Four species in a growth chamber
The 1981 study compared Euphorbia trigona, Pachypodium brevicaule, Pachypodium horombense and Pachypodium lamerei. Potted plants were transferred to growth chambers at least two weeks before measurements. Illuminance was maintained at 16000 lux, while temperature, air humidity and day length varied among experiments. The photosynthetic surface area was determined for gas-exchange measurements. [Eder et al. 1981]
The authors combined several kinds of measurement. They followed CO₂ uptake and release over the day, examined products of brief radiocarbon labelling, measured the isotope composition of dry matter and assessed daily changes in malate. They also examined stomata on the leaves and stem of Euphorbia trigona. This design connected gas-exchange rhythms with carbon metabolism inside individual organs. [Eder et al. 1981]
The leaf and stem of Euphorbia trigona
The leaves of Euphorbia trigona took up CO₂ in the light and followed a C3 pattern. The stem fixed substantial amounts of CO₂ in darkness. Malate accumulated in its tissues at night and declined during the light period, accompanied by inverse changes in starch and changes in pH. This combination of observations is characteristic of CAM in the stem. [Eder et al. 1981]
After 15 minutes of stem labelling in darkness, malate contained 86% of the recovered radiocarbon activity. In leaves briefly labelled in the light, much of the label entered 3-phosphoglycerate and hexose phosphates. Leaf stomata opened in the light, whereas stem stomata opened in darkness. The two organs of the same plant therefore maintained different gas-exchange rhythms. [Eder et al. 1981]
C3 in the tested Pachypodium
The tested Pachypodium plants produced a different pattern. Carbon isotope composition in stems and leaves was consistent with C3. The table gives δ¹³C values of −28.52‰ in the stem and −29.67‰ in the leaves of Pachypodium brevicaule. Corresponding values in Pachypodium lamerei were −27.41‰ and −26.33‰. The Pachypodium horombense stem measured −24.21‰. These values describe the experimental material, and their differences were not presented as a scale of drought tolerance. [Eder et al. 1981]
For Pachypodium lamerei, the paper also presents daily CO₂ exchange by the whole plant, the leafless stem and detached leaves. The pattern was consistent with C3. Combining these measurements with isotope evidence provides a stronger basis for identifying the pathway than the appearance of a succulent trunk. In this experiment, water storage coexisted with daytime carbon assimilation. [Eder et al. 1981]
Leaf shedding and the distribution of activity
As Euphorbia trigona dried, it gradually shed its leaves. In the illustrated experiment, only the stem remained from day ten onwards. Nocturnal CO₂ fixation initially increased and then declined as water deprivation continued. Photosynthetic activity therefore persisted in an organ with different water relations from the leaves. This observation concerns Euphorbia trigona under the experimental conditions used. [Eder et al. 1981]
The Pachypodium results cover the three named species. Pachypodium rosulatum and Pachypodium namaquanum lie outside this comparison. The 1981 measurements provide a direct example of succulents showing a C3 pattern under the conditions tested and of a plant combining different photosynthetic rhythms in leaves and stem. Each pattern is described by its organ, species and measurement conditions. [Eder et al. 1981]
Mechanismen der CO₂-Fixierung bei Euphorbia trigona Haw. und einigen Pachypodium-Arten
Eder A., Stichler W., Ziegler H., Mechanismen der CO₂-Fixierung bei Euphorbia trigona Haw. und einigen Pachypodium-Arten, Biochemie und Physiologie der Pflanzen, 1981, 176(1):1-12, doi: 10.1016/S0015-3796(81)80003-0
Physiological experiments compare Euphorbia trigona with Pachypodium brevicaule, P. horombense and P. lamerei. Gas exchange, isotopic composition and carbon metabolism contrast C3 in the tested Pachypodium with differing photosynthetic rhythms in Euphorbia leaves and stems.