Limiting Factors in Photosynthesis

Abstract

Earlier work (SE Taylor, N Terry [1984] 86) has shown that the rate of photosynthesis may be colimited by photosynthetic electron transport capacity, even at low intercellular CO2 concentrations. Here we monitored leaf metabolites diumally and the activities of key Calvin cycle enzymes in the leaves of three treatment groups of sugar beet (Beta vulgaris L.) plants representing three different in vivo photochemical capacities, i.e. Fe-sufficient (control) plants, moderately Fe-deficient, and severely Fe-deficient plants. The results show that the decrease in photosynthesis with Fe deficiency mediated reduction in photochemical capacity was through a reduction in ribulose 1,5-bisphosphate (RuBP) regeneration and not through a decrease in ribulose 1,5-bisphosphate carboxylase/oxygenase activity. Based on measurements of ATP and NADPH and triose phosphate/3-phosphoglycerate ratios in leaves, there was littie evidence that photosynthesis and RuBP regeneration in Fe-deficient leaves were limited directly by the supply of ATP and NADPH. It appeared more likely that photochemical capacity influenced RuBP regeneration through modulation of enzymes in the photosynthetic carbon reduction cycle between fructose-6-phosphate and RuBP; in particular, the initial activity of ribulose-5-phosphate kinase was strongly diminished by Fe deficiency. Starch and sucrose levels changed independently of one another to some extent during the diumal period (both increasing in the day and decreasing at night) but the average rates of starch or sucrose accumulation over the light period were each proportional to photochemical capacity and photosynthetic rate.their study also showed that rubisco activation reached a maximum at PFD well below those needed to saturate photosynthesis. This suggests that some factor other than rubisco modulation (e.g. RuBP regeneration) may have been responsible for controlling photosynthetic rates at higher PFD levels. In the present work, we explored the roles of rubisco modulation and RuBP regeneration in the limitation of photosynthesis under conditions of Fe deficiency mediated reduction in photochemical capacity.In addition, we were interested in determining whether Fe deficiency mediated reduction in photochemical capacity impaired photosynthesis by decreasing the pool sizes of the photochemical products, ATP and NADPH. By measuring the levels of other metabolites such as PGA, triose-P, FBP, and F6P, we gained further information on the effects of photochemical capacity on the conversion of RuBP to PGA, PGA to triose-P, and F6P to RuBP. By measuring the initial and total activities of key enzymes of the PCR cycle, we explored the extent to which photochemical capacity might affect RuBP regeneration through enzyme modulation. Because fluxes through the various biochemical pathways may change with time, the levels of leaf metabolites and nucleotides, as well as the carbon storage compounds, starch, sucrose and glucose, were monitored continuously during the 16 h light/8 h dark cycle. MATERIALS AND M...

Enhanced plant tolerance to iron starvation by functional substitution of chloroplast ferredoxin with a bacterial flavodoxin

“…Cleared lysates corresponding to 8 mg of fresh weight were subjected to SDS/PAGE and blotted onto nylon membranes for immunodetection of Fd, Fld, light-harvesting chlorophyllbinding protein (LHCP), and Rubisco large (LSU) and small (SSU) subunits. agreement with previous observations (10)(11)(12)(13). The presence of Fld in chloroplasts, but not in the cytosol, almost entirely prevented this decline (Fig.…”

“…Total RNA was extracted from 1 g of plant material with the TRIzol reagent (Invitrogen, Paisley, U.K.). Synthesis of cDNA was carried out on 1 g of RNA by using the SuperScript III RT kit and oligo(dT) [12][13][14][15][16][17][18] primers of Invitrogen, as indicated in the instruction manual. Semiquantitative RT-PCR analysis was performed as described (18), with the optimal number of cycles determined for each gene to analyze expression in the exponential phase.…”

“…Trx reduction is in turn linked to Fd via the Fd-Trx reductase FTR (27). Not unexpectedly, therefore, some key components of the cycle are reported to be especially vulnerable to Fe limitation (12,13). The ability of Fld to sustain Trx reduction in vitro and in planta has been documented (26).…”

“…Chloroplasts are therefore primary targets of Fe deficiency, resulting in a decrease of photosynthetic activity and of plastidic pigments and proteins (11,12). Both the utilization of ribulose-1,5-bisP by Rubisco and its regeneration by the Calvin cycle appear compromised in Fe-starved plants (12,13).…”

Sulfur starvation reduces phytosiderophores release by iron-deficient barley plants

“…This result differs from that reported by Thoiron and Briat (1999), who showed that leaf sucrose content remained unchanged under Fe deficiency. However, other authors (Arulanantham et al 1990) recorded a decrease in sucrose accumulation in leaves of sugar beet plants under moderate or severe Fe deficiency.…”