Robert E. Holloway

Plant growth depressions in arbuscular mycorrhizal symbioses: not just caused by carbon drain?

* This study investigated effects of plant density and arbuscular mycorrhizal (AM) colonization on growth and phosphorus (P) nutrition of a cultivar of wheat (Triticum aestivum) that often shows early AM-induced growth depressions. * Two experiments were conducted. Expt 1 had three plant densities and one soil P concentration. Expt 2 had two plant densities and two P concentrations. Plants were grown in calcareous P-fixing soil, inoculated with Glomus intraradices or Gigaspora margarita, or noninoculated (nonmycorrhizal (NM)). Glomus intraradices colonized well and caused a growth depression only in Expt 1. Gigaspora margarita caused large growth depressions in both experiments even though it colonized poorly. * The results showed that growth depressions were mitigated by changes in relative competition for soil P by NM and AM plants, and probably by decreasing carbon costs of the fungi. * The different effects of the two fungi appear to be attributable to differences in the balance between P uptake by the fungal pathway and direct uptake via the roots. These differences may be important in other AM symbioses that result in growth depressions. The results show that mycorrhizal growth responses of plants grown singly may not apply at the population or community level.

Naturally occurring arbuscular mycorrhizal fungi can replace direct P uptake by wheat when roots cannot access added P fertiliser

We investigated the roles of naturally occurring arbuscular mycorrhizal (AM) fungi in phosphorus (P) uptake by wheat (Triticum aestivum L.) in a calcareous, P-fixing soil. Plants grew in a main pot containing autoclaved soil (NM) or autoclaved soil mixed with non-autoclaved soil (to provide inoculum of naturally occurring AM fungi; AM). Granular (GP; monoammonium phosphate) or fluid (FP; ammonium polyphosphate) fertilisers were applied in small compartments (PCs) within a main pot, to which either roots plus hyphae (-Mesh) or hyphae only (+Mesh) had access. Controls received no additional P (NP). Inoculated plants were well colonised by AM fungi. AM growth depressions were observed in -Mesh treatments with NP and GP, but not with FP. Neither AM growth nor P responses were observed in +Mesh treatments. AM plants had much higher P uptake than NM plants, regardless of the P and mesh treatments. Total P uptake by NM plants increased with FP in -Mesh, but was unaffected by either form of P in the +Mesh treatments. Total P uptake by AM plants was similar between -Mesh and +Mesh treatments, regardless of applied P, showing that roots plus hyphae and hyphae alone have the same ability to obtain P. Thus, hyphae can take over the roles of roots in P uptake when roots are not able to access P sources.

The use of a zinc-efficient wheat cultivar as an adaptation to calcareous subsoil: a glasshouse study

Zinc (Zn) is an essential nutrient for plants with a major role in healthy root growth. Zinc is essential for maintaining root membrane integrity, but the effective Zn concentration required may depend on the crop genotype. Zinc-efficient and inefficient wheat cultivars (Triticuum aestivum cv. Excalibur and Gatcher, respectively) were grown in deep soil cores in calcareous subsoil with low micronutrient levels, and high pH and boron. Plants were grown in soil with or without basal nutrients (excluding Zn) and with or without addition of Zn. Components of yield and nutrient use efficiency were measured. Although Gatcher produced 47% more dry weight of tops and double the root length density of Excalibur at maturity, Excalibur was much more efficient in terms of Zn uptake by roots and seven-fold more efficient than Gatcher in partitioning Zn to grain production.