C. Stomatal and Nonstomatal Limitations to Leaf Photosynthesis
Under stress conditions such as water deficiency and low temperature, declines in both leaf photosyn-
thetic rate and stomatal conductance are often observed. To determine correctly the cause-effect relation
between the two declines, an analysis of the stomatal limitation of photosynthesis must be made accord-
ing to criteria suggested by Farquhar and Sharkey [14]. A decline in intercellular CO 2 concentration (Ci)
indicates that the main cause of decline in leaf photosynthetic rate is a decrease in stomatal conductance.
In contrast, an increase in Ci suggests that a decrease in photosynthetic activity of mesophyll cells, namely
a nonstomatal factor, is the main cause of the decline in leaf photosynthetic rate. It appears that the di-
rection of Ci change is the most important criterion for analysis of the stomatal limitation of photosyn-
thesis.
If some unreliable criteria are used in the analysis, an incorrect conclusion may be reached about the
cause-effect relation between changes in leaf photosynthetic rate and stomatal conductance. It has been
emphasized in a review [15] that (1) a necessary criterion of predominantly stomatal limitation is a de-
creased Ci rather than a positive correlation between leaf photosynthetic rate and stomatal conductance;
(2) an important criterion of predominantly stomatal limitation is the direction rather than the extent of Ci
decrease; (3) a reliable criterion of predominantly nonstomatal limitation is an increased rather than a con-
stant Ci; and (4) a boundary line of predominantly stomatal and nonstomatal limitation is the direction of
change in stomatal limitation value rather than the relative magnitudes of stomatal and nonstomatal lim-
itation values.
D. Relationship between Leaf Photosynthetic Rate and Crop Yield
Except for the mineral nutrient elements, accounting for about 5% of the total, all of the dry matter of crop
plants is derived from photosynthetic CO 2 assimilation. From this fact it is naturally expected that a high
photosynthetic rate will lead to a high yield or that there is a positive correlation between leaf photosyn-
thesis and crop yield. However, a positive relation is not often observed, and it has been stated that in most
cases there is no association between them and in some cases even a negative correlation between leaf
photosynthetic rate and yield [16]. This paradox puzzled many plant physiologists and agronomists for
quite a long time [17]. In fact, the apparent lack of a positive correlation is not surprising because this is
a very complex problem and leaf photosynthetic rate is an important but not the sole factor determining
crop yield.
- Reflection of the Essence—Positive Correlation
It is well known that about 95% of the dry matter of plants comes from photosynthesis. This fundamen-
tal fact determines that the essence of the relationship between leaf photosynthetic rate and crop yield is
positive but not negative or no correlation.
The economic yield (Y) of crops is a function of photosynthetic production, respiratory consumption
(R), and harvest index (HI). The amount of photosynthetic production depends on photosynthetic rate (P),
leaf area (A), and photosynthetic duration (T). Their relationship can be very roughly expressed in the fol-
lowing equation:
YHI (P A TR)From this equation, it is very clear that Y must increase when P increases and Y must decrease when P
decreases provided that HI, A, T, and R remain constant. A positive relationship between P and Y is in-
trinsic. Therefore, an increase in P caused by some treatment such as CO 2 enrichment in soybean and rice
[18–20], spraying water in wheat [13], spraying sodium bisulfide solution upon leaves of wheat and rice
[21], and improving N nutrient in soybean [22] always leads to an increase in crop yield. On the contrary,
shading treatment leads to a decline in yield due to decreased photosynthesis [23].
Positive relationships between leaf photosynthetic rates and plant productivity, indeed, have been re-
ported for dry bean [24], wheat [25,26], soybean [27], blackgram [28], green gram [29], pea [30], cassava
[31], grain sorghum [32], upland cotton [33], and asparagus [34]. It appears that a significant positive cor-
relation between light-saturated photosynthetic rate and yield among the cultivars of many crops is a re-
flection of the rule rather than the exception [35]. In addition, there have been some reports indicating that
cultivars with higher yields have higher photosynthetic rates in soybean [36,37], oats [38], and rice [39].
PHOTOSYNTHETIC EFFICIENCY AND CROP YIELD 823