Fig. 7.5. Spectral photon distribution in the 400-800 nm wavelength region on an overcast day in: (a) full daylight, and (b) filtered through a canopy of ash (Fagus) (redrawn and adapted from Pons, 2000).
□ Seedling emergence to floral primordium
□ To first flowering
■ To initiation of seed set S To completion of seed set
er p o12/12
20 40 60 80 100
Days (after emergence)
Fig. 7.6. The effect of photoperiod on the duration from emergence, to floral primordia, to first flowering, to seed set initiation and to completion of seed set in wild mustard (from data in Huang et al., 2001). Different letters within each developmental stage indicate significant differences. The lengths of the final developmental stage were not different.
er R:FR ratio promotes flowering in some species; for example, in shaded conditions, oxeye daisy allocates more resources to reproduction, resulting in higher seed production than individuals in sun (Olson and Wallander, 1999).
Photoperiod is what synchronizes many of the seasonal events observed in nature. Alternating light and dark cycles give an accurate cue as to the time of year; temperature can vary unpredictably from day to day, and light quantity and quality is altered by the surrounding vegetation. Thus, pho-toperiod is often a trigger for important phe-nological events such as reproduction. In wild mustard, for example, photoperiod influences many reproductive events (Huang et al., 2001). As photoperiod shortens, the time from seedling emergence to floral pri-mordia increases, that from flower primordia to flowering increases, but the time from flowering to initiation of seed set decreases (Fig. 7.6). When photoperiod increases above 18 h, the timing of phenological events is not affected. Photoperiod also effects the morphology of some species. In wild mustard, plants grown in 12-20 h of day length are taller than those grown at 22 h (Huang et al., 2001). Furthermore, plants at 12 and 14 h of day length have more leaves, while plants in 22 and 24 h day length have larger inflorescences than plants grown in other day lengths.
The flowering response to photoperiod can take many forms. Species responses may be triggered by short days, long days, long days and then short days, or by other variations in the sequence. Table 7.1 summarizes flowering responses to day length and gives some weed examples. A response to pho-toperiod typically requires several cycles to occur. Some species, however, only require
Table 7.1. Types of photoperiod responses and examples of representative species (adapted from Salisbury and Ross, 1985).
Red goosefoot, Chenopodium rubrum Goosefoot, Chenopodium polyspermum Common cocklebur, Xanthium strumarium Redroot pigweed, Amaranthus retroflexus Scarlet pimpernel, Anagallis arvensis White mustard, Sinapis alba Henbane, Hyoscyamus niger White clover, Trifolium repens Kentucky bluegrass, Poa pratensis Purple nutsedge, Cyperus rotundus Hooked bristlegrass, Setaria verticillata Onion, Allium cepa Wild carrot, Daucus carota Barnyardgrass, Echinochloa crus-galli Indian goosegrass, Eleusine indica Portulaca, Portulaca oleracea Itchgrass, Rottboellia exaltata one cycle to trigger a response. For example, a single short-day cycle will induce flowering in redroot pigweed. While we refer to the importance of day length, it is actually the length of dark period that usually triggers a response. For example, a 1-h interruption of fluorescent light during the dark period will inhibit flowering of redroot pigweed (Gutterman, 1985). Plants that respond to photoperiod generally go through three stages of sensitivity: a pre-inductive stage where photoperiod has no effect, an inductive phase where photoperiod triggers reproductive response, and a post-inductive phase where reproduction will continue irrespective of photoperiod (Patterson, 1995).
Because photoperiod changes with latitude, there are often 'biotypes' of species. A biotype is a group of individuals that have similar genetic structure but respond to their environment in different ways than other biotypes of the same species. For example, there are biotypes of common lambsquarters and cocklebur (Xanthium strumarium) which respond differently to photoperiod. Northern biotypes usually require shorter nights to initiate flowering. Patterson (1993) examined the differences between
Mississippi and Minnesota populations of velvetleaf. The Minnesota population produced more vegetative growth than the Mississippi in short days (12 h light), but the reverse was true for long days (16 h light). In the longest photoperiods, Minnesota plants allocated resources to reproduction, thereby limiting further vegetative growth. Northern populations of side-oats grama (Bouteloua curtipendula) are long-day ecotypes while southern populations are short-day ecotypes (Olmsted, 1944).
