Flower initiation marks the transition from vegetative to reproductive growth in seed plants. It is thus a crucial event in the life of these plants, particularly so because of the peculiar relation of vegetative and reproductive development in seed plants which is in turn an outcome of the morphological nature of the flower. Flowers are modified shoots which are produced by modified shoot meristems, the flower primordia. However, once a meristem has been determined as a flower primordium, it is usually unable — except perhaps at the very earliest stages — of reverting to vegetative growth. Vegetative growth and reproductive development in seed plants are thus in a certain sense mutually exclusive; as far as a particular meristem is concerned, flower initiation means the end of its life. The central problem of the physiology of flower initiation is to understand which factors cause a shoot meristem to become a flower primordium, and how they consummate their action.

Physiology of flower initiation

Temperate grasses of the subfamily Festucioideae can be grouped into two main categories according to their environmental control of flowering, species with regular long day (LD) induction, and those with dual induction requirements. The former group includes the temperate annual crosses and a few perennial species such as Phleum pratense and Poa nemoralis. These species base no winter requirement and require only LD to flower. Most temperature perennial grasses have a dual induction requirement for flowering, a primary induction which is brought about by low temperature (vernalization) and/or short days (SD), and a secondary induction which requires a transition to long days and is enhanced by moderately high temperatures. In most dual induction species SD and low temperature are interchangeable and independently able to fulfil the primary induction requirement. Yet, they are highly interactive in this process. Commonly the plants become day neutral at low temperature (0-6 °C) and primary induction takes place in both SD and LD. Primary induction is then identical with the common vernalization response. At higher temperatures induction becomes increasingly dependent on SD. until a critical temperature is reached, usually c. 12-18 °C, at which primary induction cannot take place regardless of the photoperiod. In a few species, e.g, Bromus inermts, Phalaris arundinacea and to some extent Dactylis glomeratca, the SD response predominates while low temperature induction is weak or absent. Critical temperatures and photoperiods for primary induction vary greatly among species and, within the species, among ecotypes of different geographical origin. Critical exposure time may vary from 3-4 wk in arctic-alpine Poa species to 20 wk in some Festuca species. Generally, ecotypes from high latitudes and especially arctic-alpine ones, have wider temperature and daylength limits and require fewer inductive cycles for primary induction than their low-latitude counterparts. In some grasses, especially- arctic-alpine species, initiation of inflorescence primordia takes place during SD primary induction, in others it requires a transition to LD. In the former group, primordia are initiated in the autumn, an important adaptation to arctic-alpine conditions. Critical photoperiods for secondary induction vary from 9-10 h in Mediterranean ecotypes to more than 16 h, and the critical number of LD cycles from four to eight, whereas 12-16 LD cycles are needed for the full saturated response. Generally, high-latitude ecotypes have longer critical photoperiods and require more LD cycles for secondary induction than do those from lower latitudes. Culm elongation, heading and inflorescence development are all promoted by LD. The more inductive cycles given and the more favourable their daylength, the greater is the response. Grasses also have efficient vegetative means of reproduction which are also environmentally controlled. Vegetative proliferation of inflorescences or 'vivipary' is readily induced in habitually seminiferous grasses of both LD and dual induction types, by marginal LD induction of flowering. On the other hand, a high proportion of normal flowering can be obtained in habitually viviparous species and ecotypes by optimal primary and secondary Moral induction. Thus, sexuality is by no means entirely suppressed in viviparous species but is under environmental control. In the high-latitude environment the primary induction requirements are met: by the decreasing daylength and temperature of autumn and winter, while the increasing daylength and temperature of spring and early summer fulfil the secondary induction requirements. Thus, the dual Horal induction control system of the temperate perennial grasses provides an efficient and important mechanism for fitting their life cycles to the dramatic seasonal changes of the high-latitude environment in which they live.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1994.tb04019.x

Control of flowering and reproduction in temperate grasses

Individual tillers of timothy (Phleum pratense) growing from seed in pots were labelled and their life history studied over a period of two years.



