Qifu Ma

Root Exudates, Rhizosphere Zn Fractions, and Zn Accumulation of Ryegrass at Different Soil Zn Levels

A glasshouse experiment was conducted using a root-bag technique to study the root exudates, rhizosphere Zn fractions, and Zn concentrations and accumulations of two ryegrass cultivars (Lolium perenne L. cvs. Airs and Tede) at different soil Zn levels (0, 2, 4, 8, and 16 mmol kg^(-1) soil). Results indicated that plant growth of the two cultivars was not adversely affected at soil Zn level≤8 mmol kg^(-1). Plants accumulated more Zn as soil Zn levels increased, and Zn concentrations of shoots were about 540 µg g^(-1) in Aris and 583.9 µg g^(-1) in Tede in response to 16 mmol Zn kg^(-1) soil. Zn ratios of shoots to roots across the soil Zn levels were higher in Tede than in Airs, corresponding with higher rhizosphere available Zn fractions (exchangeable, bound to manganese oxides, and bound to organic matter) in Airs than in Tede. Low-molecular-weight (LMW) organic acids (oxalic, tartaric, malic, and succinic acids) and amino acids (proline, threonine, glutamic acid, and aspartic acid, etc.) were detected in root exudates, and the concentrations of LMW organic acids and amino acids increased with addition of 4 mmol Zn kg^(-1) soil compared with zero Zn addition. Higher rhizosphere concentrations of oxalic acid, glutamic acid, alanine, phenylalanine, leucine, and proline in Tede than in Airs likely resulted in increased Zn uptake from the soil by Tede than by Airs. The results suggested that genotypic differences in Zn accumulations were mainly because of different root exudates and rhizosphere Zn fractions.

The effectiveness of deep placement of fertilisers is determined by crop species and edaphic conditions in Mediterranean-type environments: a review

Much of our knowledge of plant growth in response to soil nutrient supply comes from studies under homogeneous soil conditions. However, the adoption of reduced or nil tillage and shallow banding of fertilisers at the time of seeding causes spatially variable distribution and availability of soil nutrients in agricultural lands. Soil available nutrients, particularly the poorly mobile ones such as phosphorus (P), potassium (K), zinc (Zn), manganese (Mn), and copper (Cu), stratify within the fertilised topsoil. In water-limited environments where the topsoil is prone to drying, soil nutrient stratification may influence nutrient availability and plant uptake because of impeded root growth or reduced diffusion of immobile nutrients to the root surface, or more likely a combination of both factors. Placing fertilisers deeper in the soil profile could increase nutrient acquisition and utilisation by plants as fertiliser nutrients are in the moist soil for a longer part of the growing season. However, the effectiveness of deep placement of fertilisers may also be determined by soil texture, tillage, fertilising history, nutrient mobility, and crop species. In Mediterranean-type climates of southern Australia, a yield response of winter crops to deep fertiliser mostly occurs on infertile sandy soils in low rainfall regions. This contrasts with the responses of winter and summer crops in northern Australia on soils with optimum-to-high nutrients but subjected to rapid and frequent drying of topsoil because of high temperatures and high evaporation demand during the growing season. The pattern of nutrient accumulation by crop species (indeterminate v. determinate) and the mobility of mineral nutrients in the phloem would also modify the effectiveness of deep-placed nutrients under drought. The complexity of plant responses to subsoil nutrition may suggest that before adopting deep fertiliser practice in a paddock it is essential to understand the effects of edaphic and climatic conditions, soil management, and plant-soil interactions in order to achieve maximum yield benefit.

Responses of osmotic adjustment and seed yield of Brassica napus and B. juncea to soil water deficit at different growth stages

Canola (Brassica napus L.) is a major rotation crop but low yield has limited its adoption by farmers in the low-rainfall regions of southern Australia, where drought events can occur at any stage of crop development. We examined the effect of soil water deficit on osmotic adjustment and seed yield of canola and mustard (B. juncea L.) at the juvenile, elongation, anthesis, or seed-fill stage under glasshouse conditions and post-anthesis drought in the field. At the juvenile and elongation stages, leaves of both canola cv. Monty and mustard line 397-23-2-3-3 adjusted osmotically after exposure to water deficit. In comparison, only the mustard line expressed osmotic adjustment at anthesis and neither genotype adjusted at the seed-fill stage. A single drought event at the juvenile or elongation stage had little effect on growth and seed yield of either genotype, whereas water deficit at anthesis or seed-fill stage reduced seed yield of the canola cultivar by decreasing pod number, seeds per pod, and/or harvest index but largely did not affect the mustard line. In the field where rainfall diminished and plants were subjected to increasing water deficit during the reproductive stages, canola cv. Karoo and mustard line JN25 showed higher osmotic adjustment at anthesis and less yield reduction than the canola cv. Monty. This study suggests that yield sensitivity to water deficit was mainly due to its effect on concurrent formation of yield components, but could be modified by the physiological trait of osmotic adjustment.

