- False Appearance—Negative Correlation
By making a comparative study of wild and cultivated wheat, it was found that leaf photosynthetic rate de-
creased while yield increased in wheat in the development of agriculture; that is, wild wheat has a lower
yield and a higher leaf photosynthetic rate than modern cultivated wheat [40]. It was then assumed that
there are counterproductive associations with high photosynthetic rate [41]. This finding is often cited as
evidence that there is no positive association and there is even a negative association between leaf photo-
synthesis and crop yield. However, wild wheat with a higher leaf photosynthetic rate also had a smaller
leaf area, shorter photosynthetic functional duration, and lower harvest index. Therefore, the so-called neg-
ative association is actually a false appearance. Obviously, for wild wheat the lower yield is due to smaller
leaf area, shorter functional duration, and lower harvest index rather than higher leaf photosynthesis.
A negative correlation between leaf photosynthetic rate and biological yield was also reported for
three tall fescue genotypes [42]. The negative correlation is also a false appearance. For the genotype hav-
ing a higher yield, the high yield is owing to its bigger leaf area rather than its lower leaf photosynthetic
rate.
It appears that although the analysis of correlation is a useful method used frequently in biologi-
cal studies, a positive or negative correlation between two variables is only a phenomenon, and it is not
necessarily a correct reflection of an intrinsic relation, especially a cause-effect one, between them. The
correlation may be altered or masked by complex changes in other variables related to the two being
considered. Therefore, one should take care when a conclusion is based on data from a correlation anal-
ysis.
- Factors Masking the Intrinsic Relation
Yield formation of a crop plant is a complex process involving many factors. In other words, crop yield
is influenced and determined by several factors, external and internal. Among internal factors, leaf pho-
tosynthetic rate is the basic but not the sole factor. Thus, the intrinsic relation between leaf photosynthetic
rate and crop yield may be masked by other factors changed in the opposite direction.
LEAF AREA Leaf area is a very important factor for crop yield. A close association between leaf area
and yield is often observed, especially when the planting density is not high. Many species have a signif-
icant negative correlation between leaf size and photosynthetic rate calculated on a unit leaf area basis.
The “dilution” effect [43] frequently masks the intrinsic relation between leaf photosynthesis and yield,
resulting in the paradox of no positive or even a negative association. By examining the differences in
photosynthetic rates between soybean cultivars, it was found that two cultivars had lower yields although
their photosynthetic rates were 25% higher than that of another one. It was due to the fact that the latter
had a bigger leaf area (20% higher) and longer leaf functional duration (8 to 10 days longer) than the for-
mer ones [44].
LEAF FUNCTIONAL DURATION From the results mentioned above [40,44], it may be seen that
leaf functional duration is also an important factor affecting yield, and a lower yield is often related to a
shorter leaf functional duration.
HARVEST INDEX When the photosynthesis-yield relation is analyzed, the partitioning of photosyn-
thates to different organs, expressed as a partitioning coefficient or harvest index, must be considered. A
change in the harvest index opposite to that in leaf photosynthesis may mask the intrinsic relation between
leaf photosynthetic rate and yield. It was reported that there was a significant positive correlation between
leaf photosynthetic rate and biomass and seed yield for eight of nine cultivars of dry bean grown in the
field. The only exception had a lower photosynthetic rate but a higher yield because of its very high har-
vest index [24].
RESPIRATORY LOSS The size of respiration loss is closely related to crop yield. Selection for low
leaf respiration has led to yield increases in perennial ryegrass [45,46]. There were considerable varietal
differences in leaf photosynthetic rate during flowering among 11 cultivars of rice, but the low-yielding
tall varieties were not all less efficient in photosynthesis than the high-yielding dwarf varieties. The lower
yield might result from higher photorespiratory activity and inferior ability to transport the postflowering
photosynthates to the developing grains [47].
DEVELOPMENTAL STAGE For many crops, more than half of the economic yield derives from
photosynthesis after flowering. Therefore, photosynthesis at the reproductive stage is more directly re-
824 XU AND SHEN