- Day-neutral (“ever-blooming”) plants are unaffected by day length.
Examples: tomato, cucumber, cotton, sunflowers and buckwheat (some varieties are
long-day).
Some plant species have different varieties with different photoperiodic responses. For
example some cowpea varieties are short-day, some are long-day and others are day-
neutral.
The key to photoperiodic response is in fact the length of darkness during a 24-hour
period. If the light (daytime) period is interrupted with a period of darkness, this will not
affect flowering. But if short-day plants are exposed to even short periods of light
during the night, flowering is prevented even though the period of day length is correct.
The number of 24-hour cycles of suitable photoperiods to which plants need to be
exposed to trigger flowering is different for each species. One exposure is enough for
some species while other species need three or four.
Moving Seed from One Country to Another
This photoperiodic response of plants is more than just an interesting botanical feature.
Anybody who is involved with seed programmes should have some understanding of the
practical relevance of photoperiodism if seed is moved from one country (or latitude) to
another.
Photoperiod sensitive crops such as maize, sorghum and soybeans will very often
not grow successfully if they are introduced to a region that is very different in latitude
from the region to which they are adapted.
Example: if a variety of soybean which is adapted to grow in latitudes far from the
Equator is introduced and grown in a latitude much closer to the equator, it will start to
flower much earlier than normal. This means that its vegetative growth stage will be
much shorter than normal, and it will probably produce a lower yield as a result.Also known as the length of growing period (LGP), the number of days to maturity and
the maturity time, the growth period of a crop is the term used in this book to describe
the number of days from planting to harvesting. This figure, usually expressed in days,
varies with growing conditions, temperature and so on.
The growth period is a useful piece of information to know when looking at any
particular crop, for reasons given below:- If a farmer who is trying to grow food in marginal conditions chooses to grow a
variety of a crop which has a short growth period (it is early, quick growing,
precocious etc.) then they will very often be more likely to produce at least some
food than if they choose to grow a late, unimproved or local variety with a longer
growth period.
The earlier that a farmer can harvest, and feed himself and his family—and then
plant a second crop—the more he is attracted to that crop.
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For subsistence farmers, food today is much more interesting than food tomorrow.