Why Are Plants Green? 植物為何是綠色的？

伊克巴爾·皮塔瓦拉文 薛洪君譯

When sunlight shining on a leaf changes rapidly， plants must protect themselves from the ensuing sudden surges of solar energy. To cope with these changes， photosynthetic1 organisms—from plants to bacteria—have developed numerous tactics. Scientists have been unable， however， to identify the underlying design principle. 當照在葉片上的陽光急遽變化，植物就必須自我防護，以免受隨之而來的太陽能驟增的傷害。為應對這些變化，能進行光合作用的有機體——從植物到細菌——已演化出眾多策略。不過，科學家一直無法確定其背后的設(shè)計原則。

An international team of scientists， led by physicist Nathaniel M. Gabor at the University of California， Riverside， has now constructed a model that reproduces a general feature of photosynthetic light harvesting2， observed across many photosynthetic organisms.

Light harvesting is the collection of solar energy by protein-bound chlorophyll3 molecules. In photosynthesis4—the process by which green plants and some other organisms use sunlight to synthesize foods from carbon dioxide and water—light energy harvesting begins with sunlight absorption.

The researchers model borrows ideas from the science of complex networks， a field of study that explores efficient operation in cellphone networks， brains， and the power grid. The model describes a simple network that is able to input light of two different colors， yet output a steady rate of solar power. This unusual choice of only two inputs has remarkable consequences.

“Our model shows that by absorbing only very specific colors of light， photosynthetic organisms may automatic-ally protect themselves against sudden changes—or ‘noise—in solar energy， resulting in remarkably efficient power conversion，” said Gabor， an associate professor of physics and astronomy， who led the study appearing in the journal Science. “Green plants appear green and purple bacteria appear purple because only specific regions of the spectrum from which they absorb are suited for protection against rapidly changing solar energy.”

Gabor first began thinking about photosynthesis research more than a decade ago， when he was a doctoral student at Cornell University. He wondered why plants rejected green light， the most intense solar light. Over the years， he worked with physicists and biologists worldwide to learn more about statistical methods and the quantum biology of photosynthesis.

Richard Cogdell， a botanist at the University of Glasgow in the United Kingdom and a coauthor on the research paper， encouraged Gabor to extend the model to include a wider range of photosynthetic organisms that grow in environments where the incident5 solar spectrum is very different.

“Excitingly， we were then able to show that the model worked in other photosynthetic organisms besides green plants， and that the model identified a general and fundamental property of photosynthetic light harvesting，” he said. “Our study shows how， by choosing where you absorb solar energy in relation to the incident solar spectrum， you can minimize the noise on the output—information that can be used to enhance the performance of solar cells.”

Coauthor Rienk van Grondelle， an influential experimental physicist at Vrije Universiteit Amsterdam in the Netherlands who works on the primary physical processes of photosynthesis， said the team found the absorption spectra of certain photosynthetic systems select certain spectral excitation regions that cancel the noise and maximize the energy stored.

“This very simple design principle could also be applied in the design of human-made solar cells，” said van Grondelle， who has vast experience with photosynthetic light harvesting.

Gabor explained that plants and other photosynthetic organisms have a wide variety of tactics to prevent damage due to overexposure to the sun， ranging from molecular mechanisms of energy release to physical movement of the leaf to track the sun. Plants have even developed effective protection against UV light， just as in sunscreen.

“In the complex process of photosynthesis， it is clear that protecting the organism from overexposure is the driving factor in successful energy production， and this is the inspiration we used to develop our model，” he said. “Our model incorporates relatively simple physics， yet it is consistent with a vast set of observations in biology. This is remarkably rare. If our model holds up6 to continued experiments， we may find even more agreement between theory and observations， giving rich insight into the inner workings of nature.”

To construct the model， Gabor and his colleagues applied straightforward physics of networks to the complex details of biology， and were able to make clear， quantitative， and generic statements about highly diverse photosynthetic organisms.

“Our model is the first hypothesis-driven explanation for why plants are green， and we give a roadmap to test the model through more detailed experiments，” Gabor said.

Photosynthesis may be thought of as a kitchen sink， Gabor added， where a faucet flows water in and a drain allows the water to flow out. If the flow into the sink is much bigger than the outward flow， the sink overflows and the water spills all over the floor.

