Study carbohydrate transport and photosynthesis: PhD-level assistantship available in the Tree Ecophysiology Lab

Orlando Li sets up detectors to measure carbohydrate translocation in citrus stems.

We are looking for a graduate student to start in the fall to contribute to the current knowledge about sugar transport, carbon allocation, and photosynthesis.

Photosynthesis is limited by the capacity of plants to translocate sugars from leaves to sinks. Source-sink relations affect translocation and thus may be key in optimizing photosynthesis in crop plants broadly. The selected candidates will be working on a USDA AFRI-NIFA Foundational project to quantify the interacting relationships among source-sink allocation, carbohydrate translocation speeds, and the regulation of net carbon fixation. The underlying hypothesis of this work is that increased allocation to sink growth interacts with phloem transport limitations to govern the regulation of carbon fixation. Thus, carbohydrate allocation can be co-optimized to enhance net assimilation rates. This project will expand the current knowledge of functional characteristics of carbohydrate translocation, as well as its relationship with regulation of carbon fixation in trees.

The selected candidates will be involved in a variety experiments to quantify these underlying relationships and the degree to which they affect growth. Experiments will include the impact of ploidy on translocation characteristics, methods of assessing total sink demand, impact of loading on translocation speed, and population-wide growth analysis and photosynthetic regulation. The selected candidate will have the opportunity to use unique methodologies at the UF Tree Ecophysiology Lab, including a range of methods to assess photosynthesis and radioisotopic methods of assessing carbohydrate translocation and allocation, as well as collaborate in assessing genetic components regulating these processes. This project will focus on the woody subtropical genus, Citrus. The work involves combinations of field, greenhouse, and laboratory work.

The work environment is highly collaborative, and demonstration of the ability to work in diverse teams will be valued in the selection process. Critical thinking, independent judgment, and interest in the subject matter are essential. Other valued skills include:

  • Quantitative analysis
  • Written communication
  • Experience with gas exchange methods
  • Knowledge of plant carbohydrate allocation processes or phloem function

The ecophysiology lab (website here) at the Citrus Research and Education Center in Lake Alfred, Florida, uses whole-plant physiological approaches to address challenges in horticultural productivity in perennial plants. The Citrus Research and Education Center offers ample opportunities for collaboration with 25 labs working in areas as varied as genetics, plant pathology, and entomology. The PI of the Tree Ecophysiology lab places a high importance on mentorship and the development of skills of and opportunities for students and post-doctoral scholars. If you are interested, please send your questions or a resume to Christopher Vincent at

Internship: Chlorophyll fluorescence to understand complex stress responses in plants

Talent Vharachumu, undergraduate student intern in 2019, used chlorophyll fluorescence to assess heat tolerance in citrus leaves.

Florida has several fascinating subtropical and tropical flora. The abundant sunshine and mild winter of Florida prolong the physiological activities of the plants leading to a longer growth season compared to other cooler landscapes. But too much sunshine than what is metabolically desired often leads to serious physiological stress. Evolution has led to plants that can handle such surplus light energy. But what happens under more complex stress situations? For example, Florida has been witnessing a progressive climatic warming, and new disease and pest outbreaks. Plant’s defense mechanisms face a stupendous challenge when handling such multiple stressors and failure to do so can threaten species long-term sustainability. At the Tree Physiology Laboratory at the University of Florida’s Citrus Research and Education Center (Lake Alfred, FL), we are studying how the tropical evergreen citrus trees respond to the complex agroclimatic niche of central Florida that includes warming events, high sunlight energy and greening (a systemic plant disease).

One of our approaches is to decode the chlorophyll fluorescence signals that citrus leaves emit back to nature while harvesting sunlight. These long wavelength, invisible fluorescence spectrums though emitted out, they carry valuable information on leaves bioenergetic processes and overall photosynthetic performance. We use different high throughput fluorometers to monitor real-time plant fluorescence emission signals, study signal patterns and use the signal-derived variables as biophysical markers for comparative stress physiology studies.

We are looking for undergraduate student interns who are excited to work with plants and interested in taking up short-term projects on plant stress physiology using chlorophyll fluorescence approaches. We do not expect the students to have a deep understanding of chlorophyll fluorescence or photobiology as our projects are designed for hands-on learning and gradual concept building. We will offer research projects that are either experimental (in field and controlled environment settings) or meta-analysis (literature review, data analysis, and interpretation) depending on student’s interest and the availability of lab projects. Please contact or if interested in joining us for the summer. Interns are paid. We will begin the selection process after April 16, so please contact us before then.

Fasih Khalid

Fasih Khalid came to CREC from Pakistan for a six-month program to work as a horticulturist focused on stress physiology. He also wanted to learn about physiological techniques and how to use different instruments. In Pakistan, he spent time working on abiotic stresses like water deficits and salinity. In our lab, he worked on experiments dealing with sap flow and hydraulic conductance. He said with the results the lab can determine whether “the sap flow movement is OK in relation to the regular irrigation.” Because of rising prevalence of water deficit problems in the world, it is important to see how irrigation levels in plants can be adjusted to save water and lead to more efficient water practices in the future.