The leaf axil position of the first-produced tiller depended on its time of origin. A minimum of five leaves on the parent axis was necessary before its appearance, and no tiller emerged from the axil of any of the three terminal leaves of a shoot.



Ear-bearing capacity was high in early-formed tillers, but decreased progressively with later dates of origin despite favourable day length; all tillers arising after the end of July failed to produce inflorescences. A greater proportion of tillers on the main stem formed ears than secondary tillers of the same age. Time of ear emergence varied only within a few days for all tillers arising before the end of April. The number of leaves formed up to ear emergence varied linearly with date of origin and tiller position, the lowest recorded number being six.



Dry weight and length of inflorescence as well as seed yield per tiller varied linearly with the date of origin of tillers. Primary tillers generally had longer and heavier inflorescences and produced more seed per stem than secondary tillers of the same age.



Tillers arising at all times were found to have a limited span of life, not normally exceeding just over a year. The mean age of ear-bearing tillers varied linearly with their date of origin. Death-rates of all tillers generally increased with their age. Relative death-rates of fertile and vegetative tillers rose rapidly in the autumn but decreased during winter and early spring. Vegetative tillers which elongated in the year of formation survived for a shorter time than equivalent tillers in which stem elongation was delayed until the following year.



The perennial habit of grasses is discussed in relation to tiller survival. Successive new formation and death of tillers keep the plant in a condition of continuous dynamic change.

Growth and nutrition of timothy (phleum pratense)

Influence of Light and Temperature on the Growth of Ryegrass (Lolium spp.)

A comparison of the effects on vegetative growth of 8 hr. and of 16 hr. of light per day at various temperatures was carried out with 9 species of pasture plants grown in New Zealand. The temperatures used were 45°F, 55°F, 65°F, 75°F, 85°F, and 95°F. The light intensity was constant at approximately 2,700 foot candles. Hence in these experiments an increase in photoperiod from 8 to 16 hr. of light was accompanied by a doubling in quantity of light per day. The influence of the various levels of temperature on the rates of growth and tillering of individual species was similar to that found earlier for plants grown with a 12 hr. photoperiod. However, optimum temperatures tended to be lower with 8 hr. than with 16 hr. of light, particularly for rates of tillering. At the lower temperatures the increase in photoperiod from 8 to 16 hr. generally gave only a small increase for rate of tillering and in some cases possibly decreased it; but it generally more than doubled daily growth on an individual ti...

https://www.tandfonline.com/doi/pdf/10.1080/00288233.1962.10419984

Growth of pasture species under controlled environment

The responses of plants to varying photoperiod has received much attention since Garner and Allard (3, 4) first showed that flowering and fruiting of many plants is controlled by the relative length of day and night. Numerous species have been classified as "short day" or "long day" on the basis of the day length required for maximum flowering and fruiting. Temperature also may influence the flowering responses of plants (13), and the two factors may show interaction (11). The experiments with Kentucky bluegrass (Poa pratensis L.) reported here were conducted primarily for the purposes of studying the physiological response of this species to management practices which might influence forage and seed production (9t 10). The formation of flowers in May and June has placed this species in the "long-day'' class. However, Evans and Watkins (2) found that bluegrass plants produced inflorescenses in the spring regardless of length of day, and none was reported in the fall under day lengths ranging from 8.5 to 18 hours. Since plants are dependent upon their leaves for both the stimulus of photoperiod (5, ?) and reserve accumulations, any cultural treatments such as close grazing or clipping of Kentucky bluegrass could influence subsequent seed production, either through induction or through subsequent development of flowers and fruits. An interaction of nitrogen fertilization with photoperiod also was found in studies reported by Borthwick (1).

Maurice L. Peterson

Papers

Effects of photoperiod and temperature on growth and flowering of kentucky bluegrass