Loik Michael E
Environmental Studies Department, University of California, Santa Cruz, CA 95064, USA.
Physiol Plant. 2008 Sep;134(1):87-98. doi: 10.1111/j.1399-3054.2008.01110.x. Epub 2008 Apr 11.
Cactus spines reduce herbivory, direct water toward roots and reduce the impacts of high- and low-temperature extremes. Yet, shading of stems by spines reduces incident photosynthetic photon flux density (PFD), photosynthesis and growth. This study compared spinescence, PFD interception, stem temperature, Photosystem II (PSII) photochemistry and xanthophyll pigment composition for three species of cacti from the Mojave Desert, CA. The species vary in spinescence: Opuntia basilaris, which has no central or radial spines; Opuntia erinacea, which is densely covered with spines; and Opuntia phaeacantha, which has an intermediate coverage of spines. The role of spines was tested by removing spines from stems of O. erinacea. PFD interception was similar for both O. basilaris and O. phaeacantha, and about three times that for densely spined O. erinacea; removal of spines increased incident PFD three-fold. There were no effects of spines on stem temperatures. Steady-state light-response curves of chlorophyll a fluorescence from PSII indicated that PhiPSII, photochemical quenching (qP) and electron flux within PSII were lower, and non-photochemical quenching was higher, for O. erinacea in comparison to the other two species with less spines. After 2 months, qP was higher and electron flux lower, and xanthophyll pigment pool size was higher, for stems from which spines had been removed compared with intact stems. These three species allocate different amounts of biomass to spines, resulting in species-specific PFD interception, PSII photochemistry and xanthophyll pigment pool size, which may help maintain rates of photosynthesis during the hot, dry Mojave Desert summer.
仙人掌的刺可减少食草动物的侵害，引导水分流向根部，并减轻极端高温和低温的影响。然而，刺对茎的遮荫会降低光合有效辐射通量密度（PFD）、光合作用和生长。本研究比较了来自加利福尼亚州莫哈韦沙漠的三种仙人掌的刺密度、PFD截留、茎温度、光系统II（PSII）光化学和叶黄素色素组成。这三种仙人掌的刺密度各不相同：无中心刺或径向刺的基氏仙人掌（Opuntia basilaris）；被刺密集覆盖的刺梨仙人掌（Opuntia erinacea）；刺覆盖率中等的褐刺仙人掌（Opuntia phaeacantha）。通过去除刺梨仙人掌茎上的刺来测试刺的作用。基氏仙人掌和褐刺仙人掌的PFD截留情况相似，约为多刺的刺梨仙人掌的三倍；去除刺后，入射PFD增加了两倍。刺对茎温度没有影响。PSII叶绿素a荧光的稳态光响应曲线表明，与另外两种刺较少的仙人掌相比，刺梨仙人掌的光系统II实际光化学效率（PhiPSII）、光化学猝灭（qP）和电子通量较低，非光化学猝灭较高。2个月后，与完整茎相比，去除刺的茎的qP较高，电子通量较低，叶黄素色素库大小较高。这三种仙人掌将不同数量的生物量分配到刺上，导致物种特异性的PFD截留、PSII光化学和叶黄素色素库大小，这可能有助于在炎热干燥的莫哈韦沙漠夏季维持光合作用速率。
