Workshop on Pattern Formation and Functional Morphology, Wed, 09 Jan, 2008
Speaker: Ulrich Lüttge
Abstract
Crassulacean acid metabolism (CAM) is a mode of photosynthesis with nocturnal fixation of CO2 and storage in the form of malate in cell vacuoles followed by daytime remobilisation and assimilation of CO2 in the light. In the diurnal phases of CAM strong differences of CO2-partial pressures occur within the leaves [1]. This elicits heterogeneity of photosynthetic activity over individual leaves, which is depicted by chlorophyll fluorescence imaging and quantified using a nearest neighbour algorithm [2]. CO2 gas exchange in CAM leaves also shows endogenous circadian oscillations. These are strongly determined by temperature as an external control parameter [3, 4]. They are lost and the leaves become arrhythmic above an upper temperature threshold [5]. This is reversible. By using minimal models of the metabolic CAM cycle with the permeability of the tonoplast membrane, which is enclosing the cell vacuoles, as a hysteresis switch [6 - 9] this behaviour can be perfectly simulated [10 - 12]. The simulations also bear out the phenomenon of stochastic resonance, where additive white noise in the nonlinear differential equations describing the system elicits a quasi periodic behaviour in a temperature domain otherwise not allowing rhythmicity [13]. Reducing temperature from above the upper temperature threshold into the temperature domain of rhythmicity only reinstalls rhythmicity when temperature reduction is fast. Slow temperature reduction fails to reactivate the overt endogenous oscillations of CO2 exchange [14]. This suggests that each photosynthesising leaf cell has its own copy of the clock or oscillator and a strong signal is required to synchronize them [15]. After introducing coupling of oscillators in the model it perfectly simulates the behaviour observed experimentally. Creating artificial patchiness by locally preventing CO2 exchange of the leaves by silicon greasing provides evidence that lateral CO2 diffusion in the leaves is the synchronizing signal [16].
[1] Lüttge U. (2002) J. Exp. Bot. 53: 2131 – 2142
[2] Rascher U., Lüttge U. (2002) Plant Biol. 4: 671 - 681
[3] Bohn A., Rascher U., Hütt M.-T., Kaiser F., Lüttge U. (2002) Biol. Rhythm Res. 33: 151
- 170
[4] Bohn A., Hinderlich S., Hütt M.-T., Kaiser F., Lüttge U. (2003) Biol. Chem. 384: 721 –
728
[5] Lüttge U., Beck F. (1992) Planta 188: 28 – 38
[6] Blasius B., Beck F., Lüttge U. (1998) Plant, Cell and Environment 21: 775 – 784
[7] Blasius B., Neff R., Beck F., Lüttge U. (1999) Proc. R. Soc. Lond. B 266: 93 – 101
[8] Neff R., Blasius B., Beck F., Lüttge U. (1998) J. Membrane Biol. 165: 37 – 43
[9] Lüttge U. (2000) Planta 211: 761 – 769
[10] Blasius B., Beck F., Lüttge U. (1997) J. theor. Biol. 184: 345 – 351
[11] Grams T.E.E., Beck F., Lüttge U. (1996) Planta 198: 110 – 117
[12] Grams T.E.E., Borland A.M., Roberts A., Griffiths H., Beck F., Lüttge U. (1997) Plant
Physiol 113: 1309 – 1317
[13] Beck F., Blasius B., Lüttge U., Neff R., Rascher U. (2001) Proc. R. Soc. Lond. B 268:
1307 - 1313
[14] Rascher U., Blasius B., Beck F., Lüttge U. (1998) Planta 207: 76 – 82
[15] Rascher U., Hütt M.-T., Siebke K., Osmond B., Beck F., Lüttge U. (2001) Proc. Nat.
Acad. Sci. US 98: 11801 – 11805
[16] Duarte H.M., Jakovljevic I., Kaiser F., Lüttge U. (2005) Planta 220: 809 - 816
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