Abstract
Abstract Measurements of photosynthesis and respiration were made on leaves in summer in a Quercus rubra L. canopy at approximately hourly intervals throughout 5 days and nights. Leaves were selected in the upper canopy in fully sunlit conditions (upper) and in the lower canopy (lower). In addition, leaves in the upper canopy were shaded (upper shaded) to decrease photosynthesis rates. The data were used to test the hypothesis that total night‐time respiration is dependent on total photosynthesis during the previous day and that the response is mediated through changes in storage in carbohydrate pools. Measurements were made on clear sunny days with similar solar irradiance and air temperature, except for the last day when temperature, especially at night, was lower than that for the previous days. Maximum rates of photosynthesis in the upper leaves (18.7 μmol m −2 s −1 ) were approximately four times higher than those in the lower leaves (4.3 μmol m −2 s −1 ) and maximum photosynthesis rates in the upper shaded leaves (8.0 μmol m −2 s −1 ) were about half those in the upper leaves. There was a strong linear relationship between total night‐time respiration and total photosynthesis during the previous day when rates of respiration were normalized to a fixed temperature of 20°C, removing the effects of temperature from this relationship. Measurements of specific leaf area, nitrogen and chlorophyll concentration and calculations of the maximum rate of carboxylation activity, V cmax , were not significantly different between upper and upper shaded leaves 5 days after the shading treatment was started. There were small, but significant decreases in the rate of apparent maximum electron transport at saturating irradiance, J max ( P> 0.05), and light use efficiency, ɛ ( P <0.05), for upper shaded leaves compared with those for upper leaves. This suggests that the duration of shading in the experiment was sufficient to initiate changes in the electron transport, but not the carboxylation processes of photosynthesis. Support for the hypothesis was provided from analysis of soluble sugar and starch concentrations in leaves. Respiration rates in the upper shaded leaves were lower than those expected from a relationship between respiration and soluble sugar concentration for fully exposed upper and lower leaves. However, there was no similar difference in starch concentrations. This suggests that shading for the duration of several days did not affect sugar concentrations but reduced starch concentrations in leaves, leading to lower rates of respiration at night. A model was used to quantify the significance of the findings on estimated canopy CO 2 exchange for the full growing season. Introducing respiration as a function of total photosynthesis on the previous day resulted in a decrease in growing season night‐time respiration by 23% compared with the value when respiration was held constant. This highlights the need for a process‐based approach linking respiration to photosynthesis when modelling long‐term carbon exchange in forest ecosystems.
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Claims 8
Claim · model-written summary
Upper leaf maximum photosynthesis was about four times higher than lower leaf photosynthesis.
- Taxon
- Quercus rubra L.
_(14092717698).jpg?width=60)
Quercus rubrae1 - Outcome
- Maximum rates of photosynthesise0
- Direction
- highere0
- Value
- 18.7 μmol m −2 s −1e0
- Qualifier
- 4.3 μmol m −2 s −1e0
- Comparator
- lower leavese0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
_(14092717698).jpg?width=500)
Verbatim
e0“Maximum rates of photosynthesis in the upper leaves (18.7 μmol m −2 s −1 ) were approximately four times higher than those in the lower leaves (4.3 μmol m −2 s −1 )”
e1“Quercus rubra L. canopy”
Claim · model-written summary
Shaded upper leaves had about half the maximum photosynthesis of upper leaves.
- Taxon
- Quercus rubra L.Quercus rubrae1
- Treatment
- upper shaded leavese0
- Outcome
- maximum photosynthesis ratese0
- Direction
- about halfe0
- Value
- 8.0 μmol m −2 s −1e0
- Comparator
- upper leavese0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“maximum photosynthesis rates in the upper shaded leaves (8.0 μmol m −2 s −1 ) were about half those in the upper leaves”
e1“Quercus rubra L. canopy”
Claim · model-written summary
Night respiration related linearly to previous-day photosynthesis at a fixed temperature.
- Taxon
- Quercus rubra L.Quercus rubrae1
- Outcome
- total night‐time respiratione0
- Qualifier
- 20°Ce0
- Comparator
- total photosynthesis during the previous daye0
Not reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“There was a strong linear relationship between total night‐time respiration and total photosynthesis during the previous day when rates of respiration were normalized to a fixed temperature of 20°C”
e1“Quercus rubra L. canopy”
Claim · model-written summary
Leaf area, nitrogen, chlorophyll and carboxylation measurements did not differ significantly with shading.
- Taxon
- Quercus rubra L.Quercus rubrae1
- Treatment
- upper shaded leavese0
- Outcome
- specific leaf area, nitrogen and chlorophyll concentratione0
- Direction
- not significantly differente0
- Comparator
- uppere0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Measurements of specific leaf area, nitrogen and chlorophyll concentration and calculations of the maximum rate of carboxylation activity, V cmax , were not significantly different between upper and upper shaded leaves 5 days after the shading treatment was started”
e1“Quercus rubra L. canopy”
Claim · model-written summary
Shading decreased electron transport and light use efficiency.
- Taxon
- Quercus rubra L.Quercus rubrae1
- Treatment
- upper shaded leavese0
- Outcome
- rate of apparent maximum electron transport at saturating irradiance, J max ( P> 0.05), and light use efficiency, ɛ ( P <0.05)e0
- Direction
- decreasese0
- Comparator
- upper leavese0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“There were small, but significant decreases in the rate of apparent maximum electron transport at saturating irradiance, J max ( P> 0.05), and light use efficiency, ɛ ( P <0.05), for upper shaded leaves compared with those for upper leaves”
e1“Quercus rubra L. canopy”
Claim · model-written summary
Respiration in shaded leaves was lower than expected from the sugar relationship.
- Taxon
- Quercus rubra L.Quercus rubrae1
- Outcome
- Respiration rates in the upper shaded leavese0
- Direction
- lowere0
- Comparator
- fully exposed upper and lower leavese0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Respiration rates in the upper shaded leaves were lower than those expected from a relationship between respiration and soluble sugar concentration for fully exposed upper and lower leaves”
e1“Quercus rubra L. canopy”
Claim · model-written summary
Starch concentrations lacked a similar difference.
- Taxon
- Quercus rubra L.Quercus rubrae1
- Outcome
- starch concentrationse0
- Direction
- no similar differencee0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“there was no similar difference in starch concentrations”
e1“Quercus rubra L. canopy”
Claim · model-written summary
A model with prior-day photosynthesis lowered estimated growing-season night respiration by 23%.
- Taxon
- Quercus rubra L.Quercus rubrae1
- Outcome
- growing season night‐time respiratione0
- Direction
- decreasee0
- Value
- 23%e0
- Comparator
- respiration was held constante0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Introducing respiration as a function of total photosynthesis on the previous day resulted in a decrease in growing season night‐time respiration by 23% compared with the value when respiration was held constant”
e1“Quercus rubra L. canopy”