit, and marketing.
Most fruit producers generally apply a combination of hot water with synthetic fungicides to reduce potential infection by postharvest diseases. Hot water treatment could affect nutritional value and sensory properties of fruits and vegetables, whereas applying synthetic fungicides might lead to development of fungicide-resistant strains of the pathogen. Furthermore, chemical residues on the fruit as a result of fungicide application may cause serious threats to consumers and the environment. Chemical fungicides to control postharvest decays are restricted in some countries and there is a high demand by consumers for agricultural products that have not been treated with chemicals. Therefore, applying non-hazardous products for controlling anthracnose in papaya and avocado during storage is a promising consumer-friendly and environmentally safe alternative to chemical fungicides.
Research has shown that application of essential oils may be a non-toxic method of controlling post-harvest fruit diseases. In general, essential oils are complex combinations of hydrocarbon monoterpenes, oxygenated monoterpenes, hydrocarbon sesquiterpenes, oxygenated sesquiterpenes, and related compounds that originate from secondary metabolism in plants. These oils can be extracted from various plant organs such as flowers, buds, seeds, leaves, twigs, bark, herbs, wood, fruit and roots.
The potential for essential oils of Mentha piperita, Thymus daenensis, Satureja khuzistanica, Lavandula angustiolia, and Cinammon zeylanicum to be used as bio-fungicides to manage the growth and spore germination of anthracnose disease of papaya and avocado fruit during storage is being investigated.
Electrical Signaling in Response to Abiotic Stresses in Fruit Trees and Woody Vines
Plant Ecophysiology Lab in Collaboration with Dr. Pilar Gil, Facultad de Agronomía e Ingeniería Forestal, Pontificia Universidad Católica de Chile, Santiago, Chile
Studies have shown that plants generate electrical signals in response to external stimuli. These signals often originate at the root and travel through the vascular system to the leaves taking the form of root to leaf electrical potential differences (DEP). These DEP are often followed by physiological plant responses, including changes in stomatal behavior, photosynthesis and/or respiration. This suggests that electrical signals are a direct mechanism for rapid communication between plant organs in response to environmental stress. While several studies have indicated that plant physiological responses are associated with changes in DEP, little is known about the relationship between the strength of the signal and the intensity of the physiological plant response.
Research has be underway to relate electrical signals in woody perennial fruit trees and vines to different potential stress-inducing abiotic stimuli. Plant are being subjected to varying types and degrees of abiotic stresses including drought, salinity, osmotic shock, cold, heat and root hypoxia. In addition to whole-plant physiological responses (i.e. leaf gas exchange, chlorophyll fluorescence, xylem sap flow, stem water potential), DEP between different plant organs are being monitored. Physiological plant stress variables are being related to within-plant DEP generated in response to environmental stresses. A mathematical model is also being developed to explain and quantitatively represent the biophysical processes involved in the generation and transmission of electrical signals in fruit trees and woody vines in response to abiotic stresses.
Understanding the underlying mechanisms for rapid within-plant communication in responses to abiotic stresses will provide a basic research foundation that can be built-upon with the ultimate goal of relieving abiotic stresses in fruit trees and vines. Obtaining this information will not only increase our basic understanding of stress responses and signaling in woody plants, but should ultimately help in the development of strategies to mitigate stress of commercially important woody perennial plant species. Furthermore, quantifying and elucidating the relationship between the intensity of various abiotic stresses and electrical signal magnitude may open the door for developing tools for rapid and early detection and quantification of tree and vine stress in commercial orchards and vineyards.