“In photosynthesis， if the flow of solar power into the light harvesting network is significantly larger than the flow out， the photosynthetic network must adapt to reduce the sudden over-flow of energy，” he said. “When the network fails to manage these fluctuations， the organism attempts to expel the extra energy. In doing so， the organism undergoes oxidative7 stress， which damages cells.”

The researchers were surprised by how general and simple their model is.

“Nature will always surprise you，” Gabor said. “Something that seems so complicated and complex might operate based on a few basic rules. We applied the model to organisms in different photosynthetic niches8 and continue to reproduce accurate absorption spectra. In biology， there are exceptions to every rule， so much so that9 finding a rule is usually very difficult. Surprisingly， we seem to have found one of the rules of photosynthetic life.”

Gabor noted that over the last several decades， photosynthesis research has focused mainly on the structure and function of the microscopic components of the photosynthetic process.

“Biologists know well that biological systems are not generally finely tuned given the fact that organisms have little control over their external conditions，” he said. “This contradiction has so far been unaddressed because no model exists that connects microscopic processes with macroscopic properties. Our work represents the first quantitative physical model that tackles this contradiction.”

Next， supported by several recent grants， the researchers will design a novel microscopy technique to test their ideas and advance the technology of photo-biology10 experiments using quantum optics tools.

“Theres a lot out there to understand about nature， and it only looks more beautiful as we unravel its mysteries，” Gabor said.

如今，由加利福尼亞大學河濱分校物理學家納撒尼爾·M.加博爾領(lǐng)導的國際科學家小組，已經(jīng)構(gòu)建出一個模型，可以重現(xiàn)光合作用光捕獲的一般特性，這種特性廣泛存在于眾多能進行光合作用的有機體中。

光捕獲是指與蛋白質(zhì)結(jié)合的葉綠素分子吸收太陽能的過程。在光合作用（綠色植物及某些其他有機體利用太陽光把二氧化碳和水合成養(yǎng)料的過程）中，光能捕獲始于吸收太陽光。

研究人員的這一模型借鑒了復雜網(wǎng)絡學（一個探索手機網(wǎng)絡、大腦和電網(wǎng)如何高效運行的研究領(lǐng)域）的理論。該模型描述的是一個簡單網(wǎng)絡，該網(wǎng)絡能夠輸入兩種不同顏色的光，卻輸出比率穩(wěn)定的太陽能。這種只選擇兩個輸入項的非常規(guī)方式帶來了奇特的效果。

“我們的模型顯示，通過只吸收極為特定顏色的光，能進行光合作用的有機體可以自動保護自己，免受太陽能突然變化——或是‘干擾的傷害，從而產(chǎn)生超高效的能量轉(zhuǎn)換。”該項研究（成果已發(fā)表在《科學》雜志上）的帶頭人、物理與天文學副教授加博爾說，“綠色植物呈現(xiàn)綠色，紫色細菌呈現(xiàn)紫色，是因為它們從中吸收光線的光譜只有特定區(qū)域適合防護太陽能的急遽變化?！?/p>

十多年前，當時還是康奈爾大學博士生的加博爾第一次有了研究光合作用的想法。他想弄清為什么植物排斥綠色光，這是最強的太陽光。這些年來，他與世界各地的物理學家和生物學家合作，深入了解光合作用的統(tǒng)計方法和量子生物學機制。

理查德·科格德爾是英國格拉斯哥大學的植物學家，也是該項研究論文的合著者，他鼓勵加博爾擴大這一模型的適用范圍，以涵蓋更多能進行光合作用的有機體，它們生長在入射太陽光譜極為不同的環(huán)境中。

“令人振奮的是，接下來我們得以證明，這一模型對綠色植物之外的其他能進行光合作用的有機體也適用，并且該模型發(fā)現(xiàn)了光合作用光捕獲的一個重要的普遍特性?！彼f，“我們的研究表明，通過選擇在入射太陽光譜的什么位置吸收太陽能，能最大程度減少對輸出的干擾。此項信息可用于提高太陽能電池的性能。”

論文合著者里恩克·范格龍代勒是荷蘭阿姆斯特丹自由大學一位有影響力的實驗物理學家，從事光合作用基本物理過程的研究。他說，研究團隊發(fā)現(xiàn)，某些光合系統(tǒng)的吸收光譜選擇能消除干擾并盡可能多儲存能量的光譜激發(fā)區(qū)。

