The Development of Crassulacean Acid Metabolism (CAM) Photosynthesis in Cotyledons of the C4 Species, Portulaca grandiflora (Portulacaceae)

Abstract

Portulaca grandiflora simultaneously utilizes both the C4 and Crassulacean acid metabolism (CAM) photosynthetic pathways. Our goal was to determine whether CAM developed and was functional simultaneously with the C4 pathway in cotyledons of P. grandiflora. We studied during development whether CAM would be induced with water stress by monitoring the enzyme activity, leaf structure, JO2 (rate of O2 evolution calculated by fluorescence analysis), and the changes in titratable acidity of 10 and 25 days old cotyledons. In the 10 days old cotyledons, C4 and CAM anatomy were evident within the leaf tissue. The cotyledons showed high titratable acid levels but a small CAM induction. In the 25 days old cotyledons, there was a significant acid fluctuation under 7 days of water stress. The overall enzyme activity was reduced in the 10 days old plants, while in the 25 days old plants CAM activity increased under water-stressed conditions. In addition to CAM, the research showed the presence of glycine decarboxylase in the CAM tissue. Thus, it appears both pathways develop simultaneously in the cotyledons but the CAM pathway, due to anatomical constraints, may be slower to develop than the C4 pathway. Cotyledons showed the ancestral Atriplicoid leaf anatomy, which leads to the question: Could a CAM cell be the precursor to the C4 pathway? Further study of this may lead to understanding into the evolution of C4 photosynthesis in the Portulaca.

Photo-Protective Mechanisms and the Role of Poly (ADP-Ribose) Polymerase Activity in a Facultative CAM Plant Exposed to Long-Term Water Deprivation

“…Daily measurement of gas exchanges could have confirmed the evidence of both CAM and CAM-idling status in P. afra plants in response to prolonged water stress. However, even if we could not measure the gas exchanges, our hypothesis is supported by the recent work of Guralnick et al [ 22 ], which demonstrated the development of CAM activity through the titratable acidity levels and diurnal acid fluctuations. The daily acid fluctuation measured in the present study is in agreement with values found by Guralnick and Ting [ 20 ] in P. afra plants exposed to prolonged water stress.…”

“…As we observed a reduction in nocturnal organic acid (likely malic acid) accumulation with the progression of water stress, we supposed that photorespiration could be active during the CAM-idling status, contributing to keep oxidized the photosynthetic electron transport chain. The photorespiration could also provide CO 2 for the C3 metabolism during the daylight, as recently proposed by Guralnick and colleagues [ 22 ] for the C4 plant Portulaca grandiflora .…”

A high‐temperature water vapor equilibration method to determine non‐exchangeable hydrogen isotope ratios of sugar, starch and cellulose

“…Above that, δ 2 H ne values in CAM plants may indicate if a facultative CAM plant performs C 3 or C 4 photosynthesis in the absence of water stress (Guralnick, Gilbert, Denio, & Antico, 2020 ; Winter, Garcia, & Holtum, 2008 ). The higher the contribution of C 3 or C 4 photosynthesis to a CAM plant's total carbon dioxide fixation, the more depleted are the δ 2 H ne values of cellulose and NSC (Luo & Sternberg, 1991 ; Sternberg, Deniro, & Johnson, 1984 ), thus indicating the absence of water stress.…”

Responses to Salt Stress in Portulaca: Insight into Its Tolerance Mechanisms

“…P. grandiflora has been reported as salt tolerant and as an accumulator of proline [53]. Recently, in addition to the C4 photosynthesis common in the genus Portulaca, the pathway CAM was found in cotyledons of this species [54]. According to our knowledge, there are no studies regarding the salt tolerance of P. halimoides, except our previous reports on germination and early seedling growth in P. oleracea, P. grandiflora and P. halimoides [55] which revealed a greater resilience to drought of the latter, and a seemingly better response to salinity in P. grandiflora.…”