- Harvest Index
The mechanism by which increase in economic productivity has been attained is by increasing total dry
matter production and by increasing the proportion of assimilate partitioned into the economically im-
portant plant part (i.e., increasing Hl). It should be pointed out that Hl is somewhat misleading when to-
tal assimilate partitioning is considered, for Hl usually accounts for only aboveground parts of a crop.
Dunan and Zimdahl [25] reported that roots accounted for 13.6% and 16.4% of the dry mass of barley and
oats, respectively. Pate et al. [14] did the most complete carbon balance study known. Unfortunately, their
study plant was Lupinus albus, which has a rather large tap root. Also, in considering their data, one must
realize that gas exchange studies account for net photosynthesis of the shoot, yielding low values for ac-
tual photosynthesis and shoot respiration, while root gas exchange represents 24-hr root respiration. Con-
sidering those limitations, Pate and coworkers reported that about 44% of the carbon fixed by net photo-
synthesis was used by roots, with about one fourth of that going into growth. In addition, the root nodules
used about 12% of the photosynthate in growth and respiration. Nodules used additional carbon in the for-
mation of nitrogenous compounds that were supplied to the rest of the plant. These computations did not
account for carbon that may be lost as exudate, in sloughed cells, or use by other organism. Buwalda [26],
in his review of perennial crops, stated that mycorrhizal fungi account for 5 to 10% of total carbon ac-
quired by photosynthesis. He also stated, “For mature plants, root growth is... a relatively small sink for
carbon.” Reports of high root/shoot ratios for plants that have perennial roots and annual shoots [27] do
not negate Buwalda’s statement. Increasing partitioning to roots may enhance water and nutrient absorp-
tion, thereby increasing production efficiency.
The genetic component of increased crop productivity has not been assessed with regard to the pro-
portion of assimilate partitioned to roots; rather an increase in Hl based on analysis of aboveground parts
only has been demonstrated. These changes in Hl have been associated with shorter plants for both small
grains and soybeans [11,12,28] but not with increased rates of photosynthesis (carbon fixed per unit time
and leaf area).
Economic yield is also increased by increasing total dry matter yield without altering Hl. Increase in
total yield, as well as Hl, is influenced by cultural practice, environment, and genotype. Gifford [12] com-
piled data for several crops to determine the basis of increased crop production over the years. Those data
indicate that total shoot yield and Hl have increased for all of the crops he studied. Hl was increased by
genetic selection for high yield.
- Source–Sink Ratios
It is also clear that an alteration in the pattern of assimilate partitioning occurs in response to crop thin-
ning or removal of plant parts by pruning, herbivory, or violent weather. Various manipulative experi-
ments have been performed to develop an understanding of the control of assimilate partitioning. One in-
volved bean seedlings with fully expanded primary leaves as the source and a small, rapidly expanding
first trifollate as a sink. The experimenters removed the terminal leaflet of the trifoliate, leaving the two
lateral leaflets as sinks for each primary leaf, respectively [29]. Using^14 CO 2 , they demonstrated that each
leaflet received ~80% of its carbon from the nearest primary leaf. When one primary leaf was removed,
the amount of assimilate translocated into the two leaflets did not decrease. The remaining primary leaf
became the source for both, doubling its export to the leaflets without changing its rate of photosynthe-
sis.
Loss of leaf area from insect, hall, or experimental desiccation [30] during grain or seed filling does
not decrease production in proportion to the loss of leaf area because there is an increase in the utilization
of stored carbohydrates. In addition, over longer time periods than in the experiments cited, remaining
leaf tissue increases its rate of photosynthesis. Water stress can also influence partitioning [31].
How does this redirection of partitioning occur? The short-term response is as indicated by the work
already discussed [29]. We should think of the vascular system of plants as a pipeline distribution system
that runs vertically in the stem with interconnections at nodes. Pressure is greatest where loading is great-
est and least where unloading is greatest. Flow follows pressure gradients. In addition, greatest resistance
in the system is across the stem at the nodal interconnections. Therefore, with no perturbations within the
system, most translocation is vertical with little movement across nodal interconnections [32]. However,
if the system is altered by removal of sources or sinks, cross-movement becomes significant. Longer term
adjustments are made in photosynthetic rates and in utilization of stored assimilates.
PRODUCTION-RELATED ASSIMILATE TRANSPORT 423