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.

How much kaolin is too much?

Red and white kaolin on the class panes we use to measure transmittance.

One important question that growers have been asking is how much kaolin can be on a leaf before having negative consequences from too much shading. If you are considering applying another layer, and the original layer hasn’t been washed off, it is important to know how much kaolin can be on a leaf before it begins to have a negative effect. Because we have some idea of how individual leaves respond to different light levels.

To answer this question we measured transmittance with the equivalent of different rates of kaolin. You can see the response plot below. One important feature of the response is that increasing rates of kaolin beyond 50 lbs/acre doesn’t increase the degree of shade to the same extent that rate increases below that level do.

So how much shade is beneficial for citrus plants? We can start by looking at the the maximum amount of light that these rates allow to reach the leaf.

Citrus leaves saturate at approximately 700 umols/m2 s, which means that additional light does not increase photosynthesis. With red kaolin, at the highest expected sunlight intensity (about 2200 umols), 25 lbs/acre reaches the level of shading reduces the light intensity of the exterior leaves to the saturation level, but the white kaolin doesn’t reach the level of 25 lbs/acre of red until 100 lbs/acre of white, though 50 lbs per acre reaches close to saturation.

Given these differences in intensity, we still don’t know why plants covered with the red seems to grow slightly more than those treated with the white. However, levels of red greater than 25-30 lbs/acre risk excessive diminishing light levels to exterior leaves, which means that they may not be able to maximize photosynthesis.

There can be a lot of complexity in terms of how much light reaches further into the canopy, which is part of how kaolin increases whole plant photosynthesis. However, it would seem that the risk of overapplicaton of white kaolin is low. You should be careful, though with repeated applications of red kaolin, unless previous applications have been mostly washed off.

Effects of kaolin on Asian citrus psyllid

Kaolin films are showing promising results in management of Asian citrus psyllid.  I recently presented preliminary results from our trial of Surround kaolin clay product and a Surround that we have modified with a red dye in presentations to the Polk County OJ Break and to the Citrus Research and Development Foundation research lunch.  To see the complete presentation click here.   

kaolin in field 6 mo
Six-month old trees with white kaolin.

The results are promising: Over the course of the first year after planting we saw an 78% reduction in mean psyllid numbers per tree in the white kaolin treatment.  Thus far, this has also translated in lower infection rates, with a mean of 10% infection in the white kaolin versus 25% in the foliar insecticide treatment.  These results are early, so we should be cautious about jumping to conclusions.  However, other studies have produced similar results, and this means that growers should consider kaolin as a viable practice to incorporate into their management programs.

psyllid plot 1 year
Mean Asian citrus psyllid counts based on weekly counts over the first year after planting on trees treated with Red-dyed or non-dyed (White) kaolin or with foliar insecticide. Click here for full presentation.