lated to yield size [48]. The positive correlation between leaf photosynthesis and yield mentioned earlier
is observed mostly at this stage. Obviously, one should not expect to find the correlation at all stages of
crop development.
In addition, it is likely that the apparent lack of a positive relationship between leaf photosynthesis
and crop yield may also originate from unsuitable methods and techniques used in the measurement of
leaf photosynthetic rate. First, in some studies the equipment for photosynthetic measurement could not
carry out rapid and accurate determinations of the difference in leaf photosynthetic rates under field con-
ditions. Second, the number of leaves for photosynthetic measurement is sometimes too small to detect
significant differences between cultivars. It has been calculated that 34, 16, and 8 measurements are
needed for detecting a 7–9%, 15%, and 20% difference, respectively, between different soybean geno-
types in the field [16]. Third, representative leaves should be used in photosynthetic measurements. Crop
yield comes from a canopy, not several leaves. Therefore, differences in leaf photosynthesis among crop
genotypes have not been shown to be correlated with seed yield, probably because leaves selected for
measurements have not been representative of the crop canopy [49]. Moreover, conclusions about the ab-
sence of relationship between net photosynthetic rate and crop yield are often drawn from instantaneous
photosynthetic measurements conducted under standardized conditions rather than seasonal measure-
ments conducted under field conditions [50].
It was pointed out that there are several factors that may account for the apparent lack of correlation
between photosynthetic rates and yield [23]. Much of the photosynthetic data is collected on individual
leaves, whereas yield is measured on the entire plant or canopy. Many photosynthetic measurements are
point-in-time determinations made at varying developmental stages of the plants and do not take into con-
sideration the entire growing season. The relationship could be masked by any of a number of biochem-
ical and physiological events that occur between the production of photosynthates and their utilization in
the accumulation of final yield. Therefore, the concept of canopy seasonal photosynthesis was proposed
and may be estimated by integrating the area under the time course curve of canopy net photosynthetic
rate. It was found that the grain yield of soybean is strongly dependent on seasonal photosynthesis [23].
Similarly, there have been many other reports indicating positive relationships between photosynthetic
rate at the canopy level and plant productivity in barley [51], soybean [49], cotton [52], wheat [53], and
maize [54]. These may also be considered as strong evidence for an intrinsic relation between photosyn-
thetic rate and crop yield.
From the findings discussed, it is concluded that the lack of correlation or negative correlation be-
tween photosynthetic rate and crop yield is only apparent whereas the positive correlation is a reflection
of the intrinsic relation but is often masked by some factors. There has been evidence indicating that ge-
netic selection for higher photosynthetic rates could lead to increases in yield. For example, pima cotton
bred for increased yield has enhanced photosynthesis. Among the cultivated types of pima cotton, genetic
advances are closely associated with an increase in leaf photosynthetic rate and stomatal conductance, es-
pecially in the morning [55]. Thus, we could improve crop yield by selecting a higher photosynthetic rate.
However, higher photosynthetic rate is not the sole parameter that should be considered. When a higher
photosynthetic rate is selected, some unfavorable changes such as a decline in leaf area, leaf functional
duration, and harvest index and an increase in respiratory rate should be avoided. Meanwhile, the quan-
tum yield of photosynthesis in weak light is also an important parameter to be considered in crop breed-
ing [56].
III. QUANTUM YIELD
For a high yield of crop canopy, not only a high photosynthetic rate in strong light but also a high quan-
tum yield in weak light is important because in a canopy not all leaves are in strong light. Apart from
leaves in the upper layer, those of the middle and bottom layers in a canopy are often under light-limiting
conditions even on clear days. On cloudy days or in the early morning and late afternoon on clear days,
all leaves of a canopy are under weak light. So under field conditions a significant part of crop photosyn-
thesis occurs at nonsaturating light. Therefore, Ort and Baker [57] believed that future research efforts
aimed at improving crop production through improved photosynthetic performance should have a major
focus on the efficiency of operation under nonsaturating light conditions. Similarly, it was considered that
increasing net photosynthetic rate (Pn) and productivity in low light would require an increase in appar-
ent quantum yield [7].
PHOTOSYNTHETIC EFFICIENCY AND CROP YIELD 825