Aquatic Botany 121 (2015) 1–8 Contents lists available at ScienceDirect Aquatic Botany jou rn al hom ep age: www.elsevier.com/locate/aquabot Comparative responses of two water hyacinth (Eichhornia crassipes) cultivars to different planting densities Xiao Shu, Qi Deng, QuanFa Zhang, WeiBo Wang ∗ Key Laboratory of Aquatic Botany and Watershed Ecology, Wuhan Botanical Garden, Chinese Academy of Sciences, Wuhan 430074, China a r t i c l e i n f o Article history: Received 8 October 2013 Received in revised form 9 October 2014 Accepted 23 October 2014 Available online 1 November 2014 Keywords: Physiology Photosynthesis Radial oxygen loss Dissolved oxygen Microbial diversity Water hyacinth a b s t r a c t Two water hyacinth cultivars, i.e., common water hyacinth (CWH) and purple root water hyacinth (PRWH), were used to investigate the effect of planting densities (i.e., 8, 16 and 24 plants per bucket with a volume of 1.1 m × 1 m (diameter × depth)) on root traits, physiological characteristics, and microbial diversity. The results indicated that the planting density significantly influenced root traits, photosyn- thesis, radial oxygen loss (ROL), dissolved oxygen (DO), and microbial diversity of water hyacinths. The root porosity, root diameter, and root chlorophyll of PRWH were higher than those of CWH, and CWH had higher chlorophyll and Pn in leaves. The microbial diversity decreased significantly with increasing plant density for CWH, while it increased and then decreased in PRWH and peaked at 16 plants bucket -1 . The results suggested that the aerenchyma of PRWH was more developed than those of CWH, and CWH had higher leaf photosynthesis. However, higher root chlorophyll a in PRWH indicated that its capacity for photon capture was higher than in CWH. The result of ROL suggests that larger  root length and root porosity could help improve the dissolved oxygen of water column. The photosynthesis of CWH and PRWH can release oxygen into water column, and the capacity of PRWH was better than those of CWH. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Plant density depends on the genotype, environmental factors, cultural practices, etc. It should be noted that a thinner stand pro- motes the expression of an individual plant’s potential, whereas denser stands are conducive to a greater expression of the plants’ collective potential (Moravˇ cevi ´ c et al., 2011). As planting density increases, plants with leaves on or above the water surface are able to capture a large proportion of incoming sunlight and hence shade out the root; both overwater and underwater intraspecific compe- tition increases, potentially changing partitioning priorities (Steven et al., 2005; Dale and Gillespie, 1976). Higher planting density may increase biomass partitioning to fine roots to better compete for water and nutrients, or may increase partitioning to foliage to bet- ter compete for light (Steven et al., 2005). Planting density increases often results in a deficiency of oxygen (O 2 ) and essential nutrients. To adapt to a low O 2 environment, free-floating aquatic plants have developed aerenchyma tissues, which can be expressed quantita- tively as porosity. ∗ Corresponding author. E-mail address: wangweibo@wbgcas.cn (W. Wang). Porosity in plant tissues results from the intercellular gas-filled spaces formed as a constitutive part of development (Raven, 1996), and it can be further enhanced by formation of aerenchyma. The phenomenon of aquatic plant roots releasing oxygen through the aerenchyma to the rhizosphere is termed radial oxygen loss (ROL) (Armstrong, 1979). ROL is an important characteristic of aquatic plants, which may relate to their adaptability to the water envi- ronment (Stottmeister et al., 2003) and nutrient removal (Sasikala et al., 2009). Rates of ROL have been reported to be markedly different between aquatic plant species (Li et al., 2011) and also between different genotypes of the same species. Previous studies have shown that the tolerance of plants to salinity and zinc expo- sure (Yang et al., 2014) are positively related to ROL. ROL would alert mobility and bioavailability of heavy metals, both on root sur- face and in rhizosphere, by process of oxidation and by altering pH, redox potential, microbial populations, which could eventually affect metal uptake and tolerance by plants (Jacob and Otte, 2003). ROL from roots is important for aerobic microbial activity and can cause the oxidation and/or immobilization of potential phytotoxins in rhizosphere to avoid the toxicity to roots (Taggart et al., 2009). The development of rhizosphere microbial communities is influ- enced by the plant and environment, but in turn, microorganisms exert profound effects on plant growth. Rhizosphere microbial http://dx.doi.org/10.1016/j.aquabot.2014.10.007 0304-3770/© 2014 Elsevier B.V. All rights reserved.