Stomata Are Driving the Direction of CO[sub.2]-Induced Water-Use Efficiency Gain in Selected Tropical Trees in Fiji

• Published in: Biology (Basel, Switzerland) Switzerland), 2024-09, Vol.13 (9)
• Main Authors: Soh, Wuu Kuang, Yiotis, Charilaos, Murray, Michelle, Pene, Sarah, Naikatini, Alivereti, Dornschneider-Elkink, Johan A, White, Joseph D, Tuiwawa, Marika, McElwain, Jennifer C
• Format: Article
• Language: English
• Subjects: Fiji; Florida; Ireland; Pacific Islands; United Kingdom; Water use
• ISSN: 2079-7737; 2079-7737
• DOI: 10.3390/biology13090733

Understanding how plants respond to increasing atmospheric CO[sub.2] is crucial for predicting future climate interactions. However, the long-term effects of rising CO[sub.2] on plant physiology, especially in tropical regions, are not well known. To investigate this, we studied how a CO[sub.2] increase of about 95 ppm from 1927 to 2015 affected five tropical tree species in Fiji. We analysed historical leaf samples to measure the following two key traits: how efficiently the trees use water (intrinsic water-use efficiency) and the maximum rate of conductance through leaf pores (maximum stomatal conductance). Our results showed that the responses to rising CO[sub.2] varied significantly by species. Generally, the number of stomata on the leaves was more important than their size in determining the trees’ response to higher CO[sub.2] levels. While photosynthesis is a major factor in improving the water-use efficiency, changes in stomatal conductance primarily drive this trend across different species. Trees that showed greater increases in the water-use efficiency also displayed a greater reduction in stomatal conductance. Overall, our study shows the importance of considering differences in the maximum stomatal conductance when predicting how different tree species will react to increasing CO[sub.2] levels. Understanding plant physiological response to a rising atmospheric CO[sub.2] concentration (c [sub.a]) is key in predicting Earth system plant–climate feedbacks; however, the effects of long-term rising c [sub.a] on plant gas-exchange characteristics in the tropics are largely unknown. Studying this long-term trend using herbarium records is challenging due to specimen trait variation. We assessed the impact of a c [sub.a] rise of ~95 ppm (1927–2015) on the intrinsic water-use efficiency (iWUE) and maximum stomatal conductance (g [sub.smax]) of five tropical tree species in Fiji using the isotopic composition and stomatal traits of herbarium leaves. Empirical results were compared with simulated values using models that uniquely incorporated the variation in the empirical g [sub.smax] responses and species-specific parameterisation. The magnitude of the empirical iWUE and g [sub.smax] response was species-specific, ranging from strong to negligible. Stomatal density was more influential than the pore size in determining the g [sub.smax] response to c[sub.a]. While our simulation results indicated that photosynthesis is the main factor contributing to th