Can improvement in photosynthesis increase crop yield?

組別 : 第三組指導老師 : 張孟基老師組員 : 何佳勳 馮學謙 潘昶儒 李政錩

Introduction

The world’s most important crops and their total yield in 2004

1. Maize: 823 Mt.2. Rice: 725 Mt.3. Wheat: 555 Mt.4. Barley: 142 Mt.5. Sorgnum: 59 Mt.

Increased use of nitrogen fertilizer and improvement management have produced remarkable increases.

The major grain crops yield rose from 1.2t/Ha in 1951 to 2.3 t/Ha in 1993.

For maize, this increase may be attributed 50% to genetic improvement and 50% to improved management.

Monteith principle

Pn = St˙ εi˙ εc / k (1a)

Yp = Pn˙ η (1b)

How to increase Yp ?

1.Increase εi through earlier canopy development and ground cover.

2.Select cultivars able to respond to additional nitrogen fertilization without lodging.

3.Increase CO2 concentration.

Elevate CO2 concentration at wheat flag leaf

1.Increase photosynthesis 50%.

2.Increase grain yield 35%.

SPECIFIC OPPORTUNITIES FOR INCREASING PHOTOSYNTHESIS

The maximum εc for two reasons：

1. leaves become light saturated ： energy is wasted and efficiency drops.

the acceptor molecule of CO2 [ribulose biphosphate (RuBP)] εc at 25 °C closer to the theoretical 0.051.

2. decreasing photorespiration(光呼吸 ) Conversion of a C3 to a C4 crop would raise the maximum εc at 25 °C

from 0.051 to 0.060. If Rubisco can be engineered to be completely specific to CO2, this

would raise εc from 0.051 to 0.073

10%

100%

1%

1400700

75°

52° lat & 25°C

Modifying crop canopies to increase εC

Photosynthetic photon flux densities (PPFD) ： C3 about one-quarter

PPFD would be amount required to saturate photosynthesis (Fig. 1c). → other leaves is wasted

the upper leaves are more vertical and the lowermost leaves are horizontal, as plant Y (Fig. 1a) (Nobel, Forseth & Long 1993).

leaf with a 75° light energy would be 700 μmol m−2 s−1, just sufficient to saturate photosynthesis

plant Y would have over double the efficiency of light energy use than plant X at midday in full sunlight (Ort & Long 2003).

This example oversimplifies ： overhead ， tropics ， sun angle

Older varieties( horizontal leaves such as plant X ) have been replaced by newer varieties ( vertical leaves such as plant Y )(Nobel et al. 1993).

10%

100%

1%

1400700

75°

52° lat & 25°C

Relaxing the photoprotected state more rapidly to increase εC

As PPFD increases, photosynthesis saturates. (Fig. 1b)

This additional energy exceeds the capacity for photosynthesis will cause photooxidative(光氧化 ) damage →photosystem II (PSII )

induced increase in thermal dissipation of energy via the formation of epoxidated xanthophylls(葉黃素 ) (Long, Humphries & Falkowski 1994; Havaux & Niyogi 1999; Baroli & Niyogi 2000).

Photoprotection(光保護作用 ) it decreases the maximum quantum yield of PSII (Fv/Fm) and CO2 uptake (ΦCO2)( Zhu et al. 2004a)

Photoprotection(光保護作用 ) is at the level of the cell, not the leaf, light is simulated for small points of 104 μm rather than as an average for a leaf.

Temperature is important because it decreases photosynthetic capacity and rate of recovery from the photoprotected state. (chilling-tolerant←→ chilling-susceptible)

Relaxing the photoprotected state more rapidly to increase εC

Much larger losses from photoprotection result when photosynthesis is decreased by stresses (Long et al. 1994).

Photoprotection(光保護 ) fulfils a necessary function of oxidative damage to PSII, and replacement of the proteins before efficiency can be restored.

In the longer term , a continued excess of excitation energy would lead to irreversible photooxidation(光氧化 ) (Long et al. 1994).

Falkowski and Dubindky (1981) identify algae(海藻 ) associated with corals(珊瑚 ) can withstand 1.5 × full sunlight of maximum photosynthetic efficiency

Increased biomass product the ‘super-high yield’ rice cultivars. (Wang et al. (2002)

Xanthophyll(葉黃素 ) cycle capacity, including the epoxidation associated with recovery (Long et al. 1994) ： photoprotection is feasible in rice.

Photorespiration

Photorespiration

Photorespiration

30% carbonhydrate lost in C3 photosynthesis(phs) through photorespiration(PR).

Dissipate excess excitation energy.

Photorespiration

Xanthophyll - more effeciency in dissipating ecxess energy than PR.

How to block PR?

Photorespiration

Lower O2 or Higher CO2 can inhibit PR enzymes activity.

Growers - increase CO2 in greenhouse. Global [CO2] increacing - other negtive effect

s.

C4 - Kranz anatomy

Sugarcane

http://www.botany.hawaii.edu/faculty/webb/bot311/bot311-00/PSyn/PsynDark2.htm

C4

http://www.steve.gb.com/science/photorespiration.html

C4

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Long, 2006

C4

Steady-state biochemical models of C3 and C

4. Canopy radiation transfer models.

C4

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Long, 2006

C4 to C3 ?

Overexpression C4 genes in C3.

C4

Single cell form

Red dot: Rubisco

http://www.photosynthesisresearch.org/picture_gallery.html

Regeneration of RuBP

Ribulose1,5-bisphosphate(RuBP)

3-phosphoglycerate(PGA)

Glyceraldehyde 3-phosphate

regeneration

reduction

carboxylation

ATP

ADP

ATP+NADPH

ADP+Pi

+NADP+

CO2+H2O

Rubisco carboxylation rate

Jmax (RuBP regenerative capacity)

Two points in this chain limit Jmax

(1) Electron transport chain - cytochrome b6/f complex

(2) Calvin cycle – sedoheptulose-1,7-bisphosphatase (SBPase)

strongly control the rate of RuBP synthesis

(Raines C. A., 2003)

Increased SBPase activity intransgenic tobacco plants stimulates photosynthesisand growth

(Lefebvre et al. 2005)

Overview of opportunities and barriers

Conventional plant breeding-needed several years, several generations to introduce changes

Molecular transformation -> a single gene transformation -> saving conventional breeding time

direct effect of elevated [CO2]

suppress oxygenation and photorespiration

higher photosynthesis and yield

[CO2] negative effect (increase temperature, decrease soil moisture)

Increase C3 εc

-> decreasing photorespiration -> increased Rubisco specificity for CO2

-> engineering C4 photosynthesis into C3 crops

engineering C4 photosynthesis into C3 crops

introduction of the C4 photosynthetic cycle Kranz leaf anatomy associated differential expression of photosynthetic protein

conclusion

Increase the theoretical maximum εc of C3 or C4 crops do not appear realizable on a 10-20 years

Conventional breeding – require introduction of foreign genetic material

Both environmental stress and respiration improve the tolerance of εc to stress

decrease respiration to increase εc

預備用

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are needed to see this picture.