evolved per photon absorbed. The light utilization efficiency of a canopy depends not only on the factors
already mentioned but also on the total leaf area of the canopy and the canopy architecture. It is often ex-
pressed as a ratio of the energy stored in dry matter of crop plants formed in photosynthesis to the energy
received per unit ground area where the crop plants grow. For crop yield and light utilization efficiency,
both photosynthetic rate in strong light and quantum yield of carbon assimilation in weak light are im-
portant under field conditions.
In this chapter, first of all, the limiting factors of photosynthetic rate and the photosynthetic rate–crop
yield relation are reviewed with an emphasis that the positive correlation is the reflection of the essence
of the relation. Then the significance of quantum yield in yield formation and factors affecting quantum
yield are discussed. Lastly, the main characteristics of the coming new green revolution are predicted on
the basis of analysis of the limitations of the first green revolution.
II. PHOTOSYNTHETIC RATE
Photosynthetic rate in strong light is an important parameter characterizing the photosynthetic capacity of
the photosynthetic apparatus. Apparently, it is not a notion of efficiency because the efficiency is the ra-
tio of the output to input energy [6]. In fact, it is also a notion of the efficiency because it is a determinant
of crop yield and light use efficiency. Under same light intensity, especially saturating light for photo-
synthesis, leaves with a higher photosynthetic rate necessarily have a higher photosynthetic capacity and
higher light use efficiency compared with leaves with a lower one.
A. Factors Limiting Photosynthetic Rate
Many external environment factors such as low or high temperature, deficiency of water or nutrient sup-
ply, low CO 2 or high O 2 concentration, and low light intensity may limit photosynthesis, leading to a de-
creased photosynthetic rate. Meanwhile, many plant internal factors including development, hormones,
respiration, etc. may also have a significant effect on net photosynthetic rate, but the main limitation site
of net photosynthetic rate in C 3 plants is often in the reaction catalyzed by the enzyme ribulose-1,5-bis-
phosphate (RuBP) carboxylase/oxygenase (Rubisco). Therefore, reducing or eliminating its oxygenase
function or photorespiration or increasing the affinity of the enzyme for CO 2 is a long-term goal to in-
crease productivity [7]. Nevertheless, we found that the photosynthetic rate in some plants such as wheat
and rice was increased when the ATP supply was enhanced by spraying leaves with PMS (N-
methylphenazonium methosulfate) to induce cyclic photophosphorylation (PSP) or with coupling effi-
ciency improvers, for instance, polybasic acids. These findings indicate that the ATP supply from PSP
may also be a limiting factor of the photosynthetic rate [8]. Recently, it was demonstrated that photosyn-
thetic assimilation of CO 2 in water-stressed leaves of sunflower is not limited by CO 2 diffusion but by in-
hibition of RuBP synthesis, related to a lower ATP content resulting from loss of ATP synthase [9].
B. Diurnal Variation in Leaf Photosynthetic Rate
Leaf photosynthetic rate is not a constant parameter. It often varies with development of the leaf itself and
changes in the environment. So it often displays ontogenetic, seasonal, and diurnal variations.
There are two typical patterns of the diurnal course of photosynthesis under natural conditions. One
is the one-peaked pattern with the maximum net photosynthetic rate around noon on cloudy days; the
other is two peaked, with one of the peaks in the late morning and the other in early afternoon with a de-
pression around noon (11:00–14:00) on clear days, the so-called midday depression of photosynthesis.
When the depression is severe, the peak in the afternoon may fail to appear. The possible mechanisms of
the phenomenon have been reviewed in detail [10]. For wheat, the main cause of the midday depression
observed by us is attributed to the partial closure of stomata [11], although photoinhibition of photosyn-
thesis occurs simultaneously [12].
Midday depression of photosynthesis, as a regulatory process of the plant itself, is advantageous for
the survival of plants under stress conditions but is at the expense of plant productivity, as it may decrease
productivity. Alleviating it by some measures, therefore, may increase crop yield significantly. For ex-
ample, mist irrigation at the grain-filling stage increased stomatal conductance and thereby net photo-
synthetic rate in flag leaves of wheat, thus increasing grain yield by about 18% [13].
822 XU AND SHEN