Stromal Phosphate Concentration Is Low during Feedback Limited Photosynthesis

Abstract

It has been hypothesized that photosynthesis can be feedback limited when the phosphate concentration cannot be both low enough to allow starch and sucrose synthesis at the required rate and high enough for ATP synthesis at the required rate. We have measured the concentration of phosphate in the stroma and cytosol of leaves held under feedback conditions. We used nonaqueous fractionation techniques with freeze-clamped leaves of Phaseolus vulgaris plants grown on reduced phosphate nutrition.Feedback was induced by holding leaves in low 02 or high CO2 partial pressure. We found 7 millimolar phosphate in the stroma of leaves in normal oxygen but just 2.7 millimolar phosphate in leaves held in low oxygen. Because 1 to 2 millimolar phosphate in the stroma may be metabolically inactive, we estimate that in low oxygen, the metabolically active pool of phosphate is between negligible and 1.7 millimolar. We conclude that halfway between these extremes, 0.85 millimolar is a good estimate of the phosphate concentration in the stroma of feedback-limited leaves and that the true concentration could be even lower. The stromal phosphate concentration was also low when leaves were held in high C02, which also induces feedback-limited photosynthesis, indicating that the effect is related to feedback limitation, not to low oxygen per se. We conclude that the concentration of phosphate in the stroma is usually in excess and that it is sequestered to regulate photosynthesis, especially starch synthesis. The capacity for this regulation is limited by the coupling factor requirement for phosphate.During photosynthesis, phosphate is required by the coupling factor for the production of ATP from ADP. At the same time, phosphate inhibits starch (15) and so the phosphate level need not fall to such low levels to limit photosynthesis.The concentration of phosphate in the stroma has been measured in the past (3, 21, 36), but it has never been measured in feedback-limited leaves, that is leaves which exhibit 02-insensitive photosynthesis. We decided to measure the concentration of phosphate in the stroma and cytosol of leaves exhibiting feedback-limited photosynthesis. This condition can be induced by feeding the phosphate sequestering agent mannose (9) and can also occur under natural conditions ( 13,20).The measurement of stromal phosphate concentration is difficult because plants grown on luxuriant levels of phosphate, as is common practice in research, usually have a large amount of metabolically inactive phosphate in the vacuole (4, 39). This problem can be overcome by growing plants with more realistic phosphate nutrition. When the phosphate supply is restricted, the phosphate concentration in the vacuole can be substantially reduced with little or no effect on photosynthesis (5, 22

Differential Accumulation of Dimethylallyl Diphosphate in Leaves and Needles of Isoprene- and Methylbutenol-Emitting and Nonemitting Species

“…Chl, the chloroplast marker, occurred in the upper, lightest, fractions; the greatest percentage of vacuolar marker (␣-mannosidase) occurred in the lower, heaviest, fractions; and the cytosolic marker (phosphoenolpyruvate carboxylase) generally partitioned toward the middle of the gradient. This relative distribution of compartment markers is typical for plant leaves after centrifugation in nonaqueous gradients (Gerhardt and Heldt, 1984;Sharkey and Vanderveer, 1989;Stitt et al, 1989;Moore et al, 1995). In comparison with the distribution of markers, DMAPP was predominately associated with Chl.…”

“…The nonaqueous fractionation procedures were similar to those reported elsewhere (Gerhardt and Heldt, 1984;Sharkey and Vanderveer, 1989). Sunexposed cottonwood (Populus deltoides) leaves were collected directly into liquid N 2 at midday.…”

“…3.2.1.24) activity, assayed in duplicate for 60 min at 37°C as described by Sharkey and Vanderveer (1989). Total protein was determined by the method of Bradford (1976).…”

Acclimation of metabolism to light in Arabidopsis thaliana: the glucose 6‐phosphate/phosphate translocator GPT2 directs metabolic acclimation

“…Accumulation of phosphorylated intermediates following the increase in CO 2 fixation will lower the levels of free Pi in both the cytosol and the chloroplasts. Low stromal levels of Pi lead to an inhibition of photosynthesis (Sharkey & Vanderveer 1989; Rao & Terry 1995). Export of G6P would supplement TP export, increasing cytosolic substrates for sucrose synthesis.…”

Effects of nitrogen and phosphorus supply on growth and physiological traits of two Larix species

“…In addition, a previous study indicated that excessive NSC accumulation in leaves could cause a feedback inhibition of photosynthetic abilities on account of both decreasing sucrose synthesis and orthophosphate depletion in the cytosol (Araya et al, 2006). The orthophosphate depletion could suppress adenosine triphosphate synthesis and the reduction of phosphoglyceric acid to triose phosphate, and thereby suppress the photosynthesis rate (Sharkey and Vanderveer, 1989). Therefore, an excessive accumulation of starch, fructose and NSC in L. kaempferi leaves under P fertilization probably had a feedback inhibition effect on its photosynthesis rate .…”