Plant physiology is the study of how plants acquire resources such as water and nutrients in order to conduct photosynthesis that is necessary for plant growth, reproduction and survival. In the Physiology Lab at the Desert Botanical Garden, research staff specifically focuses on how desert plants have evolved to maximize photosynthesis when faced with difficult conditions such as drought, extremely warm temperatures and other environmental challenges.
“The main research theme in the Physiology lab revolves around studying the balance between water loss versus carbon uptake during photosynthesis. We do this because we are trying to understand how different plants react to environmental changes. The one common currency between all plants is carbon; if plants can’t exchange water and other resources for carbon, they will not survive,” says Kevin Hultine, plant physiologist.
The lab was created in 2011, and originally shared space with the Molecular Lab. As the research staff and projects of both labs grew, additional space was needed. The Molecular Lab was relocated, and the Physiology Lab was able to expand and increase its research capacity.
Three of the main research projects taking place in the lab are using a broad range of modern approaches including measurements of water transport efficiency through plant vessels (known as xylem hydraulic conductivity), stable isotope ratios in plant tissues to measure stress and measurements of plant photosynthetic rates and water loss.
Using a hydraulic manifold enables research staff to study the rate in which water flows through plant stems. When a plant absorbs moisture from the soil, it transports the water molecules in small vessels (called xylem vessels) towards the leaves. The hydraulic conductivity is the measure of the flow rate through a stem relative to the amount of pressure required to produce water flow. In the lab, plant stems are placed on the manifold and pressurized water is fed through the stem. By simultaneously measuring the water flow rate and the pressure applied to the water column the stem hydraulic conductivity is calculated.
Measurements of stable isotope ratios that form in the tissues of plants provides important clues for how plants respond to changing environmental conditions. For example, researchers are currently studying the ratio of carbon 13 to carbon 12 in saguaro spines. This project consists of studying saguaros in six different sites around Arizona and northern Mexico. These sites vary from relatively wet to extremely dry climates and spines are collected each spring for testing. Through 10 years of research, it has been discovered that there is a relationship between the amount of carbon 13 relative to carbon 12 in spines and the rate of plant growth. In cactus plants, a lower carbon 13 carbon 12 ratio is an indicator of greater stress and the saguaros growing in the wet locations consistently have more carbon 13 and grow faster than the saguaros in the dry locations. We anticipate that these simple measurements will indicate how saguaros are coping with drought conditions.
Researchers in the physiology lab evaluate photosynthetic rates by placing plants into sealed Plexiglas chambers and measuring changes in carbon dioxide concentrations with a gas analyzer. Potted plants are placed in a chamber fitted with electric fans that circulate the air so that carbon dioxide concentrations are uniform inside the chamber. Tubes connect the chamber to the gas analyzer and a pump circulates the air from the chamber to the analyzer and back. The healthier a plant is, the faster carbon dioxide concentrations in the chamber decrease relative to the surface area of the plant. Photosynthetic rates are often measured in the lab under different watering treatments to determine how plants cope with drying conditions.
By focusing on how environmental changes are affecting plants, the Garden can continue to help conserve the flora of the Sonoran Desert.
“I like to say that more broadly, we are conducting global change research. Yes, climate is a big emphasis, but addressing the impacts of the urban heat island, insect herbivory, and non-native plants on native plants are all major areas of emphasis in our lab. An alternative name for the lab would be Global Change Physiology Lab,” says Hultine.