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.”

Anirban Guha

Trees provide shade, but have you ever considered trees themselves needing shade? Our lab is seeking to answer this question. Anirban Guha, who is leading this effort in our lab, was able to sit down with me and answer some questions about the experiment. He joined the lab in April 2019 as a post-doctoral scholar. The experiment is attempting to determine how the trees respond to different light conditions over a period of two to three years with the use of shade nets to manipulate the environmental conditions. The lab records daily, weekly, and monthly results, and will record the yearly results when the time comes. We already know that citrus responds better in partial shade conditions, which improve yield and yield quality and photosynthesis and water status. We think that full sun has an especially bad effect on HLB trees. Anirban explained that the infected plants often cannot take in the full force of Florida sunlight; it provides them with more energy than they have the capacity to process. HLB also stunts root growth, which becomes even more of a problem when high light conditions demand more water and nitrogen than can be taken up by the roots.

The lab is testing whether shading the trees allows them to conserve more energy and require less water and nitrogen, which would help balance their functioning with the disease. Ultimately, the goal is to “[develop an] agricultural system in a way that could modify the environmental cues, and that can lead to better fitness of the plant to help sustain yield and maintain better physical performance.”

The main recipients of this experiment are scientists and citrus growers; Anirban thinks these two groups believe they have different reasons for caring about the results, but he believes their goals are actually similar and the knowledge they seek is complementary. Whether results are sought for economic reasons or a research quest, the ultimate goal is to see the trees become healthier and create more fruit yield—something both the scientific and agricultural communities can agree upon. 

Anirban takes this collaborative approach in his work life as well. The physiology lab collaborates with other CREC labs to study and test infected trees. Their results often work together to create healthier trees. For example, the entomology lab provides information on how insects spread HLB. He desires for more scientists of different disciplines to work together to achieve “functional collaborative research,” which can help the scientific community locally and worldwide. Along with scientists working together to achieve more, he also wants his research to be holistic. He wanted to study trees not just at a cellular level, but “from leaf to whole plant.” After completing his PhD in India, he found the majority of opportunities available there were for study at the cellular level. Anirban was interested in more variety and didn’t want to do what everyone else was doing. He also saw this gap in research as something he could potentially fill back home one day. He believes the study of the whole tree is important because problems tend to be linked to one another and can be better understood when a whole plant approach is taken. He enjoys his work but told me with a good-natured smile that he is not at all attached to the state of Florida and would like to return to India one day. 

Do sprayed antibiotics move inside citrus plants?

There has been discussion lately about how effective are antibiotics in citrus groves. In order for these to take down Las, the bacterium that causes greening, they need to be moved in the vascular system, particularly the phloem, where Las hangs out.  So, are leaf-sprayed antibiotics moved systemically?

This is the question some colleagues and I asked in a recent paper.  We were following up on some of Nian Wang’s research that showed how trunk-injected oxy-tetracycline (which we’ll affectionately call “oxytet”) moved around the citrus plant, eventually reaching all parts.  We had a few questions:

  1. When oxytetracycline is sprayed on the leaves, does it move to other parts of the plant? This question addressed whether there is systemic movement at all.
  2. How much moves? If there is systemic movement, what proportion of what is sprayed is moved.
  3. Does it matter whether you spray on old or young leaves? Some growers try to spray when there is new flush?  New flush has thinner cuticle (the waxy layer on the outside of the leaf). We thought that applying to new flush would allow more oxytet to get into the leaf.

In the same study we looked at some of the effects of heat treatment, but today we’ll just look at oxytet delivery. These trees did not have HLB, this is because we wanted to look at oxytet movement, not efficacy against Las.

What did we find?

  1. Oxytetracycline did move into leaves that did not receive the spray.
  2. How much moved? In one trial we found between 0.36-0.63 of the concentration in the leaves that didn’t receive the application relative to the leaves that were sprayed. But in the next trial we found between 0.27 and 0.34. So, although we did find oxytet moved into leaves that weren’t sprayed, a portion of what was sprayed stayed in the original leaf. The portion that didn’t become systemic probably is useless against Las because it isn’t moving through the vascular system.
  3. It doesn’t matter whether you spray old or young leaves. We found the same concentrations in the leaves regardless of which leaves were sprayed. At first we thought this contradicted some previous work that showed that there was a decrease in delivery of nitrogen as leaves aged. However, taking a closer look at that older work, it turns out that delivery decreases from when leaves emerge until about 6 weeks. However, after this point delivery begins to increase again. This is because, although there is more wax on the outside of the leaf, this wax ages and forms cracks, which probably allow more of whatever is sprayed in.

How did we find this?

Some plants we removed all new flush, some we removed all old flush, and some we left all the leaves. We covered about 1/4 of the canopy of small trees with impermeable plastic. Then we sprayed the rest of the canopy. After the spray had dried, we removed the plastic. About 3 weeks later we sampled both the leaves that were directly sprayed and those that weren’t. Then we tested each for oxytetracycline content.

Is it enough?

We still don’t know whether the concentrations that made it do the unsprayed leaves were enough to reduce the Las levels, because we actually don’t know how much oxytet it takes to bring Las down in the plant. This study didn’t address streptomycin, the other antibiotic that is labeled to use against Las, so we don’t know whether it would move similarly.