Malic acid accumulation, inhibition

C02 Free Air. Effects of external CO2 concentrations on acidification and deacidification during CAM have been discussed in detail by Wolf (1960). It is now well established that C02-free air inhibits nocturnal malic acid synthesis and accumulation (Kluge, 1968 b, 1969 b see also Wolf, 1960). It is reasonable to assume that this effect is due to the lack of substrate for CO2 dark fixation. However, with C02-free air malic acid accumulation at night is not prevented completely (see for example Kluge, 1969 b) because of endogenous (respiratory) CO2, which may serve as the substrate of dark CO2 fixation. 

There is overwhelming evidence that high night temperatures inhibit malic acid accumulation in CAM plants, whereas low night temperatures have the opposite effect (see reviews of Wolf, 1960 Ranson and Thomas, 1960 see Chap. 4.3.1). Because malic acid is the end product of the metabolic pathway of dark CO2 fixation, it is reasonable to predict that CO2 exchange during the night would also be affected by temperature. 

It is not known if the CO2 output rhythms described above are accompanied by corresponding oscillations in malic acid concentrations. Clearly, such acid changes would occur if the rhythmic phenomena were CAM-related. Since CO2-free air inhibits malic acid accumulation even in normal day/night cycles (Wolf, 1960 Kluge, 1968 b), the rhythmic phenomena described by Wilkins may not be CAM-related. However, Queiroz (1975) reported a circadian CO2 output rhythm by Kalanchoe blossfeldiana in normal air in darkness as did Wilkins (1959) with B.fedtschenkoi. Hence, the rhythms are probably not the result of the C02-free air, but since we do not know more about corresponding malic acid fluctuations, we cannot determine the extent to which the rhythmic CO2 output is CAM related. 

Based on these results, Goldberg et al. (2006) suggested two possibilities to explain these findings. The first is that in R. oryzae the cytosolic fumarase is kinetically different from the mitochondrial isoenzyme, due to distinct posttranslational modifications, or to specific conditions in the two compartments. The second possibility is that R. oryzae harbors two genes encoding two different fumarases, one in mitochondria, which catalyzes the conversion of fumaric to L-malic acid, and a cytosolic enzyme, which catalyzes the conversion of L-malic to fumaric acid. Upon transfer into medium C, a fumarase with unique characteristics is induced. L-malic acid s conversion to fumaric acid is enhanced by the induced fumarase and when the concentration of fumaric acid in the cell exceeds 2 mM, the reverse reaction to L-malic acid is fully inhibited. Thus, this property of the unique fumarase, whose existence was then only hypothesized, can ensure that fumaric acid is accumulated. 

After a period of dark CO 2 fixation resulting in the vacuolar accumulation of malic acid and decreased levels of stored carbohydrate, there is a rapid and marked decrease in stored malic acid when the plants experience light (Fig. 3.6). Malate, once mobilized for subsequent metabolic consumption, may inhibit P-enolpyruvate carboxylase (Ting, 1968) and reduce further carboxylation (Kluge, 1969 Queiroz, 1967 Ting and Osmond, 1973 a, b). It is generally assumed that deacidification occurs because of malate decarboxylation (or oxalacetate decarboxylation) and concomitant release of CO2. 

With malic acid synthesis being inhibited, malic acid consumption dominates, allowing net consumption of malic acid ( deacidification ). During the night, malic acid may accumulate since photosynthesis can no longer compete for CO2, i.e., PEP-C would operate at its full capacity. 

It must be mentioned that the discussion of temperature effects is complicated by the fact that CAM is modified both by direct temperatures and to a great extent by temperature pretreatment. In general, it is well known that low night temperatures favor nocturnal acid accumulation (for example DeVries, 1884 Richards, 1915). Below 5° and above 30° C, nocturnal malic acid synthesis is inhibited. 

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