Chrysanthemums for a 15 May Show at 11°N
A floriculture unit near Coimbatore (11°N) has contracted to supply 2,000 potted chrysanthemums in full bloom for a flower show on 15 May. The variety grown is a qualitative short-day plant with a critical dark period of about 11 hours. At 11°N the natural night in May is only about 9 hours 30 minutes, while in late December it is close to 12 hours 30 minutes. The unit plans to cover the plants with black polythene from 17:00 to 06:00 daily, starting when the plants are 45 days old, and to lift the cover when the buds show colour. Two problems appear. A street lamp 15 m away throws light on one block of pots every night. And a worker, to save labour, sometimes lifts the covers at 05:00 instead of 06:00. The manager also asks whether the same treatment would be needed for a day-neutral variety they plan to add next season.
1. Explain why the black cover is judged by the length of the uninterrupted dark period rather than by the length of the day, and evaluate whether the 05:00 lifting and the 17:00 covering are sound.
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A short-day plant is really a long-night plant: the pigment system measures the length of the continuous dark period, not the photoperiod. The plant will flower only if a single uninterrupted dark period exceeds the critical length - here about 11 hours.
The schedule: 17:00 to 06:00 gives 13 hours of darkness, comfortably above the 11-hour critical value, so the covering time is sound. Lifting at 05:00 gives 12 hours of darkness, which is still above 11 hours, so in principle the plants remain induced. The real danger is not the one-hour change itself but its irregularity: if the worker varies between 04:00 and 06:00 the dark period becomes unpredictable, and if the cover is ever removed before 06:30 the dark period falls below 11 hours and that night does not count towards induction. Uneven induction across the block gives staggered bud initiation and a crop that is not ready on one date - a commercial failure even though the plants eventually flower.
The correct practical rule is to give a safety margin - for example 16:30 to 06:30, a 14-hour dark period - so that small errors in timing cannot push the dark period below the critical value. The cover must also be lifted when buds show colour because photoinduction is already complete; continued covering then only reduces photosynthesis and starves the developing flowers.
2. Explain, using phytochrome, why a very short light break at night can nullify a whole night's darkness, and predict the effect of the 15 m street lamp.
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The photoreceptor is phytochrome, which exists in two interconvertible forms. Red light (about 660 nm) converts Pr to the far-red-absorbing form Pfr; far-red light (about 730 nm) converts Pfr back to Pr; and in darkness Pfr slowly reverts to Pr (and is degraded).
In a short-day plant, flowering is permitted only when Pfr falls below a threshold before the night ends. A long, undisturbed dark period allows that decline; a short night does not. The crucial point is that the response is not proportional to the light dose. Even a brief exposure - a minute or two of weak light - is enough to photoconvert a large fraction of Pr to Pfr, and the plant then reads the night as having been interrupted and effectively short. One short pulse can therefore cancel an otherwise saturating dark period.
The nearby lamp is the serious problem. A 15 m lamp delivers very low irradiance, but if it is on throughout the night the plants never experience a continuous dark period; if it is switch-on/switch-off, the interruption is precisely the damaging pattern. The affected block will show delayed, sparse or absent flowering while the rest of the crop is on schedule.
Remedies: move the block out of the lamp's spill, erect a light-proof screen, or use a blackout curtain that is genuinely opaque at the edges. Judging the risk by the brightness of the spill rather than by its spectrum and continuity is the commonest mistake.
3. Predict what will happen if the day-neutral variety is given the same black polythene treatment, and state what else must be controlled to hit the 15 May date.
Read the worked answer
A day-neutral variety does not use photoperiod as the signal for flowering; it initiates flowers after a certain vegetative phase determined mainly by accumulated temperature (thermal time) and plant age. The black cover would give it no flowering benefit at all.
Worse, it would do harm. Thirteen hours under an opaque cover removes most of the daily light integral, so photosynthesis and dry-matter accumulation fall, the plants become thin, pale and slow, and the time to flowering is delayed rather than advanced - the opposite of the intended effect. It also raises humidity under the polythene and invites botrytis and other diseases. So the treatment is both useless and costly for that block, and the manager should not extend it.
To hit 15 May with the day-neutral variety, the controls are:
- sowing/planting date - work backwards from the expected crop duration at the prevailing temperature;
- temperature management - in Coimbatore the May heat will shorten the phase; shade netting and ventilated polyhouses are more useful than blackout;
- crop scheduling in staggered batches, so that a few days of error do not lose the whole consignment;
- nutrition and pinching to synchronise branching, which affects the date of visible bud colour as much as initiation does.
The key judgement is that photoperiodic tools belong to short-day and long-day varieties only; applying them to a day-neutral crop is a category error.