背景回顾：The green leaves of plants are optimized for carbon fixation and the production of sugars, which are used as central units of carbon and energy throughout the plant. 

提出问题：However, there are physical limits to this optimization that remain insufficiently understood.

主要研究：Here, quantitative anatomical analysis combined with mathematical modeling and sugar transport rate measurements were employed to determine how effectively sugars are exported from the needle‐shaped leaves of conifers in relation to leaf length. 

结果1-限制：Mathematical modeling indicated that phloem anatomy constrains sugar export in long needles.

结果2-解除限制：However, we identified two mechanisms by which this constraint is overcome, even in needles longer than 20 cm: i) the grouping of transport conduits, and ii) a shift in the diurnal rhythm of sugar metabolism and export in needle tips. 

结果3-功能：The efficiency of sugar transport in the phloem can have a significant influence on leaf function. 

推测：The constraints on sugar export described here for conifer needles are likely to also be relevant in other groups of plants, such as grasses and angiosperm trees.

 摘 要 

植物的绿色叶片是已经被优化了的，用来高效固定碳和产生糖，进而为整个植株提供碳源和能量源。但是，我们对于这种优化的物理限制仍然没有充分理解。本文中，作者结合定量解剖分析、数学建模以及糖运输速率测量等技术，来确定针叶树的长针叶如何有效地输出糖。数学建模显示，韧皮部解剖结构限制了长针叶中的糖输出。然而，作者鉴定了两种能够克服这种限制的机制，甚至是在长度超过20厘米的针叶中也同样适用，这两种机制分别是：i）运输管道的分组，ii）针尖糖代谢和输出的昼夜节律变化。韧皮部中的糖转运效率对叶片的功能有着显著影响。本文中所描述的针叶树的糖输出限制也有可能在其它植物类群中存在，比如禾本科植物和被子植物树木。