[ti:Scientists Help Cut the Mystery Behind Pruning] [ar:Steve Ember] [al:Agriculture Report] [by:WWW.51VOA.COM] [00:12.82]This is the VOA Special English [00:03.34]Agriculture Report. [00:05.33]We talked last week about [00:07.39]when and how to prune plants. [00:11.13]Today we tell you about [00:13.04]some new understanding of [00:15.03]why cutting the main branch [00:17.32]of a plant or tree can [00:19.66]lead to better development. [00:21.85]The findings are from researchers [00:24.61]on two continents. [00:26.55]Professor Prezemyslaw Prusinkiewicz [00:29.44]of the University of Calgary [00:31.88]in Canada led the research [00:34.23]with scientists from Britain and Sweden. [00:37.32]Their study appears [00:39.17]in the Proceedings of [00:40.68]the National Academy of Sciences. [00:43.14]Researchers have known [00:45.74]since the nineteen thirties [00:48.13]that the actively growing tip [00:50.84]of a plant releases a hormone called auxin. [00:55.42]This hormone flows down the main stem. [00:59.76]Scientists say the auxin [01:02.70]has an indirect effect on buds [01:06.01]on the side of the stem [01:08.32]to prevent branching. [01:10.80]These buds themselves also produce auxin. [01:15.74]The research suggests that to grow, [01:19.18]they have to be able [01:20.89]to export the hormone [01:22.68]into the main stem. [01:25.36]But the flow from the stem tip [01:28.52]prevents them from doing this. [01:31.13]The researchers wanted to find out [01:34.11]how this blocking happens. [01:36.90]Professor Prusinkiewicz [01:39.20]is on sabbatical leave in Australia, [01:41.95]but he sent us an e-mail [01:44.09]suggesting a simple way [01:46.30]to understand the process. [01:48.85]Think of a major road crowded with traffic. [01:52.99]So many cars are on the main road [01:56.44]that the cars on the side roads cannot enter. [02:00.93]The stem is like the crowded main road. [02:04.88]The new research shows [02:07.12]that the buds on the side [02:09.57]cannot export their auxin [02:12.20]into the main stem because it is too full. [02:16.54]But if that main shoot is pruned, [02:20.19]other buds below it can start exporting. [02:24.47]They are no longer inhibited from growing. [02:28.23]Ottoline Leyser from the University of York says [02:33.06]that after a plant is pruned, [02:36.15]all the inhibited shoot tips [02:39.10]compete with each other to grow. [02:41.96]In doing this, the branches [02:44.70]influence each other's growth. [02:47.35]Nearby shoot tips are more likely [02:50.94]to affect each other than those [02:53.34]that are far apart from each other. [02:56.38]Professor Leyser says [02:59.07]the strongest branches grow best, [03:01.97]wherever they may be on the plant. [03:04.96]The study found that the main shoot [03:08.15]grows the best of all not because [03:11.53]of its position at the top of the plant, [03:14.42]but mostly because it got there first. [03:17.96]And that's the VOA Special English [03:22.24]Agriculture Report, [03:24.10]written by Jerilyn Watson. [03:26.43]Transcripts and MP3s of our reports, [03:34.24]including last week's advice about pruning, [03:37.69]can be found at 51voa.com. [03:42.94]You can also post comments. [03:45.89]We invite you to share your own experiences [03:49.17]-- good or bad [03:50.85]-- with pruning plants and trees. [03:54.49]I'm Steve Ember.