Pyridine Nucleotide Charge Reduces Photosynthesis under Short-term Oxygen Deficiency

Soil flooding reduces partial pressure of oxygen (pO 2) in the root zone and often results in a reduction in photosynthesis and growth. In greenhouse studies, rooted stem cuttings of the mango ( Mangifera indica L.) rootstock selection 13/1 were exposed to anoxia by saturating the root zone with N 2 for up to 52 h. Reduced pO2 in the root zone affected the energy status of the roots and particularly enhanced the phosphorylated and nonphosphorylated pyridine nucleotide charges—the ratio of reduced Nicotinamide-adenine-dinucleotides (NAD(P)H) to total Nicotinamide-ad- enine-dinucleotide content (oxidized NAD(P) + plus NAD(P)H)—that drive the redox reaction rates in cell metabolism. Also, the pyridine nucleotide charges in leaves were enhanced, while the photosynthetic rate decreased following reduction in pO2 in the root zone. During up to 4 h of reduced pO 2, the ratio of internal CO 2 concentration in the mesophyll to ambient CO2 concentration was unchanged. This implies a nonstomatal influence on photosynthesis. In addition, light saturation of photosystem II occurred at lower irradiance (470 µmol·m -2 ·s -1 ) resulting in reduced maximum photochemi- cal efficiency below that of the high pO 2 controls. After 28 h of reduced pO 2, NAD(P) charges in the leaves returned to normal, diminishing its potential effect on net photosynthetic rate.