“這個十分簡單的設(shè)計原則也可以用在人造太陽能電池的設(shè)計上。”對光合作用光捕獲甚為熟知的范格龍代勒說。

加博爾解釋說，植物及其他能進行光合作用的有機體具備廣泛多樣的策略——從能量釋放的分子機制到葉片追隨太陽轉(zhuǎn)動等——來防止太陽過度暴曬造成的損害。植物甚至演化出了對紫外線的有效防護，就像涂了防曬霜一樣。

“顯然，在光合作用的復雜過程中，保護有機體免于過度暴曬是成功產(chǎn)生能量的驅(qū)動因素，這是我們開發(fā)模型的靈感來源?！彼f，“我們的模型包含的物理學知識較為簡單，卻與大量生物學的觀察結(jié)果一致。這極為罕見。如果這一模型經(jīng)得起后續(xù)實驗的檢驗，我們有可能發(fā)現(xiàn)理論和觀察結(jié)果間更多的一致性，從而讓人們對大自然的奧秘有深刻的了解?！?/p>

為構(gòu)建這一模型，加博爾及其同事把簡明的網(wǎng)絡物理學應用于生物學的復雜細節(jié)，得以就多種多樣能進行光合作用的有機體得出清晰的一般定量表述。

加博爾說：“我們的模型首次用假設(shè)驅(qū)動來解釋植物為何是綠色的，我們也提供了一個通過更詳細的實驗檢驗該模型的方案?！?/p>

加博爾補充說，可以把光合作用想象成一個廚房洗滌槽，水龍頭往里注水，排水管向外排水。如果進水量遠大于排水量，水槽就會滿溢，水會流得滿地都是。

“在光合作用中，如果進入光捕獲網(wǎng)絡的太陽能量明顯大過排出量，光合作用網(wǎng)絡就必須做調(diào)適以削減陡然滿溢的能量?！彼f，“如果該網(wǎng)絡管理不好這種波動，有機體會設(shè)法排出多余的能量。這樣做時，有機體經(jīng)受氧化應激，會損害細胞?！?/p>

研究人員感到驚喜的是，他們的模型竟如此通用而又簡單。

“大自然總會讓你感到驚奇。”加博爾說，“看上去如此錯綜復雜的現(xiàn)象可能遵循幾項基本規(guī)律。我們把這一模型應用到處于不同光合作用生態(tài)位的有機體身上，繼續(xù)復現(xiàn)正確的吸收光譜。在生物學上，每一項規(guī)律都存在例外，以致發(fā)現(xiàn)一條規(guī)律通常是很難的。令人驚奇的是，我們似乎發(fā)現(xiàn)了能進行光合作用的生物的一條規(guī)律?！?/p>

加博爾指出，過去幾十年來，光合作用研究主要聚焦于光合作用過程中微觀成分的結(jié)構(gòu)和功能。

“生物學家深知，鑒于有機體對其外部條件幾無控制力，生物系統(tǒng)通常未經(jīng)精微調(diào)節(jié)?！彼f，“因為沒有使微觀過程和宏觀特性相聯(lián)系的模型，這一矛盾迄今沒有得到處理。我們的成果是解決此項矛盾的第一個定量物理模型。”

接下來，在新近幾項資金的支持下，研究人員將設(shè)計一項新的顯微鏡技術(shù)，以檢驗他們的設(shè)想，并運用量子光學工具提升光生物學實驗技術(shù)。

加博爾說：“大自然有待人們弄清的東西很多，而隨著我們揭開其神秘面紗，它只會看上去更加美麗。”

（譯者為“《英語世界》杯”翻譯大賽獲獎者）

1 photosynthetic光合的。 ?2 light harvesting光捕獲，指光合作用光反應過程中一系列光合色素分子吸收光能并傳遞到光合反應中心的過程。 ?3 chlorophyll葉綠素。 ?4 photo-synthesis光合作用。

5 incident（尤指光或其他輻射）入射的。

6 hold up（論點、理論等）經(jīng)受得住檢驗。

7 oxidative氧化的。 ?8 niche生態(tài)位。 ?9 so much so （that） 以致。

10 photo-biology光生物學。