Zangerl A R, Arntz A Michele, Berenbaum May R
Department of Entomology, University of Illinois at Urbana-Champaign, 320 Morrill Hall, 505 S. Goodwin, Urbana, IL 61801, USA e-mail:
Department of Plant Biology, University of Illinois at Urbana-Champaign, 265 Morrill Hall, 505 S. Goodwin, Urbana, IL 61801, USA, , , , , , US.
Oecologia. 1997 Feb;109(3):433-441. doi: 10.1007/s004420050103.
A recurring theme in defense allocation theories is that defenses are costly. Most studies that attempt to quantify a cost of defense seek to establish a trade-off between a component of plant fitness and the level of a constitutive defense. Such estimates are ambiguous because they cannot discount the cost of traits that are correlated with defense but are not themselves defensive. We examined the effects of damage-induced synthesis of furanocoumarins, known defense compounds, on the growth of wild parsnip. Plants that had 2% of their leaf area removed accumulated 8.6% less total biomass and 14% less root biomass than intact plants over a 4-week period. We also found that this small amount of leaf damage significantly reduced net photosynthetic rates 0.5 h after damage; the effect was temporary, as photosynthetic rates were no longer significantly different after 48 h. Lastly, we found that increases in respiration rates associated with damage coincided spatially and temporally with increases in furanocoumarin production, and that respiration increases were phenotypically correlated with furanocoumarin production. When damage-induced changes in furanocoumarin content and respiration rates were expressed in glucose equivalents and compared, the energetic cost of furanocoumarin production (12.6 μg glucose cm) accounted for all of the increase in respiration (12.0 μg glucose cm). A comparison of other secondary compounds in damaged and intact leaflets revealed that myristicin, a furanocoumarin synergist, is the only other compound aside from furanocoumarins that is inducible. The inducible defense system of wild parsnip thus appears to involve a small subset of secondary compounds. Synthesis of these compounds is tightly linked to damage-induced rates of respiration. Because the negative impact that damage had on the rate of net photosynthesis was short-lived, the impact of damage on growth observed in this study was likely due to the cost of furanocoumarin synthesis elicited by damage rather than the loss of photosynthetic tissue caused by damage.
防御分配理论中一个反复出现的主题是防御成本高昂。大多数试图量化防御成本的研究都试图在植物适合度的一个组成部分与组成型防御水平之间建立权衡。这样的估计是不明确的，因为它们无法排除与防御相关但本身并非防御性的性状的成本。我们研究了损伤诱导的已知防御化合物呋喃香豆素的合成对野生防风草生长的影响。在4周的时间里，叶面积被去除2%的植株与完整植株相比，总生物量积累减少了8.6%，根生物量减少了14%。我们还发现，这种少量的叶片损伤在损伤后0.5小时显著降低了净光合速率；这种影响是暂时的，因为48小时后光合速率不再有显著差异。最后，我们发现与损伤相关的呼吸速率增加在空间和时间上与呋喃香豆素产量的增加相吻合，并且呼吸增加在表型上与呋喃香豆素产量相关。当以葡萄糖当量表示损伤诱导的呋喃香豆素含量和呼吸速率变化并进行比较时，呋喃香豆素产生的能量成本（12.6μg葡萄糖/cm）占呼吸增加量（12.0μg葡萄糖/cm）的全部。对受损和完整小叶中其他次生化合物的比较表明，肉豆蔻醚，一种呋喃香豆素增效剂，是除呋喃香豆素外唯一可诱导的其他化合物。因此，野生防风草的诱导防御系统似乎只涉及一小部分次生化合物。这些化合物的合成与损伤诱导的呼吸速率紧密相关。由于损伤对净光合速率的负面影响是短暂的，本研究中观察到的损伤对生长的影响可能是由于损伤引发的呋喃香豆素合成成本，而不是损伤导致的光合组织损失。
