Plant Productivity and the Control of Photorespiration

Abstract

The Green Revolution of the last decade has seen the yield of some food crops at least doubled by genetic alterations of plant stature and the ability of plants to respond to increased fertilizer. Since only 5-10% of the dry weight of plants comes from minerals and nitrogen in the soil, it is becoming more difficult to obtain further increases in productivity by this approach. Even scientists associated with the Green Revolution believe they have reached a plateau by these methods (1). Therefore, the next large increases in productivity must come from increasing the 90-95% of the dry weight that comes from the assimilation of airborne CO2 during photosynthesis.The productivity of plants (dry weight per unit of ground area) is determined by the gross CO2 assimilation during photosynthesis minus the CO2 released during respiration. The "dark" respiration processes of green plants, which probably also occur as well in the light, are biochemically similar to those found in animal tissues and many microorganisms, and are essential for the growth and maintenance of plant cells. It is not certain whether all of the dark respiration is essential, or whether some portion of it may sometimes be uncoupled from ATP production. This wasteful portion of respiration could cause diminished productivity (2). However, it is well documented that many plant species and varieties respire away large portions of their recently fixed CO2 during illumination by an entirely different biochemical process of respiration, known as photorespiration. Since photorespiration is often much faster in terms of CO2 production than dark respiration, photorespiration greatly lowers plant productivity where this occurs. Some biochemical and plant breeding experiments will be described that hold promise for retention of much of the photorespired C02, leading to large increases in crop yields.Relation of net photosynthesis to productivity among speciesCrop species vary greatly in their yields of dry weight. For example, from statistics of average market yield in the United States (Table 1), one can calculate that the crop growth rate for maize silage, sorghum silage, and sugarcane (cane) is at least double that for spinach, tobacco (leaf plus stalk), and hay grasses (3). The higher-yielding leafy species all have low fluxes of photorespiration compared with the less efficient species. Table 2 contains typical values of CO2 assimilation taken from the literature; it shows that much faster rates are usually found in the higher-yielding tropical grasses and in some weeds, as compared with many common crop plantsincluding spinach, tobacco, and orchard grass-that are lower yielding. A large part of the differences in net photosynthesis between the efficient and nonefficient species can be explained by the much slower rate of photorespiration that is encountered naturally only in the efficient plants. Characteristics of photorespirationSince photorespiration is such an important process, it is interesting to speculate on why its discovery was so long delayed....

MORE EFFICIENT PLANTS: A Consequence of Rising Atmospheric CO2?

“…Second, Rubisco catalyzes the oxygenation of Ribulose-1,5-bisphosphate (RubP), a reaction that is competitively inhibited by CO 2 (18). Oxygenation of RubP is the first step of the photosynthetic carbon oxidation or photorespiratory pathway (PCO), which decreases the net efficiency of photosynthesis by 20-50%, depending on temperature (245), by utilizing light energy and by releasing recently assimilated carbon as CO 2 . CO 2 is a competitive inhibitor of the oxygenation reaction, such that a doubling of concentration at Rubisco will roughly halve the rate of oxygenation (131).…”

Involvement of cytosolic ascorbate peroxidase and Cu/Zn-superoxide dismutase for improved tolerance against drought stress

“…This increase seems to be involved in the removal of H 2 O 2 that is produced in peroxisomes during photorespiration (Mittler and Zilinskas, 1994). During drought, an increase in photorespiration has been described that can increase H 2 O 2 production in the peroxisome due to the increased activity of glycolate oxidase (Zelitch, 1973;Mittler and Zilinskas, 1994). H 2 O 2 could also diffuse through the peroxisomal membrane into the cytosol (Del Río et al, 1998), thus increasing the risk of oxidative damage in this compartment.…”

Stoichiometric analysis of the energetics and metabolic impact of photorespiration in C3 plants

“…This comes at the expense of energy and generation of photorespiratory CO 2 . Therefore, photorespiration has generally been regarded as a wasteful process, and it has long been contested that by inhibiting the oxygenase reaction, and therefore photorespiration, crop yield could be increased (Zelitch, ). This is debatable given the importance of photorespiration in plant survival and productivity (Aliyev, ; Bloom, ; Hodges et al ., ), and its interactions with multiple cellular metabolic pathways (Foyer et al ., ; Bauwe et al ., ; Florian et al ., ; Hodges et al ., ; Busch et al ., ).…”