The responses of Crassulacean acid metabolism (CAM) photosynthetic physiology and stomatal conductance to plant and leaf age and drought-stress across the diversity of the genus Kalanchoë

Abstract

The genus Kalanchoë (Crassulaceae) represents an ideal model system for studying the Crassulacean acid metabolism (CAM) adaptation of photosynthetic CO2 assimilation, both in terms of CAM evolution and the underpinning molecular-genetic blueprint used to achieve an optimised CAM system. Kalanchoë species are known to vary from C3 with drought-inducible weak CAM, through to full and strong CAM. Earlier work classified the genus into three taxonomic sections, namely Kitchingia, Bryophyllum and Eukalanchoe, and correlated CAM physiology and C3-CAM flexibility with these evolutionary groups within the genus. However, this work relied mostly on delta- 13C values to determine the level of CAM, and few Kalanchoë species have been studied in detail in terms of their daily cycle of CO2 fixation and stomatal conductance. In order to better understand the diversity of CAM physiotypes in Kalanchoë, the gas exchange patterns were measured over 24-hours for a wide diversity of Kalanchoë species. The selected species possess a broad spectrum of leaf and stem morphological traits, with representatives from each taxonomic section. Gas exchange measurements were collected from different developmental leaf ages under well-watered conditions as well as during drought-stress progression at identified soil water content (SWC), providing valuable insights into increasing levels of CAM-associated dark CO2 fixation as leaves develop and mature. Using these complementary datasets, the level of CO2 fixation in the dark and light periods was quantified, compared and contrasted across Kalanchoë species. In addition, leaf thickness, growth and biomass accumulation were measured for different leaf ages and their relation to the level of CAM was studied. The link of CAM level to the anatomical traits was also addressed. The results of gas exchange reveal that all Kalanchoë species that are selected from each taxonomic section are capable to perform CAM under well-watered conditions. In addition, Kalanchoë species that belong to each taxonomic section develop CAM in their developmental leaf ages and plant ages. The anatomical feature identified for the diverse representative Kalanchoë species that are selected from each taxonomic section revealed that Kalanchoë plants possessed compacted mesophyll cells with reduced intercellular air space. In addition, most Kalanchoë species were defined to possess uniform mesophyll cells. Gas exchange results of the selected representative Kalanchoë species provide fascinating diversity that shows their capability of expressing and inducing CAM levels in a variable level and flexible way in response to the environmental condition such that each Kalanchoë species is capable of performing multiple physiotype regardless of where they have been classified. A representative Kalanchoë species, such as K. miniata exhibited unexpectedly high levels of CAM under well-watered and drought-stress conditions. K. orgyalis, which has been identified as an obligate CAM is found to rely on CAM even in their young leaf ages under well-watered conditions, but shows its capability of inducing high levels of CAM when experiencing drought-stress progression. In addition, the results of gas exchange measurements revealed that African species are distinct from “woody” Madagascan Kalanchoë species in terms of their capability to perform higher light CO2 assimilation in their young leaf age. In addition, Kalanchoë species that possess thin leaves could perform as an obligate CAM such as K. shimperiana and K. welwistchii. This PhD project provides novel insights into CAM physiology with a detailed analysis that addresses the link of this type of photosynthesis to the level of CAM that can be used to understand the evolution of the CAM trait and provide examples of the diversity of CAM patterns in terms of the level of CO2 assimilation and stomatal conductance in diverse plants that could be a reference for future CAM projects that attempt to engineer increased Water Use Efficiency (WUE) in crop species.