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 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 0.051. 2. decreasing photorespiration( 光呼吸 ) Conversion of a C3 to a C4 crop would raise the maximum ε c at 25 °C from 0.051 to 0.060. If Rubisco can be engineered to be completely specific to CO 2, this would raise ε c from 0.051 to 0.073
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This example oversimplifies ： overhead ， tropics ， sun angle Older varieties( horizontal leaves such as plant X ) have been replaced by newer varieties ( vertical leaves such as plant Y )(Nobel et al. 1993).
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.
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
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 10-20 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