Age versus stage: does ontogeny modify the effect of phosphorus and arbuscular mycorrhizas on above‐ and below‐ground defence in forage sorghum?

Abstract

Arbuscular mycorrhizas (AM) can increase plant acquisition of P and N. No published studies have investigated the impact of P and AM on the allocation of N to the plant defence, cyanogenic glucosides. We investigated the effects of soil P and AM on cyanogenic glucoside (dhurrin) concentration in roots and shoots of two forage sorghum lines differing in cyanogenic potential (HCNp). Two harvest times allowed plants grown at high and low P to be compared at the same age and the same size, to take account of known ontogenetic changes in shoot HCNp. P responses were dependent on ontogeny and tissue type. At the same age, P-limited plants were smaller and had higher shoot HCNp but lower root HCNp. Ontogenetically controlled comparisons showed a P effect of lesser magnitude, and that there was also an increase in the allocation of N to dhurrin in shoots of P-limited plants. Colonization by AM had little effect on shoot HCNp, but increased root HCNp and the allocation of N to dhurrin in roots. Divergent responses of roots and shoots to P, AM and with ontogeny demonstrate the importance of broadening the predominantly foliar focus of plant defence studies/ theory, and of ontogenetically controlled comparisons.

Drought-induced changes in nitrogen partitioning between cyanide and nitrate in leaves and stems of sorghum grown at elevated CO2 are age dependent

“…The foliar cyanide concentrations reported here for grain sorghum are marginally higher than for published results for forage sorghum varieties of similar age Miller et al, 2014). Concentrations were well above the toxicity threshold of 750 ppm cyanide in leaves of young plants, and at the marginally toxic level (600-750 ppm) in older plants (Duncan, 1996).…”

“…In order to make meaningful ontogenetic comparisons across years (Miller et al, 2014), samples were taken from plants at the 10 leaf-stage (Day 237 in 1998 andDay 215 in 1999), and at the 19-leaf stage (Day 265 in 1998 and day 236 in 1999). FACE had relatively minor effects on leaf, stem and plant development (Rillig et al, 2001).…”

“…Recently it has been proposed that they may also act to mitigate oxidative stress (Selmar and Kleinwächeter, 2013;Gleadow and Møller, 2014). The three most important factors driving high concentrations of dhurrin are ontogeny, tissue nitrogen concentration and drought stress (Wheeler et al, 1990;Miller et al, 2014). Less is known about the effect of the environment on nitrate production, although concentrations tend to be higher in water-limited plants and plants that have been supplied with nitrogenous fertilizers (Bennet et al, 1990).…”

Interactive effects of temperature and drought on cassava growth and toxicity: implications for food security?

“…; Table ), confirming field studies showing that foliar chemistry is not a suitable proxy for estimating tuber toxicity (Bokanga et al ., ; Jørgensen et al ., ; Burns et al ., ). This result also supports the assertion that environmentally driven changes in foliar defence metabolites cannot be assumed to be representative of the whole plant, an important consideration for root food crops (Parker et al ., ; Miller et al ., ). The lower tuber CNp at high temperature may indicate a reallocation of N away from defence under conditions that stimulate growth (Herms & Mattson, ; Neilson et al ., ).…”

Leaf metabolome in arbuscular mycorrhizal symbiosis

“…It is well known that metabolite concentrations can considerably differ between plants and tissues of different age [31]. Up to now, very few studies investigating AM effects on systemic metabolites control for this aspect by adjusting the fertilisation regime in a way that AMF inoculated plants and control plants are of similar size and developmental stage [26 ], or by harvesting plants grown under high or low phosphate conditions at different time-points [32]. We highly recommend such controls for potential AM-induced growth responses to avoid confounding effects of differences in leaf development or plant reproductive state.…”