Osmotic adjustment and seed yield of Brassica napus and B. juncea genotypes in a water-limited environment in south-western Australia

The relationship between osmotic adjustment and seed yield of Brassica oilseeds was examined at a low rainfall site, Merredin, Western Australia, in 1998 and 1999. Genotypes of B. napus and B. juncea were subjected to rain-fed and irrigated treatments at the seed-fill stage. The B. juncea lines showed small or even no yield reduction under rain-fed conditions, and generally had no yield advantage over the B. napus cultivars where irrigated. In both species, an inverse correlation was found between the magnitude of osmotic adjustment and the percentage of yield reduction. Genotypes with low osmotic adjustment, under rain-fed conditions, had a yield reduction of up to 40%, whereas those with high osmotic adjustment had only 0-10% yield reduction. In contrast, seed oil concentrations decreased from 41% under irrigation to 38% under water deficits and the differences among genotypes were not related to osmotic adjustment. In 1999, osmotic adjustment was again observed for most of the genotypes, but its association with seed yield was not as obvious as in the previous year and usually only the osmotically adjusting B. juncea genotypes maintained a good yield under water deficits. Not all the B. juncea genotypes expressed osmotic adjustment despite the fact that they were generally more drought resistant than the B. napus genotypes. In both years, however, osmotic adjustment was associated with increased harvest indices of B. napus and B. juncea, indicating that this physiological trait can be beneficial to Brassica yield in a water-limited Mediterranean-type environment.

Heterogeneous distribution of phosphorus and potassium in soil influences wheat growth and nutrient uptake

Heterogeneous distribution of mineral nutrients in soil profiles is a norm in agricultural lands, but its influence on nutrient uptake and crop growth is poorly documented. In this study, we examined the effects of varying phosphorus (P) and potassium (K) distribution on plant growth and nutrient uptake by wheat (Triticum aestivum L.) grown in a layered or split soil culture in glasshouse conditions. In the layered pot system the upper soil was supplied with P and either kept watered or allowed to dry or left P-deficient but watered, whereas the lower soil was watered and fertilised with K. Greater reductions in shoot growth, root length and dry weight in the upper soil layer occurred in −P/wet than in +P/dry upper soil treatment. Shoot P concentration and total P content were reduced by P deficiency but not by upper soil drying. Genotypic responses showed that K-efficient cv. Nyabing grew better and took up more P and K than K-inefficient cv. Gutha in well-watered condition, but the differences decreased when the upper soil layer was dry. In the split-root system, shoot dry weight and shoot P and K contents were similar when P and K were applied together in one compartment or separated into two compartments. In comparison, root growth was stimulated and plants took up more P and K in the treatment with the two nutrients supplied together compared with the treatment in which the two nutrients were separated. Roots proliferated in the compartment applied with either P or K at the expense of root growth in the adjoining compartment with neither P nor K. Heterogeneous nutrient distribution has a direct decreasing effect on root growth in deficient patches, and nutrient redistribution within the plant is unlikely to meet the demand of roots grown in such patches.

Hydraulic lift by canola plants aids P and K uptake from dry topsoil

The role of hydraulic lift in nutrient uptake from dry topsoil is unclear. Vertically-split root systems were used to measure hydraulic lift and phosphorus (P) and potassium (K) uptake at the rosette and flowering stages of two canola (Brassica napus L.) cultivars (Trigold and Boomer with small and large root systems, respectively) grown in soil low in P and K. Rubidium (Rb) was used as a tracer to examine K uptake from K fertiliser banded in dry topsoil. During vegetative growth, statistically significant hydraulic lift was measured, irrespective of cultivar root size or a specific nutrient (P or K) placed in the topsoil. The Rb tracer revealed significant amounts of K were absorbed from the dry soil, but at rates five-fold lower than from the moist soil. The water lifted could not prevent cessation of P uptake by Boomer and a reduction in P uptake by Trigold. During mid to late flowering, ion uptake from the dry topsoil was impeded despite the continuation of hydraulic lift up to 28 days after the topsoil was dried. Minimal fine root growth following topsoil drying (and reduction in fine root biomass in some treatments) appeared to be the primary cause of impeded nutrient uptake.

Cd uptake in rice cultivars treated with organic acids and EDTA.

A pot experiment was conducted to examine the activity of antioxidant enzymes, the content of malondialdehyde (MDA), proline and protein, and Cd uptake in different rice cultivars exposed to Cd (0, 1 and 5 mg/kg) in the presence of organic acids and ethylenediamine tetraacetic acid (EDTA). The results showed the increase in activity of dismutase (SOD), contents of proline and protein but a decline in activities of peroxidase (POD) and catalase (CAT), and MDA content for cultivars Xiushui63 and IIyou527. The resistance to Cd was higher in Xiushui63 than that in Iyou527 under the same Cd treatment. Cadmium contents in grain, straw and roots of both cultivars were markedly reduced in the presence of organic acids and EDTA. Grain Cd contents was the highest for plants treated with organic acids, followed by organic acids + 1/2EDTA, and the lowest with EDTA; Cd contents in straw and root were the lowest for plants treated with organic acids, followed by organic acids + 1/2EDTA, and the highest with EDTA treatment when exposed to Cd.

