Photoperiodism is the response of plants to the
relative lengths of light and dark periods within a 24-hour cycle. Plant growth and development processes that are affected by
photoperiod include flowering, vegetative growth, internode elongation;
tuber, rhizome and bulb formation, sex expression, the formation of
pigments such as anthocyanin, the number and size of root nodules, fruit
set, leaf fall and dormancy.
However, the above definition is susceptible of misinterpretation. As early as 1938, Hamner and Bonner demonstrated that it is the long dark period (16 hours) that elicits the flowering response in Xanthium. By interrupting the dark period with a short light break, the plant remains vegetative and fails to flower. But the flowering response is maintained with 16 hours of dark period even though the light period (8 hours) is interrupted with a short period of darkness (Devlin 1975).
Depending on their response to daylength, plants are classified as either long-day plants (LDP), short-day plants (SDP), or day-neutral plants (DNP). LDP and SDP apply to crops that are sensitive to photoperiod while DNPs are not sensitive, that is, they do not exhibit photoperiodism. Long-day plants are those in which crop response occurs when daylength is in excess of a critical length while the response of short-day plants occurs when daylength is below a certain critical length. Photoperiodic response also differs with variety within a species.
Hopkins (1999) provided the following examples of crop types based on their sensitivity to photoperiod:
But within the long- and short-day types, there are qualitative (obligate) and quantitative (facultative) photoperiodic types. Qualitative plants are those in which a particular photoperiod is an absolute requirement for the occurrence of a response. An example is the common cocklebur (Xanthium strumarium), a qualitative short-day plant, that flowers only under a short photoperiod. In contrast, quantitative plants respond in both short and long days but performs best in either. For example, wheat (Triticum sp.) and rye (Secale cereale), which are quantitative long-day plants, will flower even under short days but flowering is more accelerated under long days (Hopkins 1999).
However, there are various special types of responses. Some plants will flower under a long photoperiod followed by a short photoperiod while others do in the opposite sequence. There are plants also in which the degree of flower induction depends on the number of photocycles or photoperiodic cycles. Moreover, photoperiodic response can be modified by some environmental factors, usually temperature.
A comprehensive review by Vergara (1978) sheds more light on photoperiodism. Depending on the desired economic yield, the effects can be either advantageous or disadvantageous and vary with species and with variety.
The induction of flowering is the most studied aspect of crop growth relative to photoperiodism. It is perhaps the most important response of crops to photoperiod. This is so in most crops in which the economic product is the fruit or seed. But in sugarcane, tobacco, and forage crops, it is desirable if reproductive development is delayed or prevented to favor vegetative development.
Light inhibits stem growth but promotes the expansion of leaves. In lettuce and raddish, short days promote higher top:root ratio. This is desirable in lettuce because it is the top that is harvested but not in raddish in which the economic organ is the taproot.
In some varieties of potato, tuber formation is induced by short photoperiod but in others, it occurs only during long days with low temperature. A similar response is exhibited by different varieties of onion in terms of bulb formation.
It has been noted also that photoperiod is associated with abnormalities in sex ratio in cucurbits and other monoecious plants. Gherkins or cucumber (Cucumis sativus) have more staminate (male) flowers but lesser fruit-bearing flowers during long days.
The poinsettia (Euphorbia pulcherrima) naturally produces colorful flowers in December where daylength period is short. According to aggie-horticulture.tamu.edu (n.d.), dark periods of 11 hours and 45 minutes will cause initiation in most cultivars, but initiation is most rapid at 14 to 14.5 hour dark periods at temperatures of 60-70oF (15.56-21.11oC) for 8 to 11 weeks in modern cultivars.
For chrysanthemum, short daylength promotes flowering while long daylength favors vegetative growth. However, Kessler (n.d.) explains that chrysanthemums have different critical photoperiods for floral initiation and for flower development. Further, the critical photoperiod can vary with cultivar and temperature.
The relative length of the light and dark periods affects the production of carbohydrates by all crops (Edmond et al. 1978). In terms of flower initiation, the length of the light period has no effect on photoperiodism. However, it appears to have a quantitative influence on the number of flower primordia initiated. In the Biloxi soybean, it was found that a photocycle with a dark and light periods of 16 and 11 hours, respectively, produced maximum number of flower primordia. Light periods in excess or less than 11 hours produced smaller number of flower primordia (Devlin 1975).
(Ben G. Bareja, May 2011)
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