Chapter 19.  Measuring Time: Photoperiodism [Day length and flowering]

• Many plant processes must respond to seasonal cues.  Examples:
• Leaf drop in deciduous trees
• Breaking dormancy
• Bolting of rosette plants
• Wood growth in trees
• Flowering
• Potential seasonal cues: temperature, day length (in high latitudes; see Figure 19-20), soil moisture (in seasonal tropical forests).
• Photoperiodic response types:
• SDPs = short day plants
• LDPs = long day plants
• DNPs = day neutral plants
• LSDPs = long short day plants
• SLDPs = short long day plants
• Day neutral plants
• Note that these are field classifications:  an SDP is a plant which is observed to flower when days are short.  Figure 19.3 shows that SDPs flower when the day length is shorter than some critical length, LDPs flower when day length is longer than some critical length.
• Are plants measuring day length, night length or the ratio between the two?  Figure 19.4 shows that night length is critical.
• Phytochrome is involved: far red light can reverse the effect of a red light interruption of night.
• Perception and transmission of the flowering stimulus (See Figure 19.5)
• Perception - in the leaves
• Response - in shoot apices (convert from vegetative to floral apices).
• Transmission of the stimulus
• Grafting an induced to an uninduced plants show transmissibility of stimulus (Figure 19.6) Occurs in phloem (sensitive to girdling, etc.)
• Induced leaf of SDP grafted to uninduced LDP will induce it (there have even been some intergeneric grafts).  This suggest universality of stimulus.
• Florigen?
• How do plants sense night length?
• Hypothesis 1: hourglass model
• Pfr high at beginning of night, must degrade during night.  Shot of red light would reverse this.
• Problem: half-life of Pfr only about 1-2 hrs., red light interruption still effective hours after Pfr is gone.
• Hypothesis 2: biological clock model -- interaction between phytochrome and a circadian clock which maintains an endogenous rhythm in plants with respect to their sensitivity to light.  This is the currently favored hypothesis.
• Insights from developmental genetics, especially with Arabidopsis (Sections 19.3.2, 19.3.3).
• Remember that conversion of vegetative apex to flowering apex must result from changes in gene expression.
• At least two steps:
• Induction of flowering (suppressed by AGAMOUS-LIKE 20 and LEAFY mutations)
• Development of floral organs (distorted by mutations in homeotic genes APETALA1, 2, 3; PISTILLATA and AGAMOUS.
• These genes apparently encode transcription factors.