Why do lower order branches show greater shoot growth than higher order branches? Considering space availability as a factor affecting shoot growth

Abstract

Many woody plants show hierarchical shoot growth: annual shoot length decreased with increasing branching order. We hypothesize that plants showing hierarchical shoot growth improve the efficiency in terms of space acquisition and use per invested shoot length. This hypothesis was tested by using a simple geometric simulation model of branch development. In this study, the effective shoot length (EL), the shoot length produced within a growth season without any overlap from other shoots, was used as the index of space availability. We compared EL among shoots on different branching orders of a “simulated” branch system. The EL decreased with an increasing branching order. The results suggested that space availability decreased with increasing branching orders. The results also showed that simulated plants with the hierarchical shoot growth showed higher efficiency in terms of space acquisition per investment than those with the non-hierarchical shoot growth. We concluded that the difference in space availability between the branching orders could be an important ultimate factor causing hierarchical shoot growth.