Photosynthetic Carbon Assimilation in a Shootless Orchid, Chiloschista usneoides (DON) LDL

Abstract

Photosynthetic carbon assimilation in the roots of a shootless orchid Chiloschista usneoides (DON) LDL involves the synthesis and accumulation of malic acid from CO2 in darkness. Malic acid is consumed in the light.The roots do not possess stomata or any means of diurnally regulating the diffusive conductance of the pathway between the internal gas phase ofthe plant and the atmosphere. Regulation of internal CO2 concentration near to atmospheric levels avoids a large net loss of CO2 to the atmosphere during malic acid consumption in the light.The water-absorbing function of the velamen conflicts with the photosynthetic function of the roots. Plants with water-saturated velamina do not acquire CO2 from the atmosphere at night.Photosynthesis in many epiphytic orchids involves the nocturnal acquisition of CO2 and accumulation of malic acid followed the next day by the release of CO2 from malic acid and its utilization in the reductive pentose phosphate pathway. Stomatal resistance is high at night and low during the day. Clearly these plants exhibit CAM (1).Another commonly observed feature of such orchids is the possession ofChl-containing aerial roots and it has recently been shown that these roots, like leaves, exhibit the diurnal fluctuation in acid content and CO2 exchange pattern characteristic ofCAM (6). The aim of the present work is to extend information on the photosynthetic metabolism of orchid roots.The experimental material used in this study is Chiloschista usneoides, a member of the group known as shootless orchids which has been eloquently described by Benzing et al. (2) CO2 Exchange. The CO2 concentration in air which had passed at a rate of 300 ml-' min-' through a glass cuvette maintained at 25°C containing a whole plant was estimated by open-system, differential analysis using a Beckman model 865 IR gas analyzer. Water vapor was removed from the usually saturated gas stream by absorption in silica gel prior to analysis. Calibration of the system utilized standard CO2 mixtures. Plants were illuminated by a 700 w Philips mercury lamp and PAR was measured using a LI-COR quantum meter with a LI-190 SB sensor. Experiments were repeated at least four times.Products of Dark '4CO2 Fixation. Radioactive CO2 (50 ACi in 1 Mmol) generated from NaHCO3 by the addition of lactic acid was injected through a serum bottle stopper into darkened 50-ml flasks containing intact plants. After incubation at 25°C for 14 h, the plants were killed and extracted in boiling ethyl alcohol.The ethanol extract was analyzed by paper chromatography and autoradiography using propan-1 -ol, ammonia, water, and phenol; water as solvents. The identity ofradioactive compounds from two experiments was confirmed by co-chromatography and relative amounts of radioactivity measured by Geiger Muller counting of the chromatograms.Internal CO2 Concentration. Using a microsyringe, I0-js1 samples of gas were removed from the internal atmosphere of plants under the conditions detailed in the results section and analyzed by GC on Porapak...

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“…In a study of the leafless orchid Campylocentrum tyrridion, Winter et al (1985) observed a nocturnal increase in titratable acidity content and a daily cycle of CO 2 uptake that resembled classical CAM. Cockburn et al (1985) found similar results in the shootless orchid Chiloschista usneoides. The major acid produced at night was malic acid, which indicates CO 2 uptake via PEPC.…”

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