Dependence of Photosynthesis of Sunflower and Maize Leaves on Phosphate Supply, Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Activity, and Ribulose-1,5-Bisphosphate Pool Size

Abstract

Sunflower (Helianthus annuus L. cv Asmer) and maize (Zea mays L. cv Eta) plants were grown under controlled environmental conditions with a nutrient solution containing 0, 0.5, or 10 millimolar inorganic phosphate. Phosphate-deficient leaves had lower photosynthetic rates at ambient and saturating CO2 and much smaller carboxylation efficiencies than those of plants grown with ample phosphate. In addition, phosphate-deficient leaves contained smaller quantities of total soluble proteins and nbulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) per unit area, although the relative proportions of these components remained unchanged. The specific activity of Rubisco (estimated in the crude extracts of leaves) was significantly reduced by phosphate deficiency in sunflower but not in maize. Thus, there was a strong dependence of carboxylation efficiency and C02-saturated photosynthetic rate on Rubisco activity only in sunflower. Phosphate deficiency decreased the 3-phosphoglycerate and ribulose-1,5-bisphosphate (RuBP) contents of the leaf in both species. The ratio of 3-phosphoglycerate to RuBP decreased in sunflower but increased in maize with phosphate deficiency. The calculated concentrations of RuBP and RuBP-binding sites in the chloroplast stroma decreased markedly with phosphate deficiency. The ratio of the stromal concentration of RuBP to that of RuBP-binding sites decreased in sunflower but was not affected in maize with phosphate deficiency. We suggest that a decrease in this ratio made the RuBP-binding sites more vulnerable to blockage or inactivation by tight-binding metabolites/inhibitors, causing a decrease in the initial specific activity of Rubisco in the crude extract from phosphate-deficient sunflower leaves. However, the decrease in Rubisco specific activity was much less than the decrease in the RuBP content in the leaf and its concentration in the stroma. A large ratio of RuBP to RuBP-binding sites may have maintained the Rubisco-specific activity in phosphatedeficient maize leaves. We conclude that the effect of phosphate deficiency is more on RuBP regeneration than on Rubisco activity in both sunflower and maize.plants was due to inhibition of nonstomatal processes rather than to stomatal factors despite a decrease in stomatal conductance. The carboxylation efficiency and the apparent quantum yield for CO2 assimilation were two parameters strongly affected by phosphate deficiency, leading to an increase in the relative mesophyll limitation of photosynthetic rate. Similar effects on carboxylation efficiency and quantum yield in phosphate-deficient leaves of spinach, soybean, and sugar beet have been observed in other studies (4,11,19).Under conditions of a nonlimiting supply of RuBP, the amount and specific activity of Rubisco determine carboxylation efficiency (24), defined as the initial slope of the curve relating A to C1, i.e. dA/dCj when A = 0 (10). Phosphatedeficient leaves produce smaller amounts of RuBP and Rubisco (4,19), and the Rubisco has a lower specific activity (

A Shoot-Specific Hypoxic Response of Arabidopsis Sheds Light on the Role of the Phosphate-Responsive Transcription Factor PHOSPHATE STARVATION RESPONSE1

“…Possibly, the ratio of 3-PGA/Pi could serve as a signal, as it increased under the two stress conditions. Levels of 3-PGA were significantly increased under hypoxia, while they tended to decrease under phosphate deficiency, a trend observed previously (low Pi, Fredeen et al, 1990;Jacob and Lawlor, 1992;Rao and Terry, 1995;and hypoxia, Biemelt et al, 1999;Shingaki-Wells et al, 2011;Ampofo-Asiama et al, 2014).…”

Soil Temperature and Phosphorus Supply Interactively Affect Physiological Responses of White Birch to CO<sub>2</sub> Elevation

“…P deficiency can limit photosynthesis either indirectly by reducing the total leaf area of a plant [14], or more directly by reducing Rubisco activity and RuBP regeneration [15][16][17][18]. Studies using isolated chloroplasts and other in-vitro systems show that P is involved in the activation of Rubisco [19], the modulation of ribulose-5-phosphate kinase and fructose-l,6-bisphosphate phosphatase [20], the transport of triosephosphate (TP) across the chloroplast membrane by the Pi-translocator and the regulation of photophosphorylation [21].…”

Suppression of Photosynthetic Gene Expression in Roots Is Required for Sustained Root Growth under Phosphate Deficiency

“…Inhibition of photosynthesis has long been observed for Pi-deficient (P-) plants (Dietz and Foyer, 1986;Sivak, 1986;Jacob and Lawlor, 1992;Rao and Terry, 1995). Severe Pi deficiency damages the energy-transducing system in the thylakoids and inhibits the activities of several key enzymes of the Calvin cycle (Preiss, 1984;Natr, 1992).…”