Photosynthesis by Flag Leaves of Wheat in Relation to Protein, RibuloseBisphosphate Carboxylase Activity and Nitrogen Supply

Abstract

The effects of nitrate supply on the composition (cell numbers, protein and chlorophyll contents) of flag leaves of winter wheat grown with two amounts of N fertilizer and of spring wheat grown in the glasshouse under controlled nitrate supply are described and related to photosynthesis. Nitrogen deficiency decreased the size of leaves, mainly by reducing cell number and, to a smaller extent, by decreasing cell volume. Protein content per unit leaf area, per cell and per unit cell volume was larger with abundant N. Total soluble protein, ribulose tophosphate carboxylase-oxygenase (RuBPc-o) protein and chlorophyll changed in proportion irrespective of nitrogen supply and leaf age. Photosynthesis per unit area of flag leaf and carboxylation efficiency in both winter and spring wheat were proportional to the amount of total soluble protein up to 7-0 g m~2 and to the amount of RuBPc-o protein up to 40 g m~2. However, photosynthesis did not increase in proportion to the amount of total soluble or RuBPc-o protein above these amounts. In young leaves with a high protein content the measured rates of photosynthesis were lower than expected from the amount and activity of RuBPc-o. Carboxylation per unit of RuBPc-o protein, measured in vitro, was slightly greater in N-deficient leaves of winter wheat but not of spring wheat. RuBPc-o activity per unit of RuBPc-o protein was similar in winter and spring wheat leaves and remained approximately constant with age, but increased in leaves showing advanced senescence. RuBPc-o protein from N-deficient leaves migrated faster on polyacrylamide gels than protein from leaves with high N content. Regulation of the rate of photosynthesis in leaves and chloroplasts with a high protein content is discussed. The conductance of the cell to the flux of CO 2 from intercellular spaces to RuBPc-o active sites is calculated, from cell surface areas and CO 2 fluxes, to decrease the CO 2 partial pressure at the active site by less than 0-8 Pa at an internal CO 2 partial pressure of 34 Pa. Thus the decrease in partial pressure of CO 2 is insufficient to account for the inefficiency of RuBPc-o in vivo at high protein contents. Other limitations to the rate of photosynthesis are considered.

Crescimento de folhas do capim-bermuda tifton 85 submetido à adubação nitrogenada após o corte

“…Andrews et al (1991) verificaram, quando as doses de nitrato variaram de 0,5 a 5,0 mol m -3 , valores de 85 e 86 mm para comprimento final da folha e de 124 e 182 mm 2 para área final da folha em festuca, enquanto, para azevém-perene, os valores respectivos foram de 91 e 90 mm e 132 e 176 mm 2 . Lawlor et al (1989) reportaram, para a área foliar de trigo (Triticum aestivum), variação de 14,8 para 33,3 cm 2 e de 3,1 para 35, 3 cm 2 , quando as doses de nitrogênio foram de 0 e 200 kg ha -1 e de 0,1 e 20 mol m -3 NO -3 , respectivamente. Herrera et al (1991) constataram, para o Coastcross 1 bermudagrass, variação de 4,49 para 6,76 e para 9,25 cm 2 em doses de nitrogênio de 0, 100 e 200 kg ha -1 na estação chuvosa.…”

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

“…A, by intact leaves, was determined as described by Lawlor et al (12) using an open gas exchange system at saturating light intensity (1200 ,.umol quanta m-2 s-'). The response of A to Ci was measured and an asymptotic function fitted because it gave high R2 values (7).…”

C4 Plants Adaptation to High Levels of CO2 and to Drought Environments

“…Even though leaf nitrogen is invested into photosynthetic components into the same fraction in both C 3 and C 4 species, C 4 plants allocate less nitrogen to Rubisco protein and more to other soluble protein and thylakoids components. In C 3 plants, the photosynthetic enzyme Rubisco accounts for up to 30% of the leaf nitrogen content (Lawlor et al, 1989), but accounts for only 4-21% of leaf nitrogen in C 4 species (Evans & von Caemmerer, 2000;Sage et al, 1987). The lower nitrogen requirement of C 4 plants results from their CO 2 -concentrating mechanism, which raises the bundle sheath CO 2 concentration, saturating Rubisco in normal air and almost eliminating photorespiration.…”