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A mutant in the maize (Zea mays) Glycolate Oxidase1 (GO1) gene was characterized to investigate the role of photorespiration in C4 photosynthesis. An Activator-induced allele of GO1 conditioned a seedling lethal phenotype when homozygous and had 5% to 10% of wild-type GO activity. Growth of seedlings in high CO2 (1%-5%) was sufficient to rescue the mutant phenotype. Upon transfer to normal air, the go1 mutant became necrotic within 7 d and plants died within 15 d. Providing [1-14C]glycolate to leaf tissue of go1 mutants in darkness confirmed that the substrate is inefficiently converted to 14CO2, but both wild-type and GO-deficient mutant seedlings metabolized [1-14C]glycine similarly to produce [14C]serine and 14CO2 in a 1:1 ratio, suggesting that the photorespiratory pathway is otherwise normal in the mutant. The net CO2 assimilation rate in wild-type leaves was only slightly inhibited in 50% O2 in high light but decreased rapidly and linearly with time in leaves with low GO. When go1 mutants were shifted from high CO2 to air in light, they accumulated glycolate linearly for 6 h to levels 7-fold higher than wild type and 11-fold higher after 25 h. These studies show that C4 photosynthesis in maize is dependent on photorespiration throughout seedling development and support the view that the carbon oxidation pathway evolved to prevent accumulation of toxic glycolate.

C4 plants such as maize partition photosynthetic activities in two morphologically distinct cell types, bundle sheath (BS) and mesophyll (M), which lie as concentric layers around veins. We show that both light and cell position relative to veins influence C4 photosynthetic gene expression. A pattern of gene expression characteristic of C3 plants [ribulose bisphosphate carboxylase (RuBPCase) and light‐harvesting chlorophyll a/b binding protein in all photosynthetic cells] is observed in leaf‐like organs such as husk leaves, which are sparsely vascularized. This pattern of gene expression reflects direct fixation of CO2 in the C3 photosynthetic pathway, as determined by O2 inhibition assays. Light induces a switch from C3‐type to C4‐type gene expression patterns in all leaves, primarily in cells that are close to a vein. We propose that light causes repression of RuBPCase expression in M cells, by a mechanism associated with the vascular system, and that this is an essential step in the induction of C4 photosynthesis.

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Sumnnmtary. There is considerable variation among species in their rate of photorespiration, and photorespiration increases greatly at higher temperattures. The addition of an inhibitor of glycolate oxidase, a-hydroxy-2-pyridinemethanesulfonic acid, to tobacco leaf disks at 350 stimulated photosynthetic 14,CO2 uptake at least 3-fold, but 14CO tuptake was not changed by the inhibitor at 250. The inhibitor did not increase photosynthesis in maize leaf disks at either temperature.The evolution of COO from glycolate was greatly enhanced in tobacco at 35°c ompared with 250. Labeling of the glycolate of tobacco with glycolate-1-1'4C and -2-'*C showed that the increased CO evolved in the light (photorespiration) arose specifically from the carboxyl-carbon atom of glycolate. Maize, a species known to have a negligible photorespiration, produced 14CO, poorly from glycolate-1-14C in comparison to tobacco.Acetate-1-14C, a substrate metabolized by dark respiration, produced similar amounts of '-CO2 in the light in both tobacco and maize. This respiration was changed little relative to photosynthesis by increasing temperature.Most plants, such as tobacco, have a high photorespiration. The loss of fixed carbon causes an increase in the internal concentration of CO, especially at higher temperatures, and results in a lower CO2 concentration gradient and therefore a lower net photosynthetic CO2 uptake. Some species, like maize, have a negligible photorespiration and are thus morc efficient photosynthetically. The use of an inhibitor of the oxidation of glycolate, the substrate for photorespiration, changed tobacco so that it behaved photosynthetically like maize. Thuls high rates of photorespiration may limit the net CO., uptake in many plant species.Ample evidence exists that photosynthetic tissues have a respiration in the light that results in CO2 evoluition (photorespiration) which occurs by reactions different from those which occur in darkness. Under normal conditions of photosynthesis, this photorespiration is often greater than the dark respiration, whether measured by 02 uptake or CO2 evolution. This paper is concerned with the nature of the substrate of this photorespiration and with the role of photorespiration in net CO, assimilation.Decker (1) observed that when leaves of several species including tobacco were transferred from the light to darkness, there was a burst of COO evolution before CO2 output resumed a lower steady rate. The CO. burst undouibtedly represented an overshoot of the greater CO2 evolution that occurred in the light. By measuiring 0 uptake with the aid of 180, labeling, Hoch, Owens, and Kok (6), have shown that several species of algae have a photorespiration which increases with increasing light intensity and that this is superimposed upon and presumably different in mechanism from the dark respiration. Ozbun, Volk, and Jackson (13) also found a greater 02 uptake by bean leaves in the light using mass spectrometric techniques.Forrester, Krotkov, and Nelson (3,4) recently found that photorespira...

Understanding the mechanism of stomatal opening in leaves is important because stomata are the avenues for CO2 and H,O diffusion. Stomata in most species open in the light when the guard cells that encompass the pores take up water and increase in turgor relative to adjacent epidermal cells (1). Thus stomatal opening is likely an osmotic plhenomenon that depends upon the accumulation of solute in the guard cells, and during opening this accumulation has been reported to range from 0.' X to 0.5 M in various experiments (2). For Tobacco leaves (Nicoliana tabacum, variety Havana Seed) were excised from greenlhouse grown plants and placed in darkness for several hr so that the stomata closed. Leaf disks 3.2 cm in diameter were cut with a sharp punch and floated upside down on water in the air at 30°in 1000 ft-c illunmination. Following illuminiation the disks were left in the dark for various periods. After these treatments, portions of the lower epidermis were quickly stripped off. They were placed on a glass slide and rapidly

Glyoxylate treatment doubles net photosynthetic carbon dioxide fixation by tobacco leaf disks because inhibition of glycolate synthesis by glyoxylate results in decreased photorespiration. These observations show that photorespiration can be metabolically regulated and suggest that genetic or chemical alteration of pool sizes of certain metabolites can produce plants with increased photosynthesis.

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