Role of environment on Initiation and development of the inflorescence in Pears
Initiation and development of the inflorescence
In the summer and autumn of the year preceding the flowering, the pear inflorescence is initiated. After differentiation of the floral primordia the sepals, petals, stamens, and carpels are formed in succession. In this way, before the start of winter, a complete inflorescence grows within the bud. In the spring the flowers continue to develop. Pollen meiosis takes place about a month before flowering, and before the anthesis begins at the end of April or the beginning of May (Staritsky, 1969), the egg-cell does not develop.
Luyten and De Vries (1926) made a detailed study at Wageningen of the pear cultivar ‘Beurré Hardy’. According to these authors, in August the inflorescence begins. In 1923 initiating took place some days later than the previous year. There were juvenile growth stages of the inflorescence up to November in both years. Felius (1954) gives information dating from 1952 on the flower initiating of six pear cultivars for the north, center and south of Holland. Differences in the initiation date of the pear cultivars were two to three weeks in the same district. The dates are given by Felius (1954) (from 16th July to 13th August) represent the approximate period of initiation. In western Germany, for five years, Zeller (1958) studied the growth of the inflorescence of three pear cultivars. The initiation date showed considerable variation from year to year. Differences of nearly a month were found in the same cultivar Depending on the research year and the cultivar, the inflorescence is initiated at Hohenheim, Germany from the beginning of July to the beginning of September.
The sub-divisions of Luyten and De Vries (1926) is shown below:
2. Expansion of the growing point (primordia of the first flowers + bracteoles appear in the form of a single growing point)
3. The first primordia differentiate into bracteoles and the primordium of the actual flower
4. The sepal primordia are initiated (the inflorescence ‘s terminal growing point begins to break off bracts)
5. The petal primordia are initiated (the terminal flower is now formed)
6. The 10 primordia of the first whorl of stamens are initiated
7. The primordia of the second (5) and third (5) whorl of stamens are initiated
8. The 5 primordia of the carpels are initiated
Role of environment in flower induction
Some external factors as temperature and day-length have a marked indirect effect on the initiation and development of the inflorescence of the cultivated pear and apple. This is clearly shown by the fact that in young apple trees the periodicity of the development may be absent, with the result that inflorescences may be initiated practically throughout the year (Zeller, 1960-1, II). It was found that the photoperiod had some effect on flower initiation in a number of apple cultivars (Gorter, 1965; Hillman, 1962), but other cultivars, on the other hand , showed no response to day-length differences (Gorter, 1955).
Plants interpret environmental conditions ( e.g., temperature and photoperiods) through complex molecular networks and endogenous signals (e.g., plant hormones, oxidative stresses and energy metabolism) that monitor growth and dormancy cycle (i.e., growth cessation, dormancy phases and budbreak) in the shoot apical meristem (SAM) and more generally in buds.
The internal factors affecting the flower initiation of fruit trees have been dealt with in detail by Kobel (1954). Generally speaking, robust vegetative production is associated with low and erratic inflorescence initiation. Inhibited growth leads, as is also the case with Pyrus communis, to successful and frequent initiation.
It seems that owing to the production of auxins, such growth centers as rapidly growing shoots, apical meristems (apical dominance), fruits, and in some cases leaves, have an inhibiting effect on flower induction and development (Feucht, 1961). Zeller (1954) found that the fruit had no effect on the initiation of inflorescences.