DROUGHT STRESS: Soil Water Availability Alters the Inter- and Intra-Cultivar Competition of Three Spring Wheat Cultivars Bred in Different Eras

Competition for water generates a classic aspect of the tragedy of the commons, the ‘race for fish’, where crops must allocate more resource to acquisition of the limiting resource than is optimal for crop yield allocation. A pot experiment using a simple additive (target–neighbour) design was conducted to examine the above-ground and below-ground growth of three spring wheat (Triticum aestivum L.) cultivars when grown alone and in mixtures at three levels of water availability. The effects of competition and water availability were compared by observing patterns of growth, biomass allocation and below-ground outcomes. Competitive interactions were investigated among cultivars ‘HST’, ‘GY602’ and ‘LC8275’, target plant of each cultivar grown without neighbouring plants are referred to herein as control plant and one target plant of each cultivar sown surrounded either by same or another cultivar as intra- or inter-cultivar competition. Competitive ability was assessed as the response ratio (lnRR) between the target plant surrounded by six other plants and the target plant in isolation. Our results showed that the cultivar ‘HST’, released over a century ago, produced a higher biomass and grain yield than the more recently released cultivars ‘LC8275’ and ‘GY602’ when grown as isolated plants with sufficient water supply. However, competition for resources from neighbours led to target plant biomass and grain yield being significantly reduced relative to controls in all three cultivars, particularly in ‘HST’. When subjected to intra-cultivar competition, the two recently released cultivars ‘LC8275’ and ‘GY602’ had higher grain yields and water use efficiency for grain than ‘HST’ in all three water regimes. The landrace ‘HST’ had better and significantly linear relationships between biomass and biomass allocation, root length and specific root length, whereas the recent and modern cultivars had much more water-related species-specific changes in root morphology and allocation patterns. These results suggest that crop traits that influence competitive ability, such as biomass allocation to roots and root plasticity in response to drought have changed in modern wheat cultivars because of breeding and selection.

Varying phosphorus supply and development, growth and seed yield in narrow-leafed lupin

Narrow-leafed lupin (Lupinus angustifolius L.) is usually grown in sandy, acidic and phosphorus (P) deficient soil with low yield and variable harvest index. This study aimed to examine the effects of varying P supply on lupin growth, seed yield and harvest index. Non-abscission plants (cv. Danja) were grown in Lancelin sand at seven rates of P supply (5, 10, 15, 20, 25, 30 or 40 mg kg−1) in a naturally-lit glasshouse. The rate of leaf emergence, flowering time and flower number were decreased or delayed by low P supply (5, 10 or 15 mg kg−1), with no differences at P rates higher than 20 mg kg−1. High P supply (25, 30 or 40 mg kg−1) increased plant seed yield and harvest index largely by increasing the number of pods and consequently yield on the lateral branches, but had less effect on the number of seeds per pod and seed size. Seed yield and seed P concentration continued to increase up to 40 mg P kg−1but harvest index plateaued at 25 mg P kg−1, indicating that low P supply decreased reproductive growth more than vegetative growth in narrow-leafed lupin.

Osmotic adjustment segregates with and is positively related to seed yield in F3 lines of crosses between Brassica napus and B. juncea subjected to water deficit

For osmotic adjustment to be used as a selection criterion for adaptation to drought there is a need to demonstrate its segregation and association with seed yield in a population. This study examined osmotic adjustment and seed yield in F3 lines derived from crosses between Brassica napus (cvv. Karoo, Monty) and B. juncea (line JN25). A clear separation of the level of osmotic adjustment was observed among 20 F3 lines of Karoo x JN25 (7 high and 13 low) and 20 F3 lines of Monty x JN25 (8 high and 12 low) under glasshouse conditions. When the 3 parents and 8 selected F3 lines were grown in a low-rainfall environment in the field, the segregation of osmotic adjustment was largely similar to that observed in the glasshouse. Yield reduction, with irrigated plots as controls, was up to 30% for genotypes with low osmotic adjustment but only 10% for those with high osmotic adjustment. Osmotic adjustment was closely correlated with the accumulation of K+ (r = 0.91), soluble sugars (r = 0.90) and proline (r = 0.96), whereas other solutes (Na+, NH 4+, Cl-, NO3-, Mg 2+ and Ca2+) made little or no contribution to osmotic adjustment. Proline concentrations were very low in well-watered plants but sharply increased by 5- to 15-fold in plants subjected to water deficit, and net proline accumulation showed a higher association with total seed number (r = 0.86) and yield (r = 0.89) than did the concentrations of K+ (r = 0.75 to 0.82) or soluble sugars (r = 0.68 to 0.72). The study suggests that leaf proline concentration could be a good indicator of osmotic adjustment in Brassica oilseeds.