Fasih loves research and learning. After his Master’s he spent about a month working in a farmer’s co-op in Pakistan, which he didn’t enjoy. As soon as an opportunity presented itself, he entered his PhD. He had done research on lychee in his Master’s and had published it, so he decided to go back to school and into the research field. After a couple of months in his PhD, he decided “…there is nothing in the world for me except research. I have to be researching.” He initially was interested in horticulture after going through his required two years of studying it in Pakistan. He thought he wanted to work in floriculture and landscaping but ultimately found pomology to be the most engaging because there are many different opportunities in that field. Plus, “The good thing of fruit is you can research or if you don’t want to research you can eat.”


During his time at CREC, he was also able to participate in several competitions, most of which were academic, though one was a t-shirt design contest for CREC, which he won! The academic competitions allowed Fasih to share the work he did at CREC, as well as help him improve his English. He liked to share that his work is important because it helps growers understand how to adapt to a world with climate change. He believes it is important for all people, not just growers, to care about changing water conditions and how this affects plants because “…we have to save water for the world and for the plants, for the next [plants].” If growers are unable to find solutions for their plants in water scarce conditions, this will cause further problems for the world as demand for fresh water reserves increases, eventually affecting our food supply, which would affect us all. He wants to take the things he’s learned during his time here back to Pakistan. His two big takeaways were time efficiency and the benefits of using hydraulics instrumentation for research. He hopes to apply these lessons as he continues horticulture research in the future.

No. 4

Mark Keeley

Mark Keeley turned a lifelong passion for plants into work that hopefully will help us better understand the way HLB affects Florida citrus trees. He worked in citrus research before starting his Master’s, but said he’s always been interested in plants and has been playing around with them for as long as he can remember. He made his way to plant research after trying out several undergrad degrees that didn’t fit quite right. Eventually, to no one’s surprise he told me, he joined the UF Horticultural Sciences Department. From there he had several jobs before landing one at a private agricultural research station. He likes his job because he said it’s, “Me and orange groves 4 or 5 days a week. Me and the plants, it’s quiet. Calming.” Through this job he decided to pursue a Masters in Agronomy. When asked why he’s pursuing his masters and working full-time for the private lab he laughed and said it was mostly for selfish, personal reasons. As a part of his master’s program he’s working with the tree physiology lab on a project that’s attempting to show a relationship between photosynthesis and citrus greening.

At the tree physiology lab, we hope to improve citrus growth based on understanding its environmental physiology. This particular experiment’s goal to describe the effects of HLB on photosynthesis. Part of this experiment also includes looking at the difference between shoot photosynthesis and leaf photosynthesis. This photosynthetic activity can’t be translated between leaf and tree, so we are trying to determine why. Studying photosynthesis on the leaf level can help garner understanding for the effects of photosynthesis on the whole canopy, but we need to understand what causes differences, too. Mark explained, “[We are] specifically looking at impacts of greening on photosynthesis as well as the amount of photosynthesis acquired from the leaf material and the stem material and what those impacts are over time on maturation.” In terms of the importance of this work for management, “It’s no longer preventing the bacteria from being there; it’s more how do we support the tree in [spite] of the bacteria,” he explained. This would be helpful because Mark believes the current approach to managing HLB isn’t working, so this could open the door to try other approaches.

The project uses an infared gas analyzer (affectionately called “IRGA”) called Li-Cor 6800. The machine cycles a known amount of CO2 and water over the leaf and measures how the concentrations change after they pass over to determine how much CO2 is being taken in and how much water is being released by the plant. This allows us to measure photosynthetic activity. Doing this in the lab with a known amount of light allows us to see the direct impacts of disease or other treatments over time without having to contend with clouds, different radiation intensities, etc. Eventually Mark would like to do work like this in the field, but while he’s still developing the methods, inside the lab is the best opt

The project uses an infared gas analyzer (affectionately called “IRGA”) called Li-Cor[RV1]  6800. The machine cycles a known amount of CO2 and water over the leaf and measures how the concentrations change after they pass over to determine how much CO2 is being taken in and how much water is being released by the plant. This allows us to measure photosynthetic activity. Doing this in the lab with a known amount of light allows us to see the direct impacts of disease or other treatments over time without having to contend with clouds, different radiation intensities, etc. Eventually Mark would like to do work like this in the field, but while he’s still developing the methods, inside the lab is the best option. 



When I asked why he believes this work is important he said, “Fascination for me. I keep going down the rabbit hole more than anything. I think it will be important if we find some of these relationships with the infection level and the tree. Like I said, it’ll change the way we culturally try to manage the disease [HLB]. Other than that, it’s more of a curiosity.”