P₇₀₀ Activity and Chlorophyll Content of Plants with Different Photosynthetic Carbon Dioxide Fixation Cycles

Abstract

Representative plants containing either the reductive pentose phosphate cycle or the C4 dicarboxylic acid cycle of photosynthetic carbon dioxide fixation have distinctly different contents of P700 and chlorophylls a and b. With leaf extracts and isolated chloroplasts from C4 cycle plants, the mean value of the relative ratio of P700 to total chlorophyll was 1. 83 pentose cycle plants saturate near 2,000 to 3,000 ft-c (5). Also the maximal rates of photosynthesis in plants with the C4 cycle, as measured by CO2 consumption, are 2-to 3-fold higher than pentose cycle plants at full sunlight (5, 6). We recently calculated a theoretical stoichiometry for CO2 fixation in C4 cycle plants of 5 moles of ATP and 2 moles of NADPH per mole of CO2 (6). In contrast, the stoichiometry in pentose cycle plants is: 3 ATP: 2 NADPH: 1 CO2 (3). We reasoned that the higher ATP requirements of C4 cycle plants could be supplied by cyclic photophosphorylation catalyzed by Photosystem I and recently presented some data to support this hypothesis (6). The reactioncenter pigment for Photosystem I is P700 according to current ideas (13). Thus this manuscript reports our findings on P700 and other pigments in these two distinct groups of higher plants. Preparation of Chloroplasts. Sodium chloride chloroplasts were isolated fresh for each experiment in 0.35 M NaCl, 0.05 M Tricine-NaOH2 buffer, pH 7.8, as previously described (19). Chloroplasts were finally suspended in 0.035 M NaCl, 0.005 M Tricine, pH 7.8. Sucrose chloroplasts were isolated and suspended in a similar manner substituting 0.4 M sucrose for NaCl in the media. Total chlorophyll was determined by the method of Arnon (1). All isolations and extracts were made in a cold room (about 4 C) and stored in an ice bucket through each experiment.Preparation of Crude Leaf Extracts. Twenty grams of freshly harvested leaves were chopped with a razor and then vigorously handground for 1 to 5 min in a cold (4 C) mortar and pestle with 1 g of acid-washed sea sand, plus 45 ml of 0.35 M NaCl, 0

Spectral, Physical, and Electron Transport Activities in the Photosynthetic Apparatus of Mesophyll Cells and Bundle Sheath Cells of Digitaria sanguinalis (L.) Scop.

“…It was previously found that C4 cycle plants including crabgrass had a higher chl a-b ratio than pentose plants (7). The extension of these measurements to the two cell types of crabgrass shows that the difference between C4 and pentose plants, in chl a-b ratio, is due primarily to the increased chl a-b ratio of the bundle sheath cells (Table I).…”

“…The P.,, absorbance change was measured in the same manner as reported previously with an Aminco-Chance dual beam spectrophotometer (7). Chl determinations were made by the method of Arnon (1) or Wintermans and DeMots (34).…”

The Azolla, Anabaena azollae Relationship

“…Correspondingly, it has been reported that Azolla freed of the symbiotic alga requires a source of combined N2 (26 (14) except that heterocysts were broken by two or more passes through the pressure cell at 18,000 psi. P700 was measured by photochemical oxidation (6) using an absorbance coefficient of 64 mmole1…”

Early Responses of Sodium-Deficient Amaranthus tricolor L. Plants to Sodium Application

“…In general, the C4 plants have a higher Chl a/b ratio and P700/Chl total ratio than C3 plants suggesting that the PSI activity is higher in the C4 plants (5). This characteristic confers a higher capacity of ATP supply required for the regeneration of phosphoenolpyruvate from pyruvate in the mesophyll chloroplasts of C4 plants (14).…”