New Phytologist, 128(2J): 347-362 (Oct 1994)
Control of Flowering and Reproduction in Temperate Grasses
O. M. Heide


Temperate grasses of the subfamily Festucioideae can be grouped into two main categories according to their environmental control of flowering, species with a regular long day (LD) induction, and those with dual induction requirements. The former group includes the temperate annual grasses and a few perennial species such as Phleum pratense and Poa nemoralis. These species have 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°) 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°, at which primary induction cannot take place regardless of the photoperiod. In a few species, e.g. Bromus inermis, Phalaris arundinacea and to some extent Dactylis glomerata, 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 arcticalpine Poa species to 20 wk in some Festuca species. Generally, ecotypes from high latitudes and especially arcticalpine 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 floral 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 floral 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.