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
The world’s most important crops and their total yield in 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 For maize, this increase may be attributed 50% to genetic improvement and 50% to improved management.
Monteith principle P n = S t ˙ ε i ˙ ε c / k (1a) Y p = P n ˙ η (1b)
How to increase Y p ? 1.Increase ε i through earlier canopy development and ground cover. 2.Select cultivars able to respond to additional nitrogen fertilization without lodging. 3.Increase CO 2 concentration.
Elevate CO 2 concentration at wheat flag leaf 1.Increase photosynthesis 50%. 2.Increase grain yield 35%.
SPECIFIC OPPORTUNITIES FOR INCREASING PHOTOSYNTHESIS The maximum ε c for two reasons ： 1. leaves become light saturated ： energy is wasted and efficiency drops. the acceptor molecule of CO 2 [ribulose biphosphate (RuBP)] ε c at 25 °C closer to the theoretical decreasing photorespiration( 光呼吸 ) Conversion of a C3 to a C4 crop would raise the maximum ε c at 25 °C from to If Rubisco can be engineered to be completely specific to CO 2, this would raise ε c from to 0.073
10% 100 % 1% ° 52° lat & 25°C
Modifying crop canopies to increase ε C 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This example oversimplifies ： overhead ， tropics ， sun angle Older varieties( horizontal leaves such as plant X ) have been replaced by newer varieties ( vertical leaves such as plant Y )(Nobel et al. 1993).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% ° 52° lat & 25°C
Relaxing the photoprotected state more rapidly to increase ε C 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 CO 2 uptake (ΦCO 2 )( 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 ε C 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.
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 O 2 or Higher CO 2 can inhibit PR enzymes activity. Growers - increase CO 2 in greenhouse. Global [CO 2 ] increacing - other negtive effects.
C 4 - Kranz anatomy Sugarcane
C4C4 Long, 2006
C4C4 Steady-state biochemical models of C 3 and C 4. Canopy radiation transfer models.
C4C4 Long, 2006
C 4 to C 3 ? Overexpression C 4 genes in C 3.
C4C4 Single cell form Red dot: Rubisco
Regeneration of RuBP
Ribulose1,5-bisphosphate (RuBP) 3-phosphoglycerate (PGA) Glyceraldehyde 3-phosphate regeneration reduction carboxylation ATP ADP ATP+NADPH ADP+Pi +NADP + CO 2 +H 2 O Rubisco carboxylation rate J max (RuBP regenerative capacity)
Two points in this chain limit J max (1) Electron transport chain - cytochrome b 6 /f complex (2) Calvin cycle – sedoheptulose-1,7-bisphosphatase (SBPase) strongly control the rate of RuBP synthesis
(Raines C. A., 2003)
Increased SBPase activity in transgenic tobacco plants stimulates photosynthesis and 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 [CO 2 ] suppress oxygenation and photorespiration higher photosynthesis and yield [CO 2 ] negative effect ( increase temperature, decrease soil moisture)
Increase C 3 ε c -> decreasing photorespiration -> increased Rubisco specificity for CO 2 -> engineering C 4 photosynthesis into C 3 crops
engineering C 4 photosynthesis into C 3 crops introduction of the C4 photosynthetic cycle Kranz leaf anatomy associated differential expression of photosynthetic protein
conclusion Increase the theoretical maximum ε c of C 3 or C 4 crops do not appear realizable on a years Conventional breeding – require introduction of foreign genetic material Both environmental stress and respiration improve the tolerance of ε c to stress decrease respiration to increase ε c