tration of Rubisco and other photosynthetic enzymes. Indeed, maximum
photosynthetic rate tends to increase linearly with leaf nitrogen concentration across
species adapted to different levels of irradiance or soil fertility (Mooney et al. 1978; Field
and Mooney 1986).
If mesophyll photosynthetic capacity and maximum photosynthetic rate increase with
total leaf nitrogen or soluble protein, why don't shade leaves also have high levels of
nitrogen or soluble protein? Mooney and Gulmon (1979) presented a conceptual model
to answer this and related questions, based on the impact of leaf nitrogen content on
whole-plant growth. They argued that as leaf nitrogen content increases, so does leaf
photosynthetic rate, but so also do the root costs the plant must pay in order to obtain
that nitrogen. The extent to which additional nitrogen-in the form of various dark
reaction enzymes, including Rubisco-can enhance photosynthesis is likely to be greater
in sunny than in shady environments, because carboxylation is more likely to limit
photosynthesis at high irradiance. Specifically, Mooney and Gulmon (1979) argued that
the leaf nitrogen level at which photosynthesis begins to plateau should be higher in
sunny than in shady environments. Thus, the optimal leaf nitrogen content-at which
the difference between photosynthesis and the energetic investment in roots needed to
obtain a given amount of nitrogen is maximised-should be higher in sun than in shade.
Based on the higher returns expected from a given investment in nitrogen if water
availability does not limit photosynthesis, and on the lower costs of obtaining a given
amount of nitrogen on more fertile soils, Mooney and Gulmon (1979) concluded that
optimal leaf nitrogen content should also be higher on moister or more fertile sites.
Gulmon and Chu (1981) present data supporting one assumption of this model as it
applies to sun v. shade adaptation: photosynthesis does increase more rapidly with leaf
nitrogen (g N m-2), and saturate at higher levels of leaf nitrogen, at higher levels of
irradiance in Diplacus aurantiacus. Provided that root costs increase with nitrogen
uptake, optimal leaf nitrogen content should thus increase with irradiance. However,
although the Mooney-Gulmon model is consistent with this finding, and accords qualitatively
with trends seen in leaf nitrogen content in plants exposed to different levels of
light, moisture, and soil fertility, the model has five important shortcomings:
(1) Quantifying the root costs associated with nutrient uptake has remained difficult,
presumably because roots have other functions (e.g. water uptake) and because nutrient
uptake involves active transport, necessitating measurements of both root construction
and maintenance costs. This has prevented any quantitative test of the model to date.
(2) The model as originally advanced does not incorporate the fact that, at least
across leaves acclimated or adapted to different irradiance levels, maximum rates of
photosynthesis are strongly correlated with rates of dark respiration (Fig. 1). Dark
respiration averages about 7% of peak photosynthesis, so that every increase in the
latter of 1 pmol rn-' s-' decreases photosynthesis at low irradiance levels by 0.07 pmol
m-z s - 1 , and increases the instantaneous leaf compensation point by 1.4pmol
m-2 s - 1 , given the average quantum yield of 0.05 mol CO2 mol- ' absorbed quanta in
CS plants (Ehleringer and Bjdrkman 1977). This raises the possibility that low irradiance
may favour low leaf nitrogen contents and peak photosynthetic rates mainly because
they maximise net leaf-level photosynthesis under shady conditions, irrespective of the
costs of obtaining a given amount of nitrogen. The impact of leaf nitrogen content on
photosynthesis at low and intermediate irradiance levels via its effect on dark respiration
should be incorporated in any updated analysis. Note, however, that dark respiration
is not tightly coupled to leaf nitrogen content in plants exposed to the same light
environment, but differing in soil nitrogen supply (Gulmon and Chu 1981) or intrinsic
leaf nitrogen content (Armond and Mooney 1978).
