I’m catching up on yardwork this weekend. I suspected my rose bushes needed to be dead-headed because they did not have very many roses. I was sort of surprised that I didn’t even find very many “hips” but some of the ones I did find were quite large. Between five bushes there were maybe 20 hips. In years past I’ve easily found 3-4 times that number and had to cut them off every few weeks. While I was pruning the roses however, I started wondering about what mechanism would cause the plant to stop blooming once it’s pollinated.
My knowledge of plants is limited to an elementary school experiment where you place lettuce in dyed water and observe that it pulls up the dye and discolors the leaves. I hypothesized that once a bloom is pollinated, it releases a hormone that causes the veins leading to that bloom to swell and draw water away from other potential buds.
Tonight I did some research to learn what the real mechanism is. Actually this turns out to be a difficult thing to look up. I found lots of “how does pollination work” resources, but only one mention that blooms release a hormone that inhibits bud formation once pollinated. Since I’m used to thinking of plants as pulling water and nutrients upwards, I’m interested in how that hormone would get spread throughout the plant.
I found my answer on the plant hormone wikipedia page. My elementary school experiment was using the xylem vessels of the stems. These are specialized tube-like structures that move water and nutrients upwards from the roots to the leaves through capillary action. Differences in pressure due to evaporation of water at the leaves and high water content near the roots allow the xylem to passively move water. Interestingly, the cells that make up the tubes are dead at maturity . There is a second set of vessels in plants called the phloem. These are living cells of the inner most layer of bark which primarily transport sugars and carbohydrates from the leaves to the roots, a process called translocation. Unlike the xylem, the phloem uses positive pressure to move molecules. Sugar producers push molecules into the phloem’s sieve tubes and sugar sinks actively remove molecules from the sieve tubes . The phloem is also responsible for moving hormones from the blooms to other buds in the plant, thus answering my question of how a bloom can communicate with other parts of the plant.
On a related note, I also learned that I should be fertilizing my roses with high phosphorus fertilizer and not high nitrogen fertilizer . In past years I have used Miracle Gro, but for my roses I should be using a rose-specific fertilizer.
Interesting fact: enormous fruits are made by cutting into the phloem of a branch and then removing all but one fruit from that branch. All the sugar produced is driven to the one fruit leading it to grow much larger than normal .