Photosynthetic Functions of Different Senescing Leaves in the Canopy of Super high-yield Rice ‘Hua-An3’

Abstract

In order to improve rice yields to feed a growing population, China has carried out a `Super High-Yield Rice Project' since 1996. Tremendous progress was made during the `Ninth Five-Year Plan' and several new varieties of rice hybrids were released. `Hua-an 3' (`X07S'×`Zihui100') is a new high yield variety that has yields of more than 10 500 kg·hm -2; this compares with yields of 7 500-8 500 kg·hm -2 in the traditional hybrid rice `Shanyou 63' (`Zhenshan 97A'×`Minghui 63'). The photosynthetic functions and pigments of the upper most 5 senescing leaves of the canopy of `Hua-an 3' were studied by techniques of fluorescence induction kinetics, low temperature (77K) fluorescence emission spectrum and HPLC. The results showed that the maximal quantum yield of PSⅡ photochemistry (F v/F m), efficiency of excitation energy captured by open PSⅡ reaction centers (F v′/F m′), quantum yield of PSⅡ electron transport (Φ PSⅡ), photochemical quenching coefficient (qP), and the content of photosynthetic pigments, especially chlorophyll (Chl), neoxanthin (N), lutein (L) and β-carotene (β-Car), of the flag leaf were higher than those of other leaves but its excitation pressure (1-qP) was lower. It was shown that the content of the core antenna complex CP47 in photosystemⅡ (PSⅡ) and photosystem Ⅰ (PS Ⅰ) of the flag leaf also was higher than in other leaves, but the content of its non-active aggregate of light harvesting complex in PSⅡ (LHCⅡ) derived from the Gaussian analysis of low-temperature (77K) fluorescence emission spectra was lower than that of the other leaves. Our research results are as follows: 1) at later growth and development stages of rice, the dry matter of grain was mainly provided by most photosynthesis occurred in the uppermost 3 leaves in the canopy; 2) in the course of leaf senescence, the senescence of photosynthetic reaction centers was quicker than that of the antenna system; 3) one of the photoprotective pathways in hybrid rice may be through an increase in the content of PS Ⅰ, absorption of more light energy, and stimulating a high speed electron transport cycle in order to protect the photosynthetic apparatus under photoinhibitory conditions; 4) in the course of leaf senescence of rice, partial chlorophyll b was probably reduced to chlorophyll a, to decrease the content of LHCⅡ, thus reducing the amount of energy absorbed by LHCⅡ and decreasing the amount of photoinhibitory damage.