解决方案——多年生植物

  目前，一年生的谷物养活着整个世界，但是也带来了一些问题。多年生植物能够更集约地利用资源。他们长长的根能够牢固地抓住土壤、水分和肥料，这也会减少了污染。

   10，000年前，在开始栽种野生植物的时候，人类做了一个可能无意但是很重大的选择：我们选择了一年生植物。今天，养活几十亿人口的谷物，像小麦、水稻、玉米等等，都是一年生植物。他们每年由种子发芽，继而长出新种子，然后死亡。“世界上，大多植物是多年生的”，美国USDA的遗传学家Buckler教授说，他在康奈尔大学研究玉米。“那么，为什么我们却驯化了一年生植物呢？”不是因为一年生植物更好，他说，而是因为新石器时代的农民使他们变好的——比如说，通过年复一年的从繁茂的植株上选种并播种，最终形成大的种子。在这样的选择条件下，多年生的植物没有优势，因为他们不需要年年播种。他们天然的优势成了不利的因素，而最终没有被选择。

   而今，一些充满激情的科学家又重回到那个岔路口：他们正在试图培育多年生的小麦、水稻和其他的谷物。堪萨斯Salina的Land Institute的共同创始人和所长，维斯.杰克逊已经推崇这个观点几十年了。没有多少研究经费，但是Salina和其他地方的育种家正在将现代谷物与他们多年生的野生亲缘物种进行杂交；他们也在试图直接对野生的多年生植物进行驯化。共同的目标是培育能够吸取多年生植物的优点的作物——深而浓密的根系，每年春天能够重新发芽，而且具有很好的适应性，并高效利用资源——而又不牺牲太多的千百年人工选择所赋予一年生植物的谷物产量。

   因为一年生植物的浅根系，我们为寻求高产付出了极高的代价，Land Institute 的土壤学家Jerry Glover说。一年生植物主要从上层根系的土壤中吸取养分，上层土壤中的养分很快被耗尽了，农民必须大量施肥来维持高产。而往往只有不到一半的肥料可以被植物吸收，很大一部分被雨水冲洗进入了墨西哥湾，那里水体富营养化而导致藻类爆发，进而在密西西比河入海口形成了一个很大的“死区”。一年生植物也需要大量施用农药和频繁耕作。而且，一年中很长时间，土地是裸露的，杂草很容易侵入。

  收获之后土壤裸露，以及播种季节的耕作都会遭成土壤侵蚀。自上世纪80年代以来，在美国，免耕和其他的保护措施至少减少了40%的土壤流失，但是每年仍有17亿吨的流失量。世界范围内，估计土壤流失的速度要十倍甚至百倍于土壤产出的速度。“除非这个病被根除，否则人类将像其他作物一样枯萎”，Jackson 30年前写道。在贫穷国家，增加的人口迫使农民在陡峭和水土流失严重的斜坡上耕作，这个“病”的威胁将更严重。

   多年生谷物将有助于解决这些问题。他们会一直覆盖在土壤上，减少侵蚀和农药的施用。他们发达的根系可以固着土壤，特别适合在边缘地块。“多年生植物能够从10 或12英尺深的土壤中 获取水和养分，并在一年的11个月份中持续生长” Glover 说。发达的根系和覆盖的土壤也能够固着肥料——减少农民和环境成本。

   Land Institute的多年生小麦-偃麦草杂交种，已经可以用来磨粉。其相比于堪萨斯的小麦，其产量很低——但是在尼泊尔或许不会。尼泊尔有陡坡田地和严酷的气候，那里，有研究者正在小区测试杂交种。多年生谷物的培育尚需十几年，但是廉价的DNA测序技术可以加快育种家的步伐。Buckler认为，利用玉米几十亿研究经费的一小部分，在短短10年内，人们将能够培育出可用于田间试验的多年生玉米。“我认为，我们应当试着去彻底变革农业”，他说。

原文：

Perennial Solution

Annual grains feed the world, but theycreate problems. Perennials are thrifty. Their long roots hold on to soil,water, and fertilizer, which means less pollution.

By Robert Kunzig

Photograph by Rebecca Hale, NGM Staff

Humans made an unwitting but fateful choice10,000 years ago as we started cultivating wild plants: We chose annuals. All the grains that feed billions of people today—wheat, rice, corn, and so on—come from annual plants, which sprout from seeds, produce new seeds, and die every year. "The whole world is mostly perennials," says USDA geneticist Edward Buckler, who studies corn at Cornell University. "So why did we domesticate annuals?" Not because annuals were better, he says, but because Neolithic farmers rapidly made them better—enlarging their seeds, for instance, by replanting the ones from thriving plants, year after year. Perennials didn't benefit from that kind of selective breeding, because they don't need to be replanted. Their natural advantage became a handicap. They became the roadnot taken.

Today an enthusiastic band of scientists has gone back to that fork in the road: They're trying to breed perennial wheat, rice, and other grains. Wes Jackson, co-founder and president of the Land Institute in Salina, Kansas, has promoted the idea for decades. It has never had much money behind it. But plant breeders in Salina and elsewhere are now crossing modern grains with wild perennial relatives; they're also trying to domesticate the wild plants directly. Either way the goal is crops that would tap the main advantage of perennials—the deep, dense root systems that fuel the plants' rebirth each spring and that make them so resilient and resource efficient—without sacrificing too much of the grain yield that millennia of selection have bred into annuals.

We pay a steep price for our reliance on high yields and shallow roots, says soil scientist—and National Geographic emerging explorer—Jerry Glover of the Land Institute. Because annual root crops mostly tap into only the top foot or so of soil, that layer gets depleted, forcing farmers to rely on large amounts of fertilizers to maintain high yields. Often less than half the fertilizer in the Midwest gets taken up by crops; much of it washes into the Gulf of Mexico, where it fertilizes algae blooms that cause a vast dead zone around the mouth of the Mississippi. Annuals also promote heavy use of pesticides or tillage because they leave the ground bare much of the year. That allows weeds to invade.

Above all, leaving the ground bare after harvest and plowing it in planting season erodes the soil. No-till farming and other conservation practices have reduced the rate of soil loss in the U.S. by more than 40 percent since the 1980s, but it's still around 1.7 billion tons a year. Worldwide, one estimate put the rate of soil erosion from plowed fields at ten to a hundred times the rate of soil production. "Unless this disease is checked, the human race will wilt like any other crop," Jackson wrote 30 years ago. As growing populations force farmers in poor countries onto steeper, erodible slopes, the "disease" threatens to get worse.

Perennial grains would help with all these problems. They would keep the ground covered, reducing erosion and the need for pesticides, and their deep roots would stabilize the soil and make the grains more suitable for marginal lands. "Perennials capture water and nutrients10 or 12 feet down in the soil, 11 months of the year," Glover says. The deep roots and ground cover would also hold on to fertilizer—reducing the cost to the farmer as well as to the environment.

The perennial wheat-wheatgrass hybrid now growing at the Land Institute can already be made into flour. Yields are too low to compete with annual wheat in Kansas—but maybe not in Nepal, which has steeper slopes and a harsher climate, and where a researcher is now testing perennial hybrids in small plots. Amber waves of perennial grain may be decades away, but the emergence of cheap DNA sequencing is allowing plant breeders to work much faster than they used to. Buckler thinks that for a tiny fraction of the billions spent annually on corn research, one could create field-testable perennial corn in as little as ten years. "I think we should take a shot at revolutionizing agriculture," he says.

链接：http://ngm.nationalgeographic.com/2011/04/big-idea/perennial-grains-text