(3) Variation in leaf nitrogen content reflects variation in both leaf mass per unit area
and nitrogen concentration per unit mass; at least in certain cases (e.g. Gulmon and
Chu 1981), most of the difference between sun and shade leaves in nitrogen content per
tration Rubisco และเอนไซม์อื่น ๆ photosynthetic แน่นอน สูงสุดphotosynthetic อัตรามีแนวโน้มการ เพิ่มเชิงเส้นกับความเข้มข้นของไนโตรเจนใบผ่านชนิดที่ปรับระดับ irradiance หรือความอุดมสมบูรณ์ของดิน (Mooney et al. 1978 แตกต่างกัน ฟิลด์ก Mooney 1986)ถ้ากำลังการผลิต photosynthetic mesophyll และอัตราสูงสุด photosynthetic เพิ่มด้วยรวมไนโตรเจนหรือโปรตีนที่ละลายน้ำได้ ทำไมไม่เงาใบไม้ยังมีระดับสูงไนโตรเจนหรือโปรตีนที่ละลายน้ำได้ Mooney และ Gulmon (1979) นำเสนอรูปแบบแนวคิดตอบซึ่งการถาม ตามผลกระทบของเนื้อหาบนใบไนโตรเจนเจริญเติบโตของพืชทั้งหมด พวกเขาโต้เถียงที่เป็นไนโตรเจนใบเนื้อหาเพิ่มขึ้น เพื่อให้ ใบไม้อัตรา photosynthetic แต่เพื่อทำต้นทุนหลักที่โรงงานต้องจ่ายเพื่อให้ได้ไนโตรเจนที่ ขอบเขตการที่ไนโตรเจนเพิ่มเติม-ในความมืดต่าง ๆปฏิกิริยาเอนไซม์ รวม Rubisco-สามารถเพิ่มการสังเคราะห์ด้วยแสงจะมีค่าในซันนี่กว่าในสภาพแวดล้อมร่มรื่น เนื่องจาก carboxylation มีแนวโน้มที่จะจำกัดการสังเคราะห์ด้วยแสงที่สูง irradiance โต้เถียงที่เฉพาะ Mooney และ Gulmon (1979)ควรสูงในระดับไนโตรเจนใบที่สังเคราะห์ด้วยแสงเริ่มที่ราบสูงซันนี่กว่าในสภาพแวดล้อมร่มรื่น ดังนั้น เหมาะสมใบไนโตรเจนเนื้อหาที่ซึ่งความแตกต่างระหว่างการสังเคราะห์ด้วยแสงและการลงทุนในรากต้องมีพลังรับจำนวนไนโตรเจนให้เป็น maximised-ควรสูงแสงแดดมากกว่าในที่ร่มBased on the higher returns expected from a given investment in nitrogen if wateravailability does not limit photosynthesis, and on the lower costs of obtaining a givenamount of nitrogen on more fertile soils, Mooney and Gulmon (1979) concluded thatoptimal leaf nitrogen content should also be higher on moister or more fertile sites.Gulmon and Chu (1981) present data supporting one assumption of this model as itapplies to sun v. shade adaptation: photosynthesis does increase more rapidly with leafnitrogen (g N m-2), and saturate at higher levels of leaf nitrogen, at higher levels ofirradiance in Diplacus aurantiacus. Provided that root costs increase with nitrogenuptake, optimal leaf nitrogen content should thus increase with irradiance. However,although the Mooney-Gulmon model is consistent with this finding, and accords qualitativelywith trends seen in leaf nitrogen content in plants exposed to different levels oflight, moisture, and soil fertility, the model has five important shortcomings:(1) Quantifying the root costs associated with nutrient uptake has remained difficult,presumably because roots have other functions (e.g. water uptake) and because nutrientuptake involves active transport, necessitating measurements of both root constructionand maintenance costs. This has prevented any quantitative test of the model to date.(2) The model as originally advanced does not incorporate the fact that, at leastacross leaves acclimated or adapted to different irradiance levels, maximum rates ofphotosynthesis are strongly correlated with rates of dark respiration (Fig. 1). Darkrespiration averages about 7% of peak photosynthesis, so that every increase in thelatter of 1 pmol rn-' s-' decreases photosynthesis at low irradiance levels by 0.07 pmolm-z s - 1 , and increases the instantaneous leaf compensation point by 1.4pmolm-2 s - 1 , given the average quantum yield of 0.05 mol CO2 mol- ' absorbed quanta inCS plants (Ehleringer and Bjdrkman 1977). This raises the possibility that low irradiancemay favour low leaf nitrogen contents and peak photosynthetic rates mainly becausethey maximise net leaf-level photosynthesis under shady conditions, irrespective of thecosts of obtaining a given amount of nitrogen. The impact of leaf nitrogen content onphotosynthesis at low and intermediate irradiance levels via its effect on dark respirationshould be incorporated in any updated analysis. Note, however, that dark respirationis not tightly coupled to leaf nitrogen content in plants exposed to the same lightenvironment, but differing in soil nitrogen supply (Gulmon and Chu 1981) or intrinsicleaf nitrogen content (Armond and Mooney 1978).(3) Variation in leaf nitrogen content reflects variation in both leaf mass per unit areaand nitrogen concentration per unit mass; at least in certain cases (e.g. Gulmon andChu 1981), most of the difference between sun and shade leaves in nitrogen content per
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