Organic acids are synthesized in plants as a result of the incomplete oxidation of photosynthetic products and represent the stored pools of fixed carbon accumulated due to different transient times of conversion of carbon compounds in metabolic pathways. When redox level in the cell increases, e.g., in conditions of active photosynthesis, the tricarboxylic acid (TCA) cycle in mitochondria is transformed to a partial cycle supplying citrate for the synthesis of 2-oxoglutarate and glutamate (citrate valve), while malate is accumulated and participates in the redox balance in different cell compartments (via malate valve). This results in malate and citrate frequently being the most accumulated acids in plants. However, the intensity of reactions linked to the conversion of these compounds can cause preferential accumulation of other organic acids, e.g., fumarate or isocitrate, in higher concentrations than malate and citrate. The secondary reactions, associated with the central metabolic pathways, in particularly with the TCA cycle, result in accumulation of other organic acids that are derived from the intermediates of the cycle. They form the additional pools of fixed carbon and stabilize the TCA cycle. Trans-aconitate is formed from citrate or cis-aconitate, accumulation of hydroxycitrate can be linked to metabolism of 2-oxoglutarate, while 4-hydroxy-2-oxoglutarate can be formed from pyruvate and glyoxylate. Glyoxylate, a product of either glycolate oxidase or isocitrate lyase, can be converted to oxalate. Malonate is accumulated at high concentrations in legume plants. Organic acids play a role in plants in providing redox equilibrium, supporting ionic gradients on membranes, and acidification of the extracellular medium.

The role of nitrate reduction to produce nitric oxide (NO) and its subsequent oxidation by oxyhaemoglobin as a mechanism to maintain plant cell energetics during hypoxia is examined. Nitrate reduction in hypoxic conditions can be considered as an alternative respiratory pathway, with nitrate as an intermediate electron acceptor, contributing to the oxidation of NADH. NO, produced in the reaction, does not accumulate due to the induction of hypoxia-induced (class 1) haemoglobins. These haemoglobins remain in the oxyhaemoglobin form, even at oxygen tensions two orders of magnitude lower than necessary to saturate cytochrome c oxidase. They act, probably in conjunction with a flavoprotein, as NO dioxygenases converting NO back to nitrate, consuming NAD(P)H in the process. The overall system oxidizes 2.5 moles of NADH per one mole of nitrate recycled during the reaction, leading to the maintenance of redox and energy status during hypoxia and resulting in the reduced production of ethanol and lactic acid.

Summary: Transgenic alfalfa root cultures expressing sense and antisense barley hemoglobin transcripts were examined under varying levels of atmospheric oxygen. Root cultures overexpressing the hemoglobin gene (Hb⁺) maintained root growth when placed under 3% oxygen, whereas control cultures or cultures underexpressing hemoglobin (Hb⁻) experienced 30–70% declines in growth under the same conditions. ATP levels and ATP/ADP ratios for Hb⁺ lines did not significantly differ in 40 and 3% oxygen, whereas the ATP levels and ATP/ADP ratios in control and Hb⁻ lines were significantly lower under 3% oxygen. Large increases in the production of nitric oxide (NO) were measured in root cultures grown under hypoxic conditions compared to aerobic conditions. The amount of NO accumulated in an Hb⁻ line was 2.5‐fold higher than that in the Hb⁺ line. Treatment of transgenic root lines under 40% oxygen with NO resulted in significant declines in the ATP levels and ATP/ADP ratio of an Hb⁻ line and the control line, with no significant change in an Hb⁺ line. The root cell structure of an Hb⁻ line showed evidence of cell breakdown under hypoxic growth, whereas an Hb⁺ line had no evidence of cell breakdown under similar growth conditions. These results lead us to hypothesize that NO is involved in the response of plants to hypoxia and that hemoglobin modulates the levels of NO in the hypoxic cell.