不同分枝数对桑树幼苗生长发育的影响

Abstract

以我国常见经济林木桑树（<em>Morus alba</em>）为试验材料，从气体交换、形态变化和地上生物量方面研究5种分枝模式（1、2、3、4和5枝）对幼苗生长发育的影响。结果显示：（1）分枝数为1的植株的净光合速率（<em>P</em>_n）最高，达到8.6 μmol·m^-2·s^-1。随着分枝数增加，<em>P</em>_n显著下降，直至分枝数达到3枝及其以上时，净光合速率保持相对稳定，为4.3 μmol·m^-2·s^-1。而气孔导度（<em>g</em>_s）、胞间CO₂浓度（<em>C</em>_i）和蒸腾速率（<em>E</em>）则不受分枝数的影响。（2）随着分枝数增加，总叶片数量、总叶面积和总枝长都显著增加，最终分别达到114.3，10481.1 cm²和457.1 cm，而平均单枝叶片数、平均单枝基径、平均单叶面积和比叶面积则显著减少。（3）随着分枝数的增加，植株的总叶生物量和总枝生物量无显著变化，但平均每枝叶干重、平均每枝枝干重和平均每枝总生物量随分枝数的增加而逐渐减少。研究结果表明了分枝数增加可能导致叶片间对光资源的竞争强度增大，引起净光合速率下降，叶片面积变小，单枝长度和生物量减小。另一方面，植株则通过生长出更多的叶片数量，以及更大的总叶片面积来尽可能地消除竞争带来的不利影响，提高对光环境资源的利用。;<em>Morus alba</em> 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 <em>M. alba </em>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<em> M. alba</em> with the different branch models. The results show that the single-branch saplings had the highest net photosynthetic rate (<em>P</em>_n) (8.6 μmol·m^-2·s^-1). As the number of branches on a plant increased, the<em> P</em>_n 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 (<em>g</em>_s), intercellular CO₂ concentration, (<em>C</em>_i) and transpiration rate (<em>E</em>) showed no significant changes in all <em>M. alba</em> saplings. Also, total leaf number (<em>TLN</em>), total leaf area (<em>TLA</em>), and total stem lengths (<em>TSL</em>) 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 (<em>LN/B</em>), basal diameter per branch (<em>BD/B</em>), mean surface area per leaf (<em>LA/L</em>) and specific leaf area (<em>SLA</em>) 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 (<em>LM/B</em>), mean stem dry mass per branch (<em>SM/B</em>), and mean dry mass per branch (<em>DM/B</em>) 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.