Bundle Sheath Diffusive Resistance to CO2 and Effectiveness of C4 Photosynthesis and Refixation of Photorespired CO2 in a C4 Cycle Mutant and Wild-Type Amaranthus edulis  

Abstract

A mutant of the NAD-malic enzyme-type C(4) plant, Amaranthus edulis, which lacks phosphoenolpyruvate carboxylase (PEPC) in the mesophyll cells was studied. Analysis of CO(2) response curves of photosynthesis of the mutant, which has normal Kranz anatomy but lacks a functional C(4) cycle, provided a direct means of determining the liquid phase-diffusive resistance of atmospheric CO(2) to sites of ribulose 1,5-bisphosphate carboxylation inside bundle sheath (BS) chloroplasts (r(bs)) within intact plants. Comparisons were made with excised shoots of wild-type plants fed 3,3-dichloro-2-(dihydroxyphosphinoyl-methyl)-propenoate, an inhibitor of PEPC. Values of r(bs) in A. edulis were 70 to 180 m(2) s(-1) mol(-1), increasing as the leaf matured. This is about 70-fold higher than the liquid phase resistance for diffusion of CO(2) to Rubisco in mesophyll cells of C(3) plants. The values of r(bs) in A. edulis are sufficient for C(4) photosynthesis to elevate CO(2) in BS cells and to minimize photorespiration. The calculated CO(2) concentration in BS cells, which is dependent on input of r(bs), was about 2,000 microbar under maximum rates of CO(2) fixation, which is about six times the ambient level of CO(2). High re-assimilation of photorespired CO(2) was demonstrated in both mutant and wild-type plants at limiting CO(2) concentrations, which can be explained by high r(bs). Increasing O(2) from near zero up to ambient levels under low CO(2), resulted in an increase in the gross rate of O(2) evolution measured by chlorophyll fluorescence analysis in the PEPC mutant; this increase was simulated from a Rubisco kinetic model, which indicates effective refixation of photorespired CO(2) in BS cells.

Evidence for a Role for NAD(P)H Dehydrogenase in Concentration of CO2 in the Bundle Sheath Cell of Zea mays

“…This upper limit for g bsc is less than oneseventh of the magnitude of g m in the MC (Table II). A study with A. edulis mutationally or chemically blocked in C 4 cycle function likewise showed conditionally higher rates of O 2 -dependent PSII linear electron transport (photorespiration) compared to controls and a similarly low conductance to CO 2 diffusion from the MC to sites of CO 2 fixation by Rubisco in the BSC (Kiirats et al, 2002). Furthermore, in this study J o at G is ,0.08 of the CO 2 -saturated J T (Table I).…”

“…A corresponding estimate of J o /J T at G in tobacco was at least 0.5 (Peterson 1989). Thus, photorespiration is indeed suppressed in C 4 relative to C 3 leaves due, in part, to the critical function of the BSC wall in restricting overcycling of CO 2 to the MC (Kiirats et al, 2002).…”

“…Likewise, a close correspondence between CO 2 assimilation and oxygenic PSII turnover was observed over a wide range of irradiances and CO 2 levels in the C 4 NAD-ME species Amaranthus edulis (Kiirats et al, 2002). An unexpected result in this study was the lower mean NPQ (Table II) for the mutants, illustrating that a NDH-dependent H + -translocating cyclic pathway could contribute to DpH formation in the MC chloroplast necessary for photoprotective quenching.…”

Feedback limitation of photosynthesis at high CO2 acts by modulating the activity of the chloroplast ATP synthase

“…Leaf gas exchange characteristics, net rates of CO 2 fixation (A), C i , PPFD and leaf temperature were determined as by Laisk and Loreto (1996). Rubisco oxygenation rate (v o ), Rubisco carboxylation rate (v c ), and PS II electron transport (J O 2 ), were calculated from simultaneous measurements of leaf CO 2 exchange and fluorescence yield as in (Kiirats et al 2002).…”

Photosynthesis and Quantum Yield of Oil Palm Seedlings to Elevated Carbon Dioxide

“…Many studies have suggested that the enriched CO 2 leaves respond to atmospheric CO 2 enrichment to a greater extent than the ambient-CO 2 leaves as a result of increased quantum yields (Hanstein & Felle, 2002). A small increase in quantum yield may increase daily carbon gain under low light conditions (Kiirats et al, 2002). In our study, elevated CO 2 increased apparent quantum yields in the 800 and 1200 µmol mol -1 CO 2 treatments.…”