Monday, August 20, 2012

The Harbinger of Summer's End

Sea squill (Drimia maritima)
My favorite plant is easily the sea squill, which in Hebrew is called hatzav (with a hard "h" like your clearing your throat). The sea squill is wondrous because it grows and flowers like a Swiss clock in August in Israel, heralding the end of the summer and the approaching fall. Out of nowhere, the hatzav sprouts and rapidly grows a two-to-three-foot stalk with hundreds of little white flowers. The flowers open over several weeks progressively from the base to the tip, resulting in a very impressive floral display.

How does the hatsav know when August has arrived? It knows this because of the lengthening nights. The sea squill is what's known in scientific terms as a "short day" plant, which is a misnomer, as they are actually "long night" plants. "Short day" plants like sea squill and tobacco flower when the length of the night surpasses a threshold specific for that plant. This is as opposed to "long day" plants like carnations and oats, which flower when the night gets shorter than a set threshold.

Plants "know how long the night is thanks to a group of photoreceptors called phytochromes. In a simple model, phytochromes are activated by red light, and are turned on in the morning; they are deactivated by far-red light, the long waves at the end of sunset, so are turned off as night begins. Plants measure the time the phytochromes are turned off, and use this information to determine season.

Sea squill leaves in winter
Getting back to the seq squill, its floral stalk has no leaves, so where does this plant get energy from photosynthesis? The sea squill has two different life cycles. In the summer and fall, it flowers, but in the winter, when there's pleanty of water for to support photosynthesis and growth it produces large green leaves. Theses leaves produce the sugars that are stored underground in a large bulb. As the dry season starts these leaves wilt and dry up. But the bulb uses these stored sugars as energy to produce the flowering stalk in August.


  1. If I understand right, two questions emerge:
    How do "plants measure the time phytochromes are turned off"? One would expect some kind of memory here...
    if the hatzav has no leaves when it sends up its flower, ostensibly it would be all underground; how then does it notice spectral changes?

    1. Todd - you're definitely on your way to being a plant scientist!
      The question of phytochrome action and memory is cutting edge research. And yes, hatzav has a large bulb underground. Actually a small tip remains at ground level. In any event red and far-red light still penetrates the earth fro several millimeters. that's how seeds know when to sprout!