It is now widely accepted in many textbooks that phytochrome (P) responds to light as represented in Fig 1.
Red light, 660nm
Pr
Pfr
Far red light, 730nm
Reversion in darkness
The schematic shows that the Pr form of phytochrome absorbs red light strongly and stabilises into Pfr. In turn, the Pfr form absorbs far-red light strongly and stabilises as the Pr form. Additionally, in the absence of any radiation, the Pfr form gradually reverts to the Pr form. The two forms of phytochrome also do not appear to be equally susceptible to conversion when exposed to the appropriate light. In red light, only 81% of Pr is converted to Pfr. Conversely, Pfr is 98% converted to Pr when exposed to far-red light. This is due to the overlap in wavelength sensitivity for both forms of phytochrome (Toole 1973). Reversion to Pr in darkness was also shown to be encouraged at temperatures higher than optimal for germination (Borthwick et al. 1954).
In seeds requiring light for germination, it is the Pfr form which triggers germination (Borthwick & Hendricks 1960; Hodson & Bryant 2012). However, because seeds of different species contain differing amounts of phytochrome, sensitivity to light also varies. Seeds with high amounts of phytochrome will require less red-light to germinate. In some cases, high phytochrome content in seeds, coupled with incomplete Pr conversion under far-red light, leads to possible germination even under poor red-light conditions (Koller et al. 1964; Toole & Borthwick 1968).
Phytochrome is a resource formed only when needed. The conditions under which phytochrome may form was shown by Kendrick et al. (1969) using Amaranthus caudatus, a species which germinates readily in darkness (>90% germination) so long as far-red light is not present to inhibit germination. Dry Amaranthus caudatus seedsdid not exhibit any photoreversible absorption activity, suggesting a lack of phytochrome. Evidence of Pfr activity was detected only in imbibed seeds, allowing germination even in darkness. Initial levels of phytochrome remain constant for about 5 hours, after which another increase in phytochrome level is observed until 72 hours after imbibition. It is postulated that the initial appearance of phytochrome may be due to a rehydration of inactive phytochrome after imbibition. The subsequent increase is hypothesised to be due to active metabolic processes. To test if seeds actively produce phytochrome de novo, seeds were exposed to 0oC after imbibition. The cooled seeds did not exhibit the second phase of phytochrome increase. The second surge in phytochrome levels was also found to occur in seeds kept in darkness but exposed to far-red light to inhibit germination; hence it is present in seeds which were not even germinating. Furthermore, this second increase in phytochrome levels occurred 10 hours before rapid water uptake of seeds as part of the germination processes were noted, indicating that it was not due simply to simple rehydration of more inactive phytochrome.
Whilst phytochrome plays an important role in signalling readiness for germination, seeds still need to be fully mature to respond to its effect on germination. Should there be ample red light formation of Pfr when the seed is not physiologically ready to germinate, it will have no effect (Fujii
1969). Seed readiness to germinate may involve factors such as presence of water, suitable temperature, and duration of suitable conditions. When seed readiness is not met, decline of total phytochrome or conversion to Pr isomer may occur to keep seeds dormant; the rate of Pfr decay is influenced by the proportion of Pfr:Ptotal (Kendrick & Frankland 1969).
It is widely demonstrated that the typical phytochrome reaction to having red light triggering germination is negated by a subsequent flash of sufficient far-red light. However, if seeds are exposed to far-red light too long after the red light exposure, there is no inhibition of germination. Hence Pfr can be deduced to require a minimum period of time for it to complete the germination triggers before conversion to Pr will no longer prevent germination. This period of time before germination cannot be prevented and varies according to species (Toole 1973), temperature (Yaniv & Mancinelli 1968), possibly pH (Anderson et al. 1969), and an aerobic environment (Fujii 1963).
