Diurnal acid rhythm

Malic acid monohydroxysuccinic acid, HOOC-CHOH-CHj- OOH, a dicarboxylic acid found in many plant juices, usually in the L(-i-)-form, m.p. 100°C, b.p. 140°C (d.). The malate ion is formed in the Thcarboxylic acid cycle (see) and in the Glyoxy-late cycle (see). Malate plays an important role in the Diurnal acid rhythm (see) of the Crassulaceae (stonecrop family). 

Fig. 6.8a and b. Performance of CAM in O. inermis on the natural stands at Goondiwini and Denman (Eastern Australia), a Diurnal cycle of CAM (malic acid rhythm, leaf resistance, and " C02 fixation). Actual temperatures and relative air humidities are indicated at top of a. b Seasonal changes in the pattern of dark CO 2 fixation and course of leaf resistance. Malic acid levels, soil water capacity (SWC) and plant water potential ( pp) are indicated in the upper parts of the figure (after Osmond et al, 1978, by permission)... 

Extensive investigations of the flora on coastal and desert habitats of Israel and the Sinai were carried out by Winter et al. (1976). Only four of the 75 species studied showed diurnal malic acid rhythms typical of CAM the halophytes Mes-embryanthemum crystallinum (see also Winter et al., 1978), M.forskalii and M. nodi-florum, and Caralluma negevensis (cf. Fig. 6.5). The latter species had 5 C values of — 13.4%o, indicating a dominant nocturnal CO2 fixation. 

Melatonin (lV-acetyl-5-methoxytryptamine) is a neuro-hormone secreted by the pineal gland from the amino acid precursor [,-tryptophan. Its endogenous secretion is photosensitive and has a circadian rhythm—plasma melatonin concentrations are highest at night in both diurnal and nocturnal animals, and fall with age (1). The nocturnal melatonin peak coincides with a drop in body temperature and increased sleepiness in healthy humans. Oral melatonin has a short half-life (30-50 minutes) and extensive first-pass metabolism. Its clearance is reduced in severe liver disease (2). 

Duane WC, Gilberstadt ml Wiegand DM (1979) Diurnal rhythms of bile acid production in the rat. Am J Physiol 236 R175-R179... 

Certain features of CAM such as the diurnal rhythm in gas exchange and C4 acid... 

Wide daily fluctuations in the concentrations of plasma tryptophan, as well as of other amino acids, occur in healthy humans.7 Feigin et al.8 reported a diurnal rhythm for total whole-blood amino acid levels in healthy humans. Daily fluctuations have been observed for all of the nutritionally important amino acids.910 A number of investigators have studied factors that may influence diurnal fluctuations of plasma tryptophan in humans.1112 Many reports cite marked variations, usually decreased levels, of plasma tryptophan, under a variety of disease states. However, it is difficult to assess the importance of these observations. 

The malic acid content of the photosynthetic tissue must fluctuate in a diurnal rhythm with accumulation of malic acid during the night period and disappearance during the day. 

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