II. CROP PRODUCTIVITY
A. Assimilate Partitioning
- General Considerations
Total biomass production of a plant is dependent on the balance between photosynthesis and respiration.
Therefore, it might seem appropriate to develop genetic or cultural strategies to control these two pro-
cesses. Cultural practices have been directed primarily toward increasing total biomass by increasing
plant density and use of fertilizer and/or water, with the assumption that increased biomass would result
in increased economic productivity. Yet it has long been known that excessive use of nitrogen or exces-
sively high plant densities often lower production of economically important plant parts.
At critical stages in the life of a plant, environment strongly influences the development of econom-
ically important plant parts. Therefore, cultural practices have the potential of influencing harvest index
(HI). Understanding the timing of development becomes increasingly important as certain inputs such as
water, pesticides, and fertilizer become more expensive, difficult to obtain, or use restricted. For the most
part, genetic selection of crop plants over the past century has altered biomass partitioning (changing HI)
but has not resulted in an increase in biomass [11,12].
Assimilate partitioning (see reviews by Wardlaw [9] and Pollock et al. [13]) includes the partition-
ing of all assimilated materials among plant parts. One of the most comprehensive models for carbon and
nitrogen partitioning was developed by Pate et al. [14]. This topic is important for both theoretical and ap-
plied plant physiology. Control of the incorporation of assimilates into economically important compo-
nents of a crop determines economic reward.
Processes that control assimilate partitioning are cell-to-cell transport, including transfer of mate-
rials between xylem and phloem, loading and unloading of vascular tissues; long-distance translocation
through vascular tissues; and metabolic sequestration of materials such that they are either temporarily
or permanently eliminated from transport processes. Several of these topics are discussed in detail in
other sections but are included here to emphasize their impact on carbon partitioning.
- Developmental Considerations
As plants develop, assimilates are partitioned differently at different times [15]. During seed germina-
tion, the radical elongates first. A day or two later, the plumule growth rate increases. Metabolic con-
trol of the development of source-sink relations in cereal seedlings was discussed by Thomas and Ro-
driguez [16]. As each leaf matures in sequence, it converts from sink to source [17] and enzyme activity
is modified to accommodate these changes [18]. In plants of determinate growth habit (e.g., corn,
wheat, barley, sunflower), essentially all vegetative growth is completed at flowering. For a short time
there are few growing sinks, so assimilate is stored in vegetative parts. As fruit and seeds enlarge, not
only is current photosynthate partitioned into reproductive parts but also, depending on circumstances,
stored materials are remobilized. This remobilization of assimilate from stems of wheat has been well
illustrated (see, e.g., Refs. 19 and 20). However, tomato plants remobilized proportionately less assim-
ilate from vegetative parts to fruit; most of what they remobilized was from leaves [21]. During these
changes, direction of translocation through the phloem often changes from primarily downward to pri-
marily upward.
Patterns for indeterminate annuals appear to be similar, although more complex, for most of their re-
productive growth occurs with the first group of flower, yet they do have a capacity for continued vege-
tative growth and to form later fruit if early fruit is lost. In perennials, there are several variations of par-
titioning patterns. In many plants, however, flowering is followed by early rapid vegetative growth using
assimilates from the previous season. The major portion of assimilate is then used in reproductive growth.
Then assimilate is stored [22]. In some plants such as elm, reproductive growth is completed in early
spring before appreciable vegetative growth occurs. Many other variations could be cited for perennial
plants. For example, Agavedevelops much like a determinate annual but with the pattern extending over
several years.
With development, the chemical mix of materials translocated is modified. Pate [23] reported in-
creasing concentrations of nitrogenous compounds in phloem sap during seed growth of lupine. Glad et
al. [24] reported a similar changing pattern of sieve tube sap composition for grape.
422 HENDRIX