Cryptic crassulacean acid metabolism (CAM) in Jatropha curcas

Jatropha curcas L. is a drought-tolerant shrub or small tree that is a candidate bioenergy feedstock. It is a member of the family Euphorbiaceae in which both CAM and C4 photosynthesis have evolved. Here, we report that J. curcas exhibits features diagnostic of low-level CAM. Small increases in nocturnal acid content were consistently observed in photosynthetic stems and occasionally in leaves. Acidification was associated with transient contractions in CO2 loss at night rather than with net CO2 dark fixation. Although the CAM-type nocturnal CO2 uptake signal was masked by background respiration, estimates of dark CO2 fixation based upon the 2 : 1 stoichiometric relationship between H+ accumulated and CO2 fixed indicated substantial carbon retention in the stems via the CAM cycle. It is proposed that under conditions of drought, low-level CAM in J. curcas stems serves primarily to conserve carbon rather than water.

Alcoholic Fermentation and Malate Metabolism in Rice Germinating at Low Oxygen Concentrations

Germinating rice was exposed, in the dark, to low or zero O2 concentrations for 4-5 days by: (1) submergence under 4-5 cm of stagnant solution (3 ppm O2); (2) exposure to a N2 atmosphere; or (3) submergence under solutions flushed with N2. These treatments completely inhibited root growth. Elongation of coleoptiles was stimulated in the stagnant solutions, but not in the N2 treatments. In most experiments, low O2 concentrations resulted in twofold to eightfold increases of malate concentrations in the shoots. Absence of O2 during exposure to H14CO3-, for 30-60 min, inhibited CO2 dark fixation. This inhibition was considerably smaller when seedlings had been raised in N2 rather than in air. Under aerobic conditions during fixation, excised shoots from seedlings raised in N2 fixed more CO2 than shoots from seedlings raised in air. Malate always contained 70% or more of the total fixed 14C, irrespective of the O2 regime during germination and during 14CO2 fixation. Ethanol in stagnant solutions was shown to be formed by the rice seedlings, rather than by bacteria. Ethanol formation during one single day was 20-30-fold greater than the highest recorded amounts of malate in the seedlings. Alcoholic fermentation also responded quickly to changes in aeration regimes, indicating it was an important adaptive factor. Another likely adaptive feature was the high K+ concentration in shoots, even of seedlings grown in the complete absence of O2. It is suggested that these high K+ concentrations have a function in maintaining turgor required for the rapid extension growth of the coleoptiles under low O2 concentrations.

RELATIONSHIPS AMONG CO2 DARK-FIXATION, SUCCULENCE, FLOWERING, AND ORGANIC ACID FORMATION IN KALANCHOE BLOSSFELDIANA VARIETY TOM THUMB

Kalanchoe blossfeldiana variety Tom Thumb was treated photoperiodically in regular and CO2-free air and examined at two widely separated time intervals for interrelationships among flowering, succulence, capacity for CO2 dark-fixation, and organic acid formation. These processes and leaf thickening are all under photoperiodic control and increase with increasing numbers of short-day treatments. They also occur in a CO2-free atmosphere on 9-hour photoperiods although CO2 dark-fixation and organic acid formation are limited to the low level of the long-day treated plants at 5 weeks' time. Only CO2 dark-fixation and organic acid formation appear to be interrelated and apparently utilize respiratory CO2 if treated in a CO2-free atmosphere. This capacity to use respiratory CO2 increases with maturity of the plants. Flowering does not control the development of succulence, organic acid formation, or the capacity for CO2 dark-fixation. Likewise, capacity for dark-fixation and organic acid formation do not control flowering and can occur in vegetative Tom Thumb plants that do not exhibit appreciable succulence.