Forty Plants, Ten Fruits: Diagnosing a Bottle Gourd Crop
A farmer in Haryana grows bottle gourd on 0.4 hectare. By week six the vines are vigorous - dark green leaves, no wilting, no leaf spots, no stunting - and carry plenty of bright yellow flowers. Yet the field sets only about one fruit per ten vines. The farmer's records show three plots grown from the same seed in the same soil:
- Plot A (0.30 ha): sprayed with a broad-spectrum insecticide every fifth day, always starting at 6 a.m. just as the flowers open. About 8 fruits per 100 vines.
- Plot B (0.05 ha): no insecticide; two beehives kept at the field edge. About 65 fruits per 100 vines.
- Plot C (0.05 ha): enclosed under fine insect-proof net from flowering onwards; hand-watered; no insecticide; no bees inside. Zero fruits.
Flowers of this crop open at dawn and close by mid-morning. Neighbouring farmers who sprayed the same insecticide only after sunset reported normal fruit set. The farmer is deciding between more urea, a boron spray, or something else entirely.
1. Which single explanation accounts for all three plots? Rule out nutrient deficiency and disease using the evidence given, and explain the significance of the 6 a.m. spraying time.
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Explanation: failure of pollination, caused by destruction of pollinators.
Ruling out nutrition and disease: nitrogen deficiency would show pale, yellowing lower leaves and stunted vines; boron deficiency would show distorted growing tips, hollow stems and poor flower development. The field has dark green, unstunted foliage and an abundance of flowers, so the plants are not starving and the flowers are forming normally. Leaf spotting or wilt would be visible if disease were causing flower drop. More urea or boron therefore cannot fix a problem that is not nutritional.
Why the plots differ: B (bees present, no spray) sets 65%, C (no bees, netted) sets 0%. The only difference between B and C is access to pollinators, so pollen transfer is the limiting factor. A sits in between because the bees are being killed but not eliminated.
The timing is the clue: flowers open at dawn, which is when bees forage. Spraying at 6 a.m. puts insecticide directly on open blossoms and kills the pollinators at the moment they are needed. Farmers who spray after sunset, when flowers are closed and bees are in the hive, keep their pollinators and get normal set. The insecticide is not harming the plant; it is removing the vector.
2. Using the flower biology of bottle gourd, explain why the netted plot set no fruit at all despite producing many flowers.
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Flower structure: bottle gourd is monoecious - it bears separate male flowers and female flowers on the same vine. A female flower has an inferior ovary and a stigma; a male flower has stamens and no ovary. A fruit can form only after pollen lands on the stigma of a female flower and a pollen tube grows down to fertilise the ovules. Until that happens the small ovary simply abscises and drops.
Why the net matters: the pollen of cucurbits is heavy and sticky, not the fine dry dust carried by wind. It must be moved physically - almost entirely by bees, which visit for nectar and carry pollen on their bodies between male and female flowers. Under the fine net, bees and larger insects are excluded, and wind cannot substitute because the grains are too heavy to travel. So pollen never reaches a stigma, no fertilisation occurs, the ovary is not stimulated to develop, and every female flower drops. The abundance of flowers makes the failure more obvious, not less: more flowers without a pollen vector simply means more flowers falling.
This is why the netted plot is the decisive evidence - it isolates pollinator access as the only variable.
3. The farmer asks whether spraying a synthetic auxin on the female flowers would give a crop. Predict the outcome, explain the mechanism, and give one genuine advantage and one genuine limitation compared with restoring pollination.
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Prediction: the treated female flowers would develop into fruits, so the farmer would get a harvest. These fruits would be seedless, because no fertilisation has occurred.
Mechanism: after fertilisation, the developing seeds release auxin, which makes the ovary enlarge into fruit. Auxin is the signal for ovary growth, not for fertilisation itself. Applying auxin externally substitutes for the seed-derived signal, so the ovary develops parthenocarpically (fruit without seeds). This is the same principle used commercially for seedless grapes and bananas.
Advantage: fruit set is rescued without pollinators, which is genuinely useful where bees have been wiped out or the crop is under net or in a polyhouse, and it works immediately.
Limitations: (i) the fruit has no seeds, so the farmer cannot save seed from the crop and must buy fresh seed each season; (ii) seedless parthenocarpic fruits are often smaller or misshapen and need correct dose and timing, since excess auxin can distort or drop young fruit; (iii) it treats the symptom, not the cause - the pollinator population stays destroyed, which will hit the next crop (and neighbouring crops) as well.
The sound decision is to restore pollinators and shift spraying to after sunset; auxin is a stopgap.