Denison R Ford, Fedders James M, Harter Barry L
Ecology Evolution and Behavior, University of Minnesota Saint Paul, MN, USA.
USDA, ARS Beckley, WV, USA.
Evol Appl. 2010 Sep;3(5-6):466-72. doi: 10.1111/j.1752-4571.2010.00148.x. Epub 2010 Aug 3.
Despite the optimism of some molecular biologists, natural selection among the wild ancestors of crops is unlikely to have missed simple genetic improvements that would consistently have enhanced individual fitness. Tradeoff-free opportunities for further improvement of crop traits like photosynthetic efficiency or drought tolerance may therefore be elusive. Opportunities linked to acceptable tradeoffs may be abundant, however. Tradeoffs between individual competitiveness and the collective productivity of plant communities (e.g. those linked to height) have been key to past increases in yield potential. Solar tracking by leaves could involve such tradeoffs, if photosynthetic benefits to tracking leaves are outweighed by increased shading of leaves lower in the canopy. This hypothesis was tested using rotation in the horizontal plane to disrupt solar tracking in alfalfa. In sparse canopies, solar tracking increased net canopy photosynthesis, but rarely by more than 3%. As leaf area increased, solar tracking tended to decrease net canopy photosynthesis, despite edge effects in our 1-m(2) artificial communities, which probably exaggerated net photosynthetic benefits of tracking. Computer modeling suggested that the season-long effects of solar tracking on community productivity can be negative. Solar tracking may have persisted, nonetheless, because individuals whose leaves track the sun increase shading of competitors.
尽管一些分子生物学家持乐观态度，但作物野生祖先之间的自然选择不太可能错过那些能持续提高个体适应性的简单基因改良。因此，像光合效率或耐旱性等作物性状的无权衡进一步改良机会可能难以实现。然而，与可接受权衡相关的机会可能很多。个体竞争力与植物群落集体生产力之间的权衡（例如与高度相关的那些）一直是过去产量潜力提高的关键。如果叶片进行太阳追踪所带来的光合益处被冠层中较低位置叶片所增加的遮荫所抵消，那么叶片的太阳追踪可能涉及此类权衡。我们利用在水平面上的旋转来扰乱苜蓿的太阳追踪，对这一假设进行了测试。在稀疏冠层中，太阳追踪增加了冠层净光合作用，但增幅很少超过3%。随着叶面积增加，尽管在我们1平方米的人工群落中存在边缘效应（这可能夸大了追踪的净光合益处），太阳追踪仍倾向于降低冠层净光合作用。计算机建模表明，太阳追踪对群落生产力的整个季节影响可能是负面的。尽管如此，太阳追踪可能仍然存在，因为叶片追踪太阳的个体增加了对竞争者的遮荫。
