Study source-sink and transport attenuation of photosynthesis

PhD-level assistantship and post-doctoral scholar position available in the Tree Ecophysiology Lab

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. 

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 process. This project will focus on the woody subtropical genus, Citrus, but opportunities to collaborate across disciplines or species groups will be sought and encouraged. 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 in an academic setting
  • 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 interested please send inquiries or a resume to Christopher Vincent at

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.


Welcome Myrtho Pierre


Myrtho Pierre is the new biological scientist in the lab.  She is already taking charge of many of our specialized measurements.  Myrtho worked for years in horticultural production and consulting in Haiti, and she has been working “behind the scenes” here at CREC for 4 years (3.5 years longer than I have!).  She brings organization and friendliness to the lab, and she is already knee deep in greenhouse management and calibration of chlorophyll fluorescence and root respiration measurement.  I look forward to good things to come.