Plant Responses to Stimuli

Tropism – movement toward or away from a stimuli Phototropism – growth toward light Gravitropism – growth against or with gravity Thigmotropism – growth toward touch

Plant Hormones – Chemicals that control the growth, development, and responses to stimuli Auxin phototropism Gibberelin growth Cytokinin cell division Abscisic Acid environment stress Ethylene ripening

Photoperiodisms – the physiological reaction to the length of day or night. They are sensitive to UV light. Long-day plants: will only flower if the night is short enough. They will flower in the late Spring early Summer. Short-day plants: Need a long night to flower. Day-neutral plants: These are sensitive to temperature

Biological Clocks Circadian rhythms – (“circa” approximately “dies” day) physiological cycles based on 24 hour periods. The leaves of plants will rise and fall every 24 hours, even in constant light or darkness.

Other cycles Ultradian cycle - Many in one day. Seconds to minutes Infradian cycle – longer than a day – months and years

Time Cycles Annuals – complete life cycle in one year (zinnia) Biennials – life cycle in 2 years (foxglove) Perennials – live many years (tulips)