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Fig. 1.4.4. The energy charge of lettuce seeds (A) or rice grains (B) during germination In air or under nitrogen. (After Pradet et al. 1985)

switched off by mutation or genetic modification. Acidification of the cytosol by lactic acid is another metabolically detrimental effect of hypoxia. When oxygen deficiency sets in, lactate dehydrogenase (LDH) is the first enzyme to be activated and the pH in the cytosol decreases as lactic acid forms and accumulates. As the pH optimum of LDH is in the neutral range, it inhibits itself upon acidification of the cytosol. Pyruvate decarboxylase is less susceptible to acidity and therefore takes over, producing acetalde-hyde and, together with alcohol dehydrogenase, ethanol.

Biochemical Adaptation to Oxygen Deficiency Besides anatomical-morphological strain reactions to the shortage of oxygen (as explained later), biochemical adaptation can take place: In addition to lactate, rice seedlings and sweet flag (Acorus calamus) rhizomes mainly produce basic amino acids [asparagine, arginine, 7-amino butyric acid (GABA)]. Synthesis of GABA from glutamate releases C02 and consumes protons, and is a reliable marker for metabolism under hypoxia:

ooc - ch2 - ch2 - chnh2 - coo + h+ — ooc - ch2 - ch2 - ch2nh2 + co2

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