Use of 13CO2 as a Tool to Investigate Carbon Partitioning in Field and Greenhouse-grown Apple Trees

Abstract

Instrumentation to measure soil respiration is currently readily available. However, the relationship between soil respiration and root activity or root mass is not known. Herein we report on preliminary result using a 13 CO 2 pulse to the foliage to determine if 13 C respiration can be related to either root activity or root mass. An experiment was performed in the field on a 5-year-old apple tree (cv. Jonagold on M7). The tree canopy was enclosed in a Mylar® balloon and 2.1 g 13 CO 2 were pulsed in the balloon for 1 hr. After the pulse, air emitted by the soil and selected roots was collected every 6 hr for 8 days, by bubbling it in 2 M NaOH. 13 C/ 12 C ratios were measured with the mass spectrometer. The emission of 13 CO 2 from the roots gradually increased after the pulse reaching a peak after 100 hr. The emission trend was not linear, but it seemed related to soil temperature. Leaves and fruit were also collected daily. 13 C content in leaves was 1.15% right after the pulse, but it progressively decreased to 1.09% at the end of the experiment. The experiment was then repeated on 12 potted apple trees (cv. Redcort on M7) in greenhouse conditions. Six of them were maintained well-watered, whereas six plants were subjected to a mild water stress, by watering them with half of the volume of water used for well-watered plants. After the two soil moisture levels were achieved, the tree canopies of all the 12 trees were pulsed. Leaves, stems, and roots were ground and run in the mass spectrometer. The results of root emission rate were found to be similar to the field experiment. Results also indicated that, in our experiment, stress did not affect root respiration rate. Specific details of the physiology data will be presented.