Comparative effects of growth irradiance on photosynthesis and leaf anatomy of Flaveria brownii (C4-like), Flaveria linearis (C 3-C 4) and their F 1 hybrid

Functional analysis of corn husk photosynthesis

The husk surrounding the ear of corn/maize (Zea mays) has widely spaced veins with a number of interveinal mesophyll (M) cells and has been described as operating a partial C(3) photosynthetic pathway, in contrast to its leaves, which use the C(4) photosynthetic pathway. Here, we characterized photosynthesis in maize husk and leaf by measuring combined gas exchange and carbon isotope discrimination, the oxygen dependence of the CO(2) compensation point, and photosynthetic enzyme activity and localization together with anatomy. The CO(2) assimilation rate in the husk was less than that in the leaves and did not saturate at high CO(2), indicating CO(2) diffusion limitations. However, maximal photosynthetic rates were similar between the leaf and husk when expressed on a chlorophyll basis. The CO(2) compensation points of the husk were high compared with the leaf but did not vary with oxygen concentration. This and the low carbon isotope discrimination measured concurrently with gas exchange in the husk and leaf suggested C(4)-like photosynthesis in the husk. However, both Rubisco activity and the ratio of phosphoenolpyruvate carboxylase to Rubisco activity were reduced in the husk. Immunolocalization studies showed that phosphoenolpyruvate carboxylase is specifically localized in the layer of M cells surrounding the bundle sheath cells, while Rubisco and glycine decarboxylase were enriched in bundle sheath cells but also present in M cells. We conclude that maize husk operates C(4) photosynthesis dispersed around the widely spaced veins (analogous to leaves) in a diffusion-limited manner due to low M surface area exposed to intercellular air space, with the functional role of Rubisco and glycine decarboxylase in distant M yet to be explained.

Growth of the C4 dicot Flaveria bidentis: photosynthetic acclimation to low light through shifts in leaf anatomy and biochemistry

In C(4) plants, acclimation to growth at low irradiance by means of anatomical and biochemical changes to leaf tissue is considered to be limited by the need for a close interaction and coordination between bundle sheath and mesophyll cells. Here differences in relative growth rate (RGR), gas exchange, carbon isotope discrimination, photosynthetic enzyme activity, and leaf anatomy in the C(4) dicot Flaveria bidentis grown at a low (LI; 150 micromol quanta m(2) s(-1)) and medium (MI; 500 micromol quanta m(2) s(-1)) irradiance and with a 12 h photoperiod over 36 d were examined. RGRs measured using a 3D non-destructive imaging technique were consistently higher in MI plants. Rates of CO(2) assimilation per leaf area measured at 1500 micromol quanta m(2) s(-1) were higher for MI than LI plants but did not differ on a mass basis. LI plants had lower Rubisco and phosphoenolpyruvate carboxylase activities and chlorophyll content on a leaf area basis. Bundle sheath leakiness of CO(2) (phi) calculated from real-time carbon isotope discrimination was similar for MI and LI plants at high irradiance. phi increased at lower irradiances, but more so in MI plants, reflecting acclimation to low growth irradiance. Leaf thickness and vein density were greater in MI plants, and mesophyll surface area exposed to intercellular airspace (S(m)) and bundle sheath surface area per unit leaf area (S(b)) measured from leaf cross-sections were also both significantly greater in MI compared with LI leaves. Both mesophyll and bundle sheath conductance to CO(2) diffusion were greater in MI compared with LI plants. Despite being a C(4) species, F. bidentis is very plastic with respect to growth irradiance.

Effects of Light and Sucrose Levels on the Anatomy, Ultrastructure, and Photosynthesis of Gardenia jasminoides Ellis Leaflets Cultured in vitro

This article reports the effect of growing conditions on the anatomical and ultrastructural changes associated with the development of photoautotrophy in Gardenia jasminoides Ellis plantlets during shoot multiplication in vitro. Two photosynthetic photon flux densities (PPFD) (50 and 100 µmol m-2 s-1 PPFD: L50 and L100, respectively) and two sucrose concentrations in the culture medium (5 and 30 g L-1: S5 and S30, respectively) were assayed. An increase in PPFD stimulated the development of photosynthetic tissues and led to higher photosynthesis and dark respiration regardless of the sucrose level assayed. However, the effect of sucrose in the medium depended on the PPFD. For the high-PPFD treatment, a low sucrose concentration in the medium stimulated the development of photosynthetic tissues, whereas the opposite effect of sucrose was observed at low PPFD. This study demonstrated that an increase in light intensity to moderate values such as L100 has a beneficial effect on the development of structural changes associated with photoautotrophy. Such an effect is stimulated by low sucrose (S5) levels in the medium. These modifications of usual growing conditions (such as L50 combined with S30) for micropropagation may prove to be useful in mass propagation of gardenia. However, the use of a low sucrose level in addition to conventional growing PPFD may be counterproductive.

Intraspecific variation for CO2 compensation point and differential growth among variants in a C3-C4 intermediate plant

CO₂ compenstation point (Γ), the concentration of CO₂ at which photosynthesis and respiration are at equilibrium, is a commonly used diagnostic for the C₄ photosynthetic pathway, since it reflects the reduced photorespiration that is a property of C₄ photosynthesis. Geographic variation for Γ was examined within Flaveria linearis, a C₃-C₄ intermediate species. Collections from four widely separated Floridian populations were propagated in a greenhouse and measured for Γ. Little differentiation among populations was found, but significant within-population variation was present. Temperature is a hypothesized selective agent for the C₄ photosynthetic pathway. To test this hypothesis, plants exhibiting a range of Γ were cloned and placed in growth chambers at 25°C and 40°C. After 7 weeks, Γ valves were remeasured and plants were harvested and weighed. There was a poor correlation between initial and final measures of Γ for a given genotype (r=0.38, P>0.1). Broad sense heritability for Γ was computed to be 0.10. At 25°C, there was no relationship between final size and Γ. At 40°C, more C₄-like plants, as indicated by their low Γ, had grown larger. Differences in relative growth rate were attributable more to differences in net assimilation rate than in leaf area ratio. Taken together, these results demonstrate that although significant plasticity exists in the amount of photorespiration in this C₃-C₄ species, high temperature appears to be an effective selective agent for the reduction of photorespiration and the enhancement of C₄-like traits.