Photosynthetic Efficiency and Crop Yield

Photosynthesis, the most important biochemical process on the earth, is of such vital importance that no plant, animal, or human can live without it because they all depend on the energy, organic matter, and oxygen provided by it. Photosystem II (PSII) of the photosynthetic apparatus has been regarded as the engine of life [1]. However, considering both the cooperative relation between it and photosystem I (PSI) in photosynthesis and the key role of photosynthesis in the biosphere, we prefer to consider the two photosystems together, even the whole photosynthetic apparatus, which includes the carbon assimilation enzyme systems, as the engine of life driven by the energy from sunlight. Photosynthesis is the cornerstone of all crop production practices, and the aim of crop production is to maximize it [2]. Agriculture is basically a system of exploiting solar energy to synthesize organic matter through photosynthesis. The yield of crop plants ultimately depends on the size and efficiency of their photosynthetic system [3]. Two important determinants of biomass production of any crop are the quantity of radiation intercepted by the crop and the efficiency of using the radiation in dry matter production [4]. As the economic yield of a crop is related not only to the dry matter production but also to the harvest index, crop productivity depends primarily on how efficiently incident light is used for assimilating carbon dioxide and how efficiently this assimilated carbon is partitioned among plant parts [5]. The notion of photosynthetic efficiency in the literature involves some different terms including photosynthetic rate; quantum yield of carbon assimilation; photochemical efficiency of PSII, which is often expressed as a ratio of variable to maximal fluorescence, Fv/Fm; light utilization efficiency; etc. These terms are different but linked to each other. From the light response curve of photosynthesis it may be understood that the limiting factors of photosynthesis are different at different light intensities. In weak light, photosynthetic rate increases linearly with an increase in light intensity because radiation energy is the main limiting factor. In stronger light with an increase in light intensity, its increase lowers gradually and finally ceases because the main limiting factor has become the capacity to use light energy of the photosynthetic apparatus. In weak light one is concerned mainly with quantum yield, whereas photosynthetic rate is more noted in strong light. Both photosynthetic rate and quantum yield are related to characteristics of the leaf, cell, and chloroplast itself and environmental conditions. Photosynthetic rate is often expressed as number of molecules of CO2 fixed or O2 evolved per unit leaf area per unit time (for example, mol CO2 m 2 s ), while quantum yield is expressed as number of molecules of CO2 fixed or O2