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Tapping into reserves: the route to survival?
When seeds (acorns) of Quercus species germinate, the emerging radicle grows quickly to form a well-developed and well-anchored tap root. Shoot emergence follows a few days after radicle emergence; elongation of the shoot brings the hypogeal cotyledons, the seed?s major storage organs, above ground. Leaves begin to open within 2-3 weeks of shoot emergence and start to photosynthesize: the seedling is on its way to becoming a mighty oak. An interesting feature of these early developmental stages is the accumulation of carbohydrate reserves in the tap root. Kabeya and Sakai (Tohoku University, Sendai, pp. 537-545) have investigated the role of this stored carbohydrate in the carbon economy of Q. crispula seedlings. They established seedlings at light intensities of 40 % full daylight, mimicking a typical open patch on the woodland floor, and at 3 % full daylight. We focus here on the former. They first noted that the tap root carbohydrates accumulate before photosynthesis starts and thus cannot come from newly fixed carbon. The cotyledons export storage reserves relatively rapidly over the first 4 weeks, first to the tap root and then to the shoot, but thereafter more slowly as photosynthesis is initiated. The authors then mimicked herbivory by clipping the shoots. This leads to re-sprouting of shoots from dormant buds either in the axils of scaly leaves on the epicotyl, or from the below-ground root collar. Obviously photosynthesis is now not available as a carbon source, so what fuels re-sprouting? It is not the carbohydrates in the root. Once again the cotyledons supply the need. The authors surmise that the root reserves are set aside for major emergencies such as when the cotyledons are eaten by birds or mammals. In such a situation the seedling would still have carbohydrates to fuel shoot emergence. It is thus abundantly clear what experiments the authors should do next!
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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