Changes in Levels of Intermediates of the C₄ Cycle and Reductive Pentose Phosphate Pathway under Various Light Intensities in Maize Leaves

Abstract

The rate of CO2 assimilation and levels of metabolites of the C4 cycle and reductive pentose phosphate pathway in attached leaves of maize (Zea mays L.) were (17,18,25). However, recent reports revealed that there were up to 30-fold differences in PEP carboxylase activity during dark to light transitions in maize when assayed in simulated conditions of dark or light or in the presence of effectors (5, 13). Studies of metabolite levels during induction in maize leaves (9, 20, 28) provided a basis for in vitro simulation ofconditions in vivo. The total level of the C4 cycle intermediates (malate, aspartate, pyruvate, PEP, and alanine) increased during induction (28) and also with increasing CO2 concentration (31,32) as the photosynthesis rate increased. These findings suggested that input of carbon into the C4 cycle and an increase in the total level of the C4 cycle intermediates is required to increase the photosynthetic rate in maize leaves (9,28,31,32).Although the influence of light intensity on metabolite levels in C4 plants has not been reported, this is an important environmental factor controlling C4 photosynthesis. Thus, the purpose of this study was to measure changes in photosynthetic metabolite levels in maize leaves under steady-state photosynthesis with varying light intensities.Recently, studies have been conducted on changes in photosynthetic metabolite levels during the induction period (9,20,28), and under steady-state photosynthesis with normal atmospheric conditions (19, 26) and with varying CO2 concentration (31) in maize leaves. These studies have increased our understanding of C4 photosynthesis and they provide a new approach to research in C4 plant. In part they have shown that there are sufficient concentration gradients between the mesophyll and bundle sheath to facilitate intercellular transport of certain C4 cycle intermediates by diffusion during C4 photosynthesis (19,

Alanine Synthesis by Bundle Sheath Cells of Maize

“…be expected to take place in the BS cells, because 3-PGA is formed there by photosynthesis. Moreover, previous studies on photosynthetic induction in leaves of Zea mays (8,24) indicated that in the BS cells the glycolytic pathway from 3-PGA to pyruvate is operating and glutamate is found in BS cells at high concentrations (19). In the present report we investigated the capacity of BS cells to produce alanine from 3-PGA.…”

Diurnal Changes in Photosynthesis in Sugarcane Leaves: I. Carbon dioxide exchange rate, photosynthetic enzyme activities and metabolite levels relating to the C4pathway and the Calvin cycle

“…Sharkey and Raschke ( 1981) found that intercellular C0 2 pressure in ,Zea mays leaves was approximately the same under low PAR (50 W m-2 ) and under high PAR (620 W m-2 ). Mainly based on the above results, it has been suggested that g 8 does not limit ,Zea mays photosynthesis under atmospheric conditions (Usuda, 1987). Although the slower response of g 8 to changing PAR observed in this study, implies that g 8 may limit CER in the morning and afternoon, i.e., under a low PAR, this response needs to be studied futher because it is the inconsistent with the results from ,Zea mays (Sharkey and Raschke 1981;Usuda, 1987).…”

“…Therefore, the results in this study suggest that PPDK does not limit sugarcane photosynthesis under a low PAR, i.e., before CER was saturated in the morning and when CER was decreasing in the afternoon. In this study, the maximum PEP level in sugarcane leaves was 55 nmol mg-1 Chl, similar to that detected in ,Zea mays (50 to 150 nmol mg-1 Chi) leaves under high light and atmospheric C0 2 conditions (Usuda, 1985a ;Usuda, 1987;Leegood and von Caemmerer, 1989). This PEP concentration in plant leaves is considered to be kinetically insufficient for PEPcase (Leegood and von Caemmerer, 1989).…”

“…Mainly based on the above results, it has been suggested that g 8 does not limit ,Zea mays photosynthesis under atmospheric conditions (Usuda, 1987). Although the slower response of g 8 to changing PAR observed in this study, implies that g 8 may limit CER in the morning and afternoon, i.e., under a low PAR, this response needs to be studied futher because it is the inconsistent with the results from ,Zea mays (Sharkey and Raschke 1981;Usuda, 1987). Usuda (1987) reported that PPDK activity was able to limit the assimilation rate in ,Zea mays leaves under a low PAR, but Leegood and von Caemmerer (1989) suggested that PEPcase activity was the limiting factor on the assimilation rate in ,Zea mays and Amaranthus edulis leaves under a low PAR.…”

Elements Required for an Efficient NADP-Malic Enzyme Type C4 Photosynthesis

“…A low rate of starch breakdown can replenish fructose-6-phosphate in BSCs, and this has enabled the model to correctly predict the increasing response of the RuBP pool with decreasing [CO 2 ] (Fig. 3) as observed in experiments (Usuda, 1987;Leegood and Kinetic parameters for C3 and C4 homologs of the enzymes involved in C4 photosynthesis were used as inputs to the model to predict CO 2 uptake rates under low and high CO 2 conditions. The CO 2 uptake ratio is the ratio of CO 2 uptake rates predicted using kinetic parameters from a C3 homolog to that using a C4 homolog.…”