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Photosynthetic Efficiency and Crop Yield
Da-Quan Xu and Yun-Kang Shen
Shanghai Institute of Plant Physiology, Chinese Academy of Sciences, Shanghai, People’s Republic of China
821I. INTRODUCTION
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 en-
gine 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 photo-
systems together, even the whole photosynthetic apparatus, which includes the carbon assimilation en-
zyme 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 mat-
ter 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 quan-
tity 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 har-
vest 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 pho-
tosynthetic 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 un-
derstood 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 photo-
synthetic 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 characteris-
tics of the leaf, cell, and chloroplast itself and environmental conditions. Photosynthetic rate is often ex-
pressed as number of molecules of CO 2 fixed or O 2 evolved per unit leaf area per unit time (for example,
mol CO 2 m^2 s^1 ), while quantum yield is expressed as number of molecules of CO 2 fixed or O 2