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.
When I first spoke with Orlando Li he told me he wanted to work in the field with farmers; I knew I had the right candidate for my lab. Many students who contact me have an interest in biology that takes them to what we refer to as “lab work.” But I sometimes tell folks that the word “lab” for our group is a bit of a euphemism. What we have is more of a scientific field crew than a lab. Orlando’s interests fit perfectly with our group and our research aims.
Orlando is just finishing his proposal and preparing preliminary experiments to address how citrus plants regulate flushing on a whole plant basis. He’s looking into the environmental cues of flushing, relative timing of citrus root and shoot growth and of changes in carbohydrate movement, as well as what signals plants to make these changes happen.
“What does this have to do with field and farmers?” you ask.
Flushing patterns determine when the two major citrus pests in Florida can reproduce. Both Asian citrus psyllids and citrus leafminer lay their eggs on new flush, so if growers understood these cues they could use them to both manipulate them and to time other management decisions. If a grower knew a major flush was coming insecticides could reduce adult insect populations before they could lay eggs- even better if the grower could force the flush to be concentrated and keep it from spreading out over time. The answers to Orlando’s questions will help us design grower practices that will contribute to managing the two major disease challenges Florida citrus growers face. Orlando, whose given name is Sheng-yang, earned his Master’s degree in horticultural sciences at the National Taiwan University working on pear fruit thinning. He subsequently worked in Belize helping develop a certified budwood source for citrus growers there. I expect his contribution to Florida citriculture to be even more impactful.
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:
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.
How much moves? If there is systemic movement, what proportion of what is sprayed is moved.
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?
Oxytetracycline did move into leaves that did not receive the spray.
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.
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.