Crop plants acquire a characteristic shape or form by correlated growth of component parts. Component parts also have a characteristic shape or form that is repeatable in time and space. A favorable environment can enhance growth quantitatively, but the geometry of the parts and the whole is relatively constant.
Allometry:
The relationship between the growth rates of individual parts of an organ or organism is referred to as allometry. The relationship between two variants (X and Y) may be expressed as Y = bxK, where x and y are physical parameters, and b and K are constants, K being the allometric constant. The quantity K can be calculated from the equation log y = log b + K log x. It may be obtained by plotting y against x on a double logarithm scale, which produces a straight line, the slope of which is K.
It also may be determined by linear regression analysis of the data set y and x. If the length and breadth of an organ, such as a leaf, expand at the same rate, the slope of the regression line (the coefficient of allometry, or K) is 1.0; the growth rates of the two parameters are perfectly correlated. Hammond (1941) showed that the allometry of normal and okra-type leaf of cotton was highly heritable and controlled by a single gene.
Allometry coefficients of top and root relationships are based on dry weights rather than dimensions and usually exhibit a lower K. The harvest index (proportion of seed weight to whole plant weight) has a relatively high coefficient of allometry and is a stable parameter in time and space.
Although allometry usually deals with physical parameters of the plant, it logically follows that physiological processes are correlated. Allometry calculations of various correlations can provide useful approximations, but it can be shown mathematically that they are not exact.
Shoot-Root Ratio:
The allometry of top growth to root growth, usually expressed as the shoot-root (S-R) ratio, has physiological significance, since it can reflect one type of tolerance to drought stress. Although the S-R ratio is under genetic control, it is also strongly influenced by environment.
Murata (1969) showed that N fertilization had a pronounced influence on the S-R ratio of rice (Table 8.2). Under a high-N regime approximately 90% of the photosynthate was partitioned to the shoot, compared with only 50% to the shoot under low N. New shoot growth, stimulated by N, was a stronger assimilate sink than were roots.
A deficiency of water, while curtailing both top and root growth, had a relatively greater effect on top growth. Tops are favored differentially when N and water are plentiful; roots are favored when these factors are limited, as reflected by S-R ratios. Roots have the first access to water, N, and other edaphic factors. Tops have the first access to light, CO2 or climatic factors.
Apical and Lateral Growth:
Plants assume a characteristic form or geometry largely due to the extent of growth from apical and lateral buds. Growth from lateral buds can profoundly change plant shape and appearance.
Lateral growth as new shoots normally arises from buds in leaf axils, frequently from the compacted nodal section of the basal stem referred to as the crown. New shoots can also emerge adventitiously from any position. The end result is that plants tend to fill the space available to them, an advantage in natural survival and productivity. Light is a primary factor controlling growth from lateral buds.
Vegetative and Reproductive Growth:
Reproductive growth in annual plants appears to make virtually absolute demands on assimilate. In annuals vegetative growth is generally terminated by reproduction. Leaves, stems, and other vegetative parts not only fail to compete for current assimilate during ripening of fruits but to some extent may sacrifice previously accumulated carbon and minerals via mobilization and redistribution. This process accelerates senescence and eventually results in the death of the plant.
Perennials appear to make only a partial commitment to reproduction; that is, shoots that bear fruits may remain healthy or even if they die, new vegetative shoots are generated from axillary buds to replace them at senescence of the fruiting shoots.
Perennials such as apple and citrus trees do not appear to be greatly affected by the presence of ripening fruits. The shoots of perennial herbaceous grasses and legumes that bear fruits usually senesce like annual plants, but new shoots arise from crown buds, which results in perennation.
Growth and Differentiation:
Plant development is a combination of a host of complex processes of growth and differentiation that leads to an accumulation of dry matter.
Differentiation processes have three requisites:
(1) Available assimilate in excess of most metabolic uses.
(2) A favorable temperature.
(3) A proper enzyme system to mediate the differentiation process.
If these requisites were met, one or more of three differentiation responses occurred:
(1) Cell wall thickening
(2) Deposit of cell inclusions
(3) Hardening of the protoplasm
The latter is important in preventing protoplasmic damage from natural stresses, such as cold, heat, or drought. For example, well-hardened nursery stock or transplants can be set out with more success than non-hardened or tender stock.
The first essential to differentiation processes is availability of carbohydrate, assuming the necessary enzyme system is in place. Assimilate in excess of growth requirements normally results from factors that curtail growth without curtailing photosynthesis.
Factors that limit growth more than they limit photosynthesis, such as water or N deficiencies, result in surplus photosynthate to drive the differentiation process, given a favorable temperature and necessary enzymes.
Cell wall thickening, secondary-product accumulation (e.g., alkaloids and starches), and protoplasm hardening may occur, depending on enzymes and temperature. These chemical changes can result in changes in anatomy and morphology.
The production of quality crop products often requires production strategies that achieve an appropriate balance between growth and differentiation. Growth is essential but generally should not be favored (for example, with water and N) as to preclude differentiation.
Cereal crops grown under high-water and high-N regimes, especially with low irradiance (as in thick stands), have thin- walls in the stems and tend to lodge. Limiting these factors will cause the reverse. Thin cell walls are desirable in the petioles of celery for tenderness, so the object is to promote petiole growth with adequate supplies of water and N and to reduce differentiation by these treatments and by shading the petioles to reduce assimilate.
Management strategies for cell inclusions are essentially the same. Sugar beet accumulates sugar poorly if N and water are excessive. While N promotes biomass yield, the percentage of sucrose is negatively correlated with soil N. High rates of N decrease the yield of sucrose per land unit. Cool nights are also necessary for sugar accumulation.
After production of good assimilatory and sugar-storage systems in the vegetative growth, which requires most of the growing season, then high radiation, cool temperatures, and less-than- optimum N and water levels are needed (as are typical of northern latitudes).
Production of quality sweet melons requires similar management strategies, except for the cool temperatures. Sandy soils are used for commercial production of melons in humid areas in order to control the levels of water and N during ripening. Melons produced on heavy soils in humid areas will usually be larger but lack sweetness (cell inclusions).
Alfalfa accumulates starches in fleshy taproots in sunny, cool fall days. High irradiance during the day and cool nights favor excess assimilate, since photosynthesis output exceeds growth and maintenance respiration requirements. The result was accumulation of carbohydrate food reserves in the taproots and hardening of the protoplasm for overwintering.
Harvest Index:
Harvest index reflects the proportion of assimilate distribution between economic and total biomass. Harvest index is similar to the partitioning coefficient. It is important to stress, as has Stoy (1969), that translocation to the metabolic sinks (e.g., roots, new shoots, and developing fruits) is extremely complex and that the mechanism or attracting force that directs or regulates partitioning to the metabolic sinks is not known.
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