Changes in Photosynthetic Net CO2 Assimilation Rates on Theobroma cacao L. From In Situ Artificial Leaf Warming: A Study in the Biosphere 2 Tropical Rainforest

Abstract

A 33-week long in situ leaf warming experiment was performed to determine the change in the rate of photosynthetic net CO2 assimilation rates (A) in the tropical understory species, Theobroma cacao L. (cacao) in the controlled environment of the tropical rainforest biome of Biosphere 2 (B2), Oracle, Arizona, USA. A sustained multi-week in situ leaf level temperature increase of 3 °C was performed to understand rate changes in A, stomatal conductance (gsw), evapotranspiration (E), and vapor pressure deficit at leaf temperature (VPDleaf). Three warming experiments were performed on two mature, fruit producing cacao trees from 17 November 2023 to 8 May 2024 . One leaf was heated using a custom designed leaf heating device where it was continuously heated to 3 °C above ambient leaf temperature; another had the same heating structure installed but not heated, and an ambient leaf used as a control. Sustained leaf warming at 3 °C caused a decrease in photosynthetic net assimilation rates (A), evapotranspiration (E), and stomatal conductance (gsw), and an increase in vapor pressure deficit at leaf temperature (VPDLeaf). Together, these indicated that heating the leaf caused a net photosynthetic decline, supported by the decreased gas exchange that would limit CO2 uptake, reduce A rates, and increase VPDleaf due to reduced E. The presence of the heating apparatus in the Unheated treatment showed mixed results, and no statistical conclusions could be drawn about its effect on the response of A and results did not significantly vary from the control leaf.