Photoperiod is the factor that exclusively affects reproduction in seasonal breeders, both plants and animals.



Photoperiod refers to the duration of light and darkness in a 24-hour period. It is a crucial environmental cue that influences the reproductive cycles of many organisms, especially seasonal breeders. Here's how photoperiod affects reproduction in seasonal breeders:

1. Definition of seasonal breeders:
- Seasonal breeders are organisms whose reproductive activity is triggered by specific environmental conditions, typically changes in photoperiod.
- These organisms have specific breeding seasons or periods during which they mate and reproduce.

2. Role of photoperiod in plant reproduction:
- Many plants are seasonal breeders and rely on photoperiod to regulate their flowering and reproductive processes.
- Photoperiod acts as a signal that helps plants determine the appropriate time to flower and produce seeds.
- For example, long-day plants require a longer duration of light (short darkness) to initiate flowering. They typically flower during the spring or summer when days are longer.
- On the other hand, short-day plants require a longer duration of darkness (shorter light) to initiate flowering. They typically flower during the autumn or winter when nights are longer.

3. Role of photoperiod in animal reproduction:
- Photoperiod also plays a crucial role in regulating the reproductive cycles of many animals.
- In response to changes in photoperiod, animals undergo physiological and behavioral changes to prepare for mating and reproduction.
- For example, many mammals, such as deer and sheep, have breeding seasons triggered by changes in photoperiod.
- The lengthening or shortening of daylight hours can influence the secretion of hormones like melatonin, which in turn affects the production of reproductive hormones like estrogen and testosterone.

4. Other factors:
- While water and temperature are important environmental factors for the overall health and survival of organisms, they do not exclusively affect reproduction in seasonal breeders.
- Water availability and temperature can influence general physiological processes and behavior but do not have a direct and exclusive impact on the timing of reproductive events in seasonal breeders.

Therefore, the correct answer is option 'C', photoperiod. It is the primary environmental cue that determines the reproductive cycles in seasonal breeders, both plants and animals.