Seeds which do not have light requirements for germination may also be under the effects of phytochrome. This was shown by red and far-red light effects on the germination of tomato seeds consistent with phytochrome behaviour (Mancinelli et al. 1966). Subsequent experiments with seeds of tomato mutants deficient in the phytochrome gene confirmed the role of phytochrome for germination in seeds which are light-independent (Appenroth et al. 2006). The presence of Pfr in dark-germinating seeds led to some speculation that there was a form of phytochrome which had an “inverse reversion” in the dark where Pfr was the preferred isomer in the absence of irradiation (Spruit & Mancinelli 1969). This is inconsistent with thermodynamics since the Pfr isomer was unstable in the absence of radiative energy. The presence of Pfr in dark germinating seeds is now known to be derived from incomplete reversion of Pfr to Pr , with the process unable to proceed beyond the first intermediate compound due to dehydration. This intermediate slowly reforms Pfr in darkness (Kendrick & Spruit 1974). In effect, seed dehydration acts to “store” Pfr during dark periods by preventing reversion. It therefore appears that phytochrome is present in seeds regardless of whether germination for that species is dependent on light.
Some seeds are inhibited from germination by light exposure. Well-known examples of this anomaly are Phacelia sp.,Nemophilia sp., and Nigella sp. This is an adaptation to dry summers and wet mild winters of their respective native regions (Baskin & Baskin 1971; Cruden 1974). The mechanism was postulated to be purely physical resistance from the hard endosperm, whereby light prevents necessary development by the embryo from exerting sufficient force to break out (Chen 1968). However, Rollin et al. (1970) showed that maximum inhibition occurred upon exposure to 717 nm far-red light, which strongly indicates phytochrome involvement, although the study did not
offer any proposed mechanism. Similar results for Nigella seeds with 720 nm far-red exposure are documented (Pamukov & Schneider 1978).
Underneath a leaf canopy, soil level light is typically filtered through foliage above. As most leaves contain pigments which strongly absorb red light for photosynthetic purposes, it is the norm for leaf-filtered light to be richer in far-red light and deficient in red light. With the low height levels of many ground cover plants, the likelihood of ambient light to be reflected in under the canopy from nearby surfaces is also reduced. Thus the area under ground cover foliage is likely to have low overall light levels which are particularly poor in red light content after filtration by leaves, producing an environment in which light-sensitive seeds would be starved of the light trigger needed for germination.
In addition, research has shown that many plant species, including those which have demonstrated an ability for their seed to germinate in total darkness, can be induced to a state of seed dormancy when placed in otherwise suitable conditions under leaf shade (King 1975; Górski & Górska 1979; Fenner 1980; Silvertown 1980). This shows phytochrome to be sensitive not only to the detection of light, but also to the quality of ambient light, especially the ratio of red to far-red light (R:FR) (Batlla et al. 2000). This sensitivity to poor light quality is also extended to detection of incipient canopy encroachment as shown by Batlla’s (2000) work where seeds of some species failed to germinate despite conditions where leaf area index of shading canopy was less than 1 and R: FR of ambient light was above 0.8. In addition, inhibition by plant shade seems to be conditional upon prolonged exposure similar to a photoperiod rather than by short pulses or intermittent exposure (Batlla et al. 2000; Benech-Arnold et al. 2000).
Such a development might prevent a seed from germinating not only when there is insufficient light, but also when the seed is likely to be under shade from other plants, unless the species has developed adaptations for a shady understorey habitat. King (1975) further postulated that it would be of particular advantage for smaller-seeded plant species to possess this germination safeguard, given the smaller food store in smaller seeds, and that most species identified in his study are smaller-seeded. As many weed species are opportunistic in reproductive strategy and may also be wind-dispersed, being smaller-seeded is common. The environment under ground cover foliage shade may thus prove additionally challenging for species which have discriminatory seed preferences for germination. The complexity of phytochrome responses to light is not fully understood, but molecular research has shown at least two different forms of phytochrome coded by different genes and differing in protein sequences (Bewley & Black 1994).
While being principally a light-sensitive protein molecule, phytochrome is the principal determinant with regards to germination response when seeds are confronted by multiple combinations of dynamic environmental factors. An understanding of phytochrome response and its underlying mechanisms may be usefully manipulated to assist in weed control, by avoiding weed seed germination altogether.