将桑树和大豆分别单独种植(单作)或两种隔行种植(间作)，测得两种植物的光合速率如图所示(注：光饱和点是光合速率达到最大值时所需的最低光照强度)，据图分析，下列叙述正确的是（ ）。
- A.与单作相比，间作时两种植物的呼吸强度均没有受到影响
- B.与单作相比，间作时两种植物光合作用的光饱和点均增大
- C.间作虽然提高了桑树的光合速率但降低了大豆的光合速率
- D.大豆植株开始积累有机物时的最低光照强度单作大于间作
正确答案及解析
正确答案
解析
图中桑树和大豆间作时的光合速率与单作时的光合速率不是一致的，说明间作对于植物来讲是有影响的，曲线显示桑树间作时呼吸作用变大，大豆间作时呼吸作用变小，故A选项错误；图中显示与间作相比，桑树间作时光合作用的光饱和点增大，而大豆减小，故B选项错误；大豆在光照强度达到两条曲线相交之前(如5)时，间作的光合速率是比单作还要高的，故C选项错误；曲线与横坐标的交点(即光补偿点)代表光合作用与呼吸作用相等，此点之后植物开始积累有机物，右图显示大豆单作光补偿点大于间作，所以大豆植株开始积累有机物时的最低光照强度单作大于间作，故D选项正确。
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