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.
Because the life cycles of two important pests, Asian citrus psyllid and citrus leaf miner, are closely linked to the emergence of new flush. Growers could benefit from spraying only when there is new flush. But if the late flush is spread out over a month or even more, then what’s a body to do?
There may be ways we can manipulate the timing of flush directly. But to do that we need to understand how the plant regulates its own growth patterns.
Citrus trees have a different growth cycle than most common fruits. Like many fruits, they flower in the spring and then fruit develops over the next several months. Deciduous plants add vegetative growth (leaves and shoots) during a solid, extended period of the year. Citrus plants grow in several cycles of vegetative flushes over the year. The number and the size of these flushes depends on the environment. In Florida there are 2-3 vegetative flushes per year. Usually this begins with a flush in May, with later flushes in July and September or October. These flushes become less coordinated from plant to plant in a grove as the year waxes on, with May flush being relatively concentrated and the late flush being very sporadic.
Citrus plants’ “distribution of wealth”
The plant has to balance the needs of the leaves for water and nutrients with the needs of fruits and roots for sugars. This gets even more complicated when the plant has to send sugars to a new shoot to new flushes that will need to grow more than 1 cm in a day. So the plant needs to to start flushing at just the right time so that its fruits and roots don’t starve. It also needs to not start a new flush when the old one is not yet mature. What researchers in the 1990s observed is that root growth stops when shoots are growing.
How do they do it?
We humans have the benefit of brains that coordinate all the different ends of our body, so that our heart beast the right amount for our feet to be able to run. Plants don’t have benefit of brains, so they rely on chemical signals. These signals can be molecules that move from roots to leaves or that accumulate in one part of the plant when the other part stops using that compound. Most of these signals are called phytohormones, because their effects are very large compared their very small concentrations. Growing shoots send out signals called auxins that keep other buds from growing so that there aren’t too many flushes. Meanwhile, gibberelic acid and cytokinins coordinate between roots and shoots. Not all of these dynamics have been completely explained in the case of citrus flush phenology, but we are gaining ground!
Why does it matter?
Knowing the “code” the roots, leaves, and shoots use to communicate, allows us to grab the microphone and give a few orders. Thus it may be possible to manage the flush timing, to decrease its sporadic nature and improve the efficacy of our pest management, ultimately getting healthier plants with greater yields.
Because the Asian citrus psyllid stakes its reproduction on new citrus flush, there is a lot of interest in tailoring management to citrus phenology. “Phenology” is an uncommon word, but it boils down to how plant development changes over time. For instance the development of the spring flowering flush is a phenological process and names like “feather flush,” “popcorn,” and “full bloom” describe phenological stages.
Gene Albrigo has been involved in phenological modeling to predict flowering intensity and bloom time since well before the HLB era. He has recently turned to using this model to help improve psyllid management in two ways: reducing psyllid reproduction on new flush through pre-emptive psyllid management, and reducing negative impacts of insecticides on bee pollinations. In other words his goal is to kill adult psyllids before they can lay eggs on tender new flush but not hurt pollinator bees with applications late in the flush, when flowers have emerged. This can be done by using the models he and collaborators developed and have maintained for more than 10 years.
Gene has worked with several regional growers, selecting some blocks to manage psyllids based on phenological predictions, leaving others as controls with calendar or sampling-based sprays.
Gene recently reported results from the first two years of developing this approach. Results are positive, with reductions in adult psyllid numbers and egg-laying using the phenology-based approach, spraying once just prior to budbreak and again about 4 weeks later. This also allowed a bloom period that was free of insecticide applications, leaving the pollinators to range at the appropriate time. These results are promising for psyllid management during the floral flush, and I expect this approach to expand to become a standard practice.