Growth and development of a plant proceeds by a sequence of phases or stages that follow in a necessary order. A plant cannot bear a ripe fruit until it has produced an immature fruit. And it cannot produce the immature fruit until it has flowered. And it cannot flower until various conditions — whether internal or external — have been completed.
By comparing two or more specimens, strains or species, we can see that the lengths of the corresponding stages differ from one strain/species to another. One may begin flowering earlier, but take longer to grow and ripen. Another may start late but finish quickly. When we cross two varieties, selected for their differences in phase-length, we may expect some of the progeny to be earlier than either. For example, one strain of cereal may flower earlier than another, but the other may ripen its seeds faster following pollination. Among the F2 or later progeny we may find some that flower as early as one parent, but ripen it seeds as quickly as the other.
As a plant passes through the various phases, its response to external conditions may change. It may respond strongly to certain fertilizers at one phase, but little or not at all in a different phase. There may be differences in susceptibility to infection at different stages, or in the response to photoperiod and temperature.
Ball: Selecting Early Kafirs and Milos (1911)
Earliness can be developed only by continuous selection. Such selections can be made either at heading time or at the time of ripening, but are preferably the results of records made at both periods. When the field or seed plat of the variety begins to head, a number of the earliest heads which are otherwise suitable for selection should be marked by means of tags on which is recorded the date of heading. When the heads on these selected stalks begin to show the characteristic colors and texture of the hard dough, or ripening stage, the date of ripening should be added to the tags. Other things being equal, those heads for which the shortest time has elapsed between heading and ripening are to be considered the earliest. These heads should be carefully saved separately and used for continuing the work another season.
Whyte: Phasic Analysis and Breeding (1960)
Powers and Lyon (1941) studied the inheritance of the duration of three stages of development in certain crosses involving varieties of tomato (Lycopersicon esculentum and L. pimpinellifolium). The three stages recognized by these workers were: (1) number of days from sowing to first bloom; (2) number of days from first bloom to first fruitset; and (3) number of days from first fruit set to first complete change of colour of any fruit. The sum of these three stages is regarded as a measure of the earliness-of-maturity character.
Whyte: Phasic Development (1960)
Whyte: Growth and Development - I (1960)
Whyte: Growth and Development - II (1960)
...we proposed that rust-resistant varieties be produced by means of intraspecific hybridization. This principle consists in crossing varieties which develop rust-resistant properties at different phases and ages. By this method we produced the valuable economic varieties Kubanskaya 131, 133 and others, distinguished for their high yield and high rust-resistant properties.
Int. J Plant Sci. 159(5):695. (1998)
Restoration of Juvenility in Maize Shoots by Meristem Culture
Erin E. Irish and Sarah Karlen
Phase change (the change from nonreproductive to reproductive status) and heteroblasty (ontogenetic changes in vegetative metamers) are two determinants of longitudinal asymmetry in plants. These concepts are critically important to understanding the regulation of plant development as well as morphological evolution and life-history variation.
Development, 124(3): 645-654. (1997)
Phase change and the regulation of trichome distribution in Arabidopsis thaliana
A Telfer, KM Bollman and RS Poethig
Higher plants pass through several phases of shoot growth during which they may produce morphologically distinct vegetative structures. In Arabidopsis thaliana this phenomenon is apparent in the distribution of trichomes on the leaf surface. Leaves produced early in rosette development lack trichomes on their abaxial (lower) surface, leaves produced later have trichomes on both surfaces, and leaves in the inflorescence (bracts) may have few or no trichomes on their adaxial (upper) surface.
Genetics, Vol 119, 959-973 (1988)
Heterochronic Mutations Affecting Shoot Development in Maize
R. S. Poethig
Three semidominant, nonallelic mutations of maize, Teopod 1 (Tp1), Teopod 2 (Tp2) and Teopod 3 (Tp3), have a profound effect on both vegetative and reproductive development. Although each mutation is phenotypically distinct, they all (1) increase the number of vegetative phytomers; (2) increase the number of phytomers producing ears, tillers and prop roots; (3) increase the number of leaves bearing epidermal wax; (4) decrease the size of leaves and internodes; (5) decrease the size of both the ear and tassel; and (6) transform reproductive structures into vegetative ones. The analysis presented here suggests that this phenotype reflects the prolonged expression of a juvenile, vegetative developmental program which overlaps with the reproductive developmental program.
See also: Patterns of Growth and Development
J. Hered., September 1966; 57: 201-202.
Linkage Between Genes for Reaction to Powdery Mildew and for Length of Vernalization Requirment in Wheat
Darrell G. Wells
In the growth chamber difficulty was experienced in crossing the mildew resistant plants, which were invariably late in heading, with normal-heading Chancellor. Backcross plants were either susceptible to mildew and of normal-heading response, or they were resistant and tended to remain vegetative like the resistant parent. The two types of backcross plants occurred in nearly equal numbers. The expected phenotypic classes, however, were four in number: early resistant, early susceptible, late resistant, and late susceptible. The missing classes were early, resistant and late, susceptible, which were recombinations types assuming pleiotropism was not involved.
There was an apparent linkage in Michigan Amber of a factor for resistance with a factor for a long-time vernalization requirement.
CybeRose note: The above note is not exactly on-topic, but relates to the general problem. The author does not state (in the extract) when the early strain is susceptible—before or after floral initiation. It is at least possible that the late-heading variety escapes the mildew season simply because it is late. If so, there is no linkage; one trait (resistance) is a direct consequence of the other. It is also interesting to compare this case with results of raspberry-blackberry hybrids, in which there is no segregation of characters affecting the prickles. See: Heredity 3:103-106 (1949) On An Integrated Species Difference by C. D. Darlington
It is worth noting that the rose 'Peace' originated as a rather ordinary seedling. It did not express its famous vigor until it had matured (i.e., had passed through its juvenile phase).