Temperature Dependence of Photosynthesis in Agropyron smithii Rydb.

Abstract

ABSTRACTof photosynthesis observed at above-optimum temperatures (3).Temperature has also been shown to affect both the Km (CO2) and V,= of RuBP carboxylase in vitro (2,24). Weis (35) has suggested that a temperature-dependent inactivation of RuBP carboxylase occurs at above-optimal temperatures in intact spinach chloroplasts. Such temperature effects may be partly responsible for the reversible temperature limitations on net photosynthesis observed for intact leaves. However, difficulties arise in interpreting these data due to the uncertainties encountered in using in vitro studies to describe processes in vivo.Photosynthetic inhibition at analysis temperatures beyond the point of reversibility is due in part to irreversible inhibition of the quantum yield for CO2 fixation, and decreased activity of certain enzyme reactions (4). An irreversible reduction in the quantum yield has been used as an indication of heat damage to the photosynthetic apparatus (29), and is presumably related to the integrity of the thylakoid membranes (1).Agropyron smithii is a C3 perennial grass which constitutes a major biomass producer of the short-and mixed-grass prairie ecosystems. Plants of this species initiate growth during the early spring months when seasonal temperatures are relatively cool (6). (P2%0-P21%0,/P2%0,) 100(1)Leaf transmittance and reflectance measurements were conducted on freshly cut leaf discs, at 5-nm wavelength intervals between 400 and 700 nm, with the integrating sphere described by Robberecht and Caldwell (28 min prior to initiating each assay, a 2-to 3-ml aliquot of the frozen extract was removed from the mortar and allowed to thaw at room temperature. The thawed extract was pressed through a 40-,um nylon net attached to a 20-ml syringe and irnmediately assayed.The assays were conducted in small glass vials which were half immersed in a temperature-controlled water bath. The assay buffer (100 mM C02-free Tricine, pH 8.0), which contained 5 mm DTT and 20 mm MgCl2, was added to each reaction vial approximately 2 min prior to each assay in order to insure temperature equilibration. N2 was bubbled through the assay medium for the entire 2-min equilibration period. Carbonic anhydrase (100 units) and RuBP (0.4 mm) were added to the assay medium at 60 and 45 s prior to initiating each assay, respectively. The appropriate concentration of NaH4C03-(7.0 mCi mm-) was added 5 s before initiating the reaction, thus minimizing losses of 14CO2 to the atmosphere above the reaction medium. The reaction was initiated by the addition of 20 IAl of crude extract. The final volume of the reaction mixture was 1 ml. The reaction was terminated after 30 s (1 min at 10 and 15°C) by the addition of 200 ,ul of 6 N HCI. The contents of each reaction vial were transferred to a scintillation

Temperature Dependence of Photosynthesis in Agropyron smithii Rydb.

“…The conclusions ofthis study are correlated with the conclusions in the accompanying study (12).of the temperature response of whole leaf photosynthesis in A. smithii. In the accompanying study, reversible decreases in the quantum yield of photosynthesis were found to occur between 35 and 42°C.…”

“…In Figure 4, the rate of gram- The temperature range that produces this apparent uncoupling in A. smithii thylakoids (35-40°C) is identical to the temperature range that has been reported to induce uncoupling in spinach thylakoids (13,21). This temperature range is also the range in which C02-saturated photosynthesis in whole leaves of S. smithii becomes limited by temperature (12). Inasmuch as the enzymes of the reductive pentose phosphate pathway have been found to be stable at these temperatures (4), high temperature effects on photophosphorylation may limit photosynthesis in vivo by limiting the regeneration of the substrate for RuBP carboxylase.…”

“…This makes it unlikely that a decrease in alkalinization of the stromal pH is the cause of decreased activation of RuBP carboxylase or another enzymes in the reductive pentose phosphate pathway in this temperature region. The large drop in proton uptake between 35 and 40°C (-I 10o/degree) could, however, prevent alkalinization of the stroma and decrease stromal Mg2e levels which could inhibit the carbon reduction cycle at either of the bisphosphatases or RuBP carboxylase steps, causing the decreased photosynthetic capacity observed in the previous study (12).…”

“…In a previous study (12) A number of investigations on the temperature sensitivity of the electron transport chain have been reported (1, 4, 6, 7, 9-11, 13-15, 19-23, 27), but few have attempted to correlate the temperature sensitivity of carbon fixation in intact leaves with the temperature sensitivity of the light reactions in isolated chloroplast membranes (1,4). In this report, the temperature dependence of electron transport and photophosphorylation in isolated chloroplast thylakoids from A. smithii was investigated to test the hypothesis that temperature effect on electron transport and photophosphorylation contribute to the decline in whole leaf net photosynthesis at temperatures above 35°C.…”

Light and Temperature Regulation of Early Morning Crassulacean Acid Metabolism in Opuntia erinacea var Columbiana (Griffiths) L. Benson

“…Whole plant CO2 exchange was measured using an open IR gas analysis system as described previously (8). Ingoing CO2 and 02 concentrations were maintained at 330 Wl/l and 21%, respectively, by mixing gases from cylinders containing 21% 02 in N2, and 1% CO2 in N2 using Wosthoff mixing pumps.…”