Feedback Inhibition Of Photosynthesis Research Articles

Testing a Mechanistic Model of Forest-Canopy Mass and Energy Exchange Using Eddy Correlation: Carbon Dioxide and Ozone Uptake by a Mixed Oak-Maple Stand

A big-leaf model of C3-canopy mass and energy exchange was used to predict hourly CO2 and O3 uptake by a mixed deciduous Quercus-Acer (oak-maple) stand in central Massachusetts, USA. The model is based on canopy-radiation interactions, leaf mesophyll metabolism (photosynthetic electron transport, carboxylation and oxygenation of ribulose 1,5-bisphosphate [RuP2] by RuP2 carboxylase/oxygenase [Rubisco], and respiration), physical transport conductances of mass and heat above and within the canopy, conductances of mass at the leaf surface and in the mesophyll, and mass and energy exchange at the soil surface (forest floor). Predictions of hourly CO2 and O3 uptake were compared to independent whole-forest CO2 and O3 exchange measurements made by the eddy correlation method during a 68 day period in the summer and early autumn of 1992. Predicted hourly CO2 exchange rate was strongly correlated (r ≈ +0.91) with measured hourly CO2 exchange, but mean day-time predicted whole-forest CO2 uptake was c. 13% (c. 1.13 μmol CO2 m-2 s-1) greater than CO2 uptake measured by eddy correlation. The model tended to overpredict CO2 uptake during late afternoon, but was accurate during the rest of the day. Predicted and measured O3 uptake rates also were positively correlated (r ≈ +0.76). The diurnal patterns of predicted and measured O3 uptake indicated that stomata1 conductance (gs) was accurately predicted during the morning, but in the afternoon the model overpredicted gs. This pattern was consistent with the overprediction of afternoon CO2 uptake, and suggested that a feedback inhibition of photosynthesis occurred in the afternoon. This might have been related to source-sink imbalance following several hours of photosynthesis. On the whole, and in spite of the simplifications inherent in the big-leaf representation of the canopy, the model is useful for predicting forest-environment interactions and for interpreting mass and energy exchange measurements.

The Effect of Cooling on Photosynthesis, Amounts of Carbohydrate and Assimilate Export in Sunflower

Sunflower plants (Helianthus annuus L.) grown at 30 °C were cooled to 13 °C in the light in atmospheric C02 or low C02, or in darkness. Photosynthetic rate at 30 °C after cooling whole plants in atmospheric C02 for 12 h during a photoperiod was significantly lower than at the start of the photoperiod compared to plants cooled at low C02, those cooled in the dark and those maintained at 30 °C. Amounts of sucrose, hexoses and starch in leaves at 13 °C increased throughout a 14 h photoperiod to levels higher than in leaves at 30 °C, where amounts of sucrose and hexoses were stable or falling after 4 h. Carbohydrate accumulation at 13 °C during this photoperiod was more than twice that at 30 °C. After three photoperiods and two dark periods at 13 °C carbohydrate levels in leaves were still as high as at the end of the first photoperiod, but less carbohydrate accumulated during the photoperiods than during the first photoperiod, and more was partitioned as starch. Amounts of soluble carbohydrate in roots were greater after 14 h at 13 °C than in roots of plants at 30 °C. Loss of 14C from leaves at 30 °C as a proportion of 14C02 fixed by them at 30 °C, decreased after exposure of plants to 13 °C in the light for 30 min prior to 14C02feeding. Results indicate an effect of cold on the transport process that was light-dependent. It is inferred that the reduction in the proportion of 14C lost from leaves after 10 h cooling was due to reduced sink demand, whereas the rise in the proportion of 14C lost from leaves after 24 h reflects reduced photosynthetic rate. The coincidence of reduced photosynthetic rate with raised carbohydrate levels in leaves maintained at 30 °C throughout, whilst the rest of the plant was cooled to 13 °C in the light implies feedback inhibition of photosynthesis. This may reduce the imbalance between source and sink in sunflower during the first days of long-term cooling.

Factors influencing the capacity for photosynthetic carbon assimilation in barley leaves at low temperatures

The aim of this work was to examine the effect upon photosynthetic capacity of short-term exposure (up to 10 h) to low temperatures (5° C) of darkened leaves of barley (Hordeum vulgare L.) plants. The carbohydrate content, metabolite status and the photosynthetic rate of leaves were measured at low temperature, high light and higher than ambient CO2. Under these conditions we could detect whether previous exposure of leaves to low temperature overcame the limitation by phosphate which occurs in leaves of plants not previously exposed to low temperatures. The rates of CO2 assimilation measured at 8° C differed by as much as twofold, depending upon the pretreatment. (i) Leaves from plants which had previously been darkened for 24 h had a low content of carbohydrate, had the lowest CO2-assimilation rates at low temperature, and photosynthesis was limited by carbohydrate, as shown by a large stimulation of photosynthesis by feeding glucose, (ii) Leaves from plants which had previously been illuminated for 24 h and which contained large carbohydrate reserves showed an accumulation of phosphorylated intermediates and higher CO2-assimilation rates at low temperature, but nevertheless remained limited by phosphate, (iii) Maximum rates of CO2 assimilation at low temperature were observed in leaves which had intermediate reserves of carbohydrate or in leaves which were rich in carbohydrate and which were also fed phosphate. It is suggested that carbohydrate reserves potentiate the system for the achievement of high rates of photosynthesis at low temperatures by accumulation of photosynthetic intermediates such as hexose phosphates, but that this potential cannot be realised if, at the same time, carbohydrate accumulation is itself leading to feedback inhibition of photosynthesis.

Effect of Pod Number on Dry Matter and Nitrogen Accumulation and Distribution in Soybean1

Greenhouse and field studies were undertaken to document the effects of a decreased pod number on dry matter and N accumulation and distribution during grain filling in soybean [Glycine max (L.) Merr.]. Removal of about 40% of the pods significantly shifted dry matter partitioning in favor of vegetative tissues. Total dry matter and N accumulation were not affected, indicating that partial depodding did not cause feedback inhibition of photosynthesis. The crop compensated for the lower pod number with an increased 100‐ seed weight, so that final grain yield was depressed only 7 to 14% below controls.Vegetative tissue N of control field‐grown plants decreased rapidly during grain filling, and the majority of this loss was attributed to redistribution of N to the growing seed. Partial depodding greatly decreased vegetative N loss, and stem N concentrations actually increased toward the end of grain filling. Apparently, current N2 fixation roughly equalled N demand by the seed in these plants so that little net redistribution occurred. The decreased pod load may also have made more photosynthate available to support greater late‐season N assimilation than occurred in the controls.