We have looked at how plants respond to changes in temperature and light separately. Data of this type are usually derived from controlled experiments where one variable is changed and all others remained constant. Such results do not necessarily reflect real situations for several reasons. First, environmental variables tend to fluctuate in tandem (when the sun comes out, light and temperature increase); therefore manipulation of one variable is not realistic. Secondly, plants respond in a complex fashion to the array of environmental factors they face and this may not be predictable by looking at one factor at a time. The response to one environmental factor will affect how an individual responds to another factor.
Reproduction is often determined by an interaction between photoperiod and temperature. For example, at low temperatures, poinsettia (Euphorbia pulcherrima) and morning glory (Ipomoea purpurea) are long-day plants but they flower in short days at high temperatures. At intermediate temperatures they are day neutral. Interactions of temperature and light occur for other processes. Kikuyugrass (Pen-nisetum clandestinum) exemplifies the interactions of environmental factors and how they can affect weeds (Wilen and Holt, 1996). Kikuyugrass is an introduced turfgrass from the tropics that became a weed in temperate climates. The reason is that it is able to maximize photosynthesis during warm temperatures (25-40°C), as expected from a tropical grass, but it also has high photosynthetic rates during the still-long day lengths of the Mediterranean zone in spring and autumn despite cooler weather. The physiology of kikyugrass is such that lower temperatures do not act to inhibit photosynthesis. We might expect that the result of an interaction of lower temperatures with abundant light would inhibit a tropical grass but this is not the case. Most successful weeds are able to accommodate and maximize growth over a wide range of light and temperature interactions (see also Plowman and Richards, 1997; Roche et al., 1997; Kibbler and Bahinsch, 1999).
Water stress can effect the plant's ability to respond to other environmental triggers. For example, water stress can limit or prevent flowering in single-cycle photoperi-od species such as common cocklebur and rye grass (Lolium temulentum) (Chiariello and Gulmon, 1991). Conversely, water stress may promote flowering in some species (e.g. siratro, Macroptilium atropurpureum, a tropical pasture legume).
Before a plant can respond to an environmental cue, it must reach a phenological state where it is able to sense the cue. Cocklebur, for example, must reach a certain size before it responds to photoperiod cues. In this species, an individual leaf must be at least 1 cm long before it will respond to light, and the most sensitive leaves are the fastest growing ones that are half-expanded (Salisbury and Ross, 1985). The cotyledons of lambsquarters (Chenopodium spp.) will respond to photoperiod and thus even tiny seedlings may flower given the appropriate light conditions. The long-day plant scarlet pimpernel (Anagallis arvenis) is most sensitive as a seedling; new leaves on older plants will respond, but they are less sensitive (Salisbury and Ross, 1985).
The same environmental cue will trigger different phenological events depending on the species. Mahal and Bormann (1978) identified four phenological patterns of understorey species in New Hampshire northern hardwood forests. Spring ephemer-als germinate and complete their life cycles early in the spring, taking advantage of higher light conditions before tree canopy closure. Summer green species emerge soon after the death of spring ephemerals and maintain their leaves until the autumn. They are shade-tolerant species. Late summer species develop slowly over the summer but persist into the autumn taking advantage of higher light conditions after the tree canopy opens. Finally, evergreen species are perennials that maintain their leaves for up to 3 years. They produce new leaves in the spring under higher light conditions and these are then maintained throughout the summer and into the following years. These four phenological patterns are subject to the same environmental triggers, but respond to them in different ways. Weeds also exhibit similar phenological patterns. In North America, dandelion (Taraxacum officinales) and coltsfoot (Tussilago farfara) flower in the spring, ox-eye daisy (Chrysanthemum leucanthemum) and vetch (Vicia sp.) in the summer, and common ragweed (Ambrosia artemisiifolia) and goldenrods (Solidago spp.) in the autumn.
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