Effect of branch number on the growth and development of Morus alba saplings

Abstract

Morus alba L. is a valuable multipurpose species and is widely distributed in central, northern and southwestern China. This species is one of the most economically important cultivated tree species in China and also provides basic raw materials supporting vigorous development of China's sericulture and textile industries. Most previous studies related to M. alba have primarily focused on cultivation techniques, pest control, quality improvement and development of resources. Previous studies have not yet provided data related to the photosynthetic capacity of this species or leaf growth patterns, and in particular, no studies have addressed the change of biomass in plants with a different number of branches. In this study, 3-year -old mulberry seedlings were transplanted from the garden of the Sichuan Academy of Agricultural Sciences to investigate the effects of branch number on plant growth. Five branch number models were used; that is, one, two, three, four, or five branches were left on the stem of saplings for experimental purposes, and the saplings were allowed to grow for 6 months. We investigated differences in various gas exchange factors, including net photosynthetic rate, stomatal conductance, intercellular CO2 concentration and transpiration rate for plants using the five branch models during the growing season. From the aspect of plant morphological growth, we measured the growth rates based on leaf number, branch length and basal diameter for the five branch models, once every ten days. After the growing season, we measured the differences of leaf biomass, above ground stem biomass and total biomass among plants used for the five branch model experiments. We also analyzed biomass allocation in M. alba with the different branch models. The results show that the single-branch saplings had the highest net photosynthetic rate (Pn) (8.6 μmol·m-2· s-1). As the number of branches on a plant increased, the Pn was observed to decrease significantly and remained relative stable (4.3 μmol·m-2· s-1) when the number of branches was three or more. In contrast, stomatal conductance (gs), intercellular CO2 concentration, (Ci) and transpiration rate (E) showed no significant changes in all M. alba saplings. Also, total leaf number (TLN), total leaf area (TLA), and total stem lengths (TSL) of saplings increased significantly as the number of branches increased to a maximum of 114.3 leaves, 10481.1 cm2, and 457.1 cm, respectively. However, the number of leaves per branch (LN/B), basal diameter per branch (BD/B), mean surface area per leaf (LA/L) and specific leaf area (SLA) obviously decreased in multiple-branch saplings and to 22.9 leaves, 7.57 mm, 87.3 cm2 and 48.91 cm2/g, respectively. Moreover, branch number had no effect on the total dry biomass accumulation and allocation, but mean leaf dry mass per branch (LM/B), mean stem dry mass per branch (SM/B), and mean dry mass per branch (DM/B) gradually decreased with the increase in branch number, with values of 8.61 g, 9.51 g and 18.12 g, respectively. The results suggest that an increase in branch number may result in more intense competition for light resources between leaves, resulting in a lower net photosynthetic rate, smaller leaf area, and shorter stem length per branch as leaves compete for light and resources used to produce biomass. To eliminate the negative effects of such competition as much as possible, saplings can grow additional leaves as well as enlarge their total leaf area to make better use of limited light resources.