S. N. Nichols

Pasture plants and soil fertility management to improve the efficiency of phosphorus fertiliser use in temperate grassland systems

Abstract. Phosphorus (P) fertilisers are important for productivity in many grassland systems. Phosphorus is a non-renewable and finite resource, and there are environmental and economic reasons for using P more effectively. We review the P balance of temperate pastures to identify the factors contributing to inefficient use of P fertiliser and discuss ways to improve P-balance efficiency. Immediate gains can be made by ensuring that P fertiliser inputs are managed to ensure that the plant-available P concentrations of soil do not exceed the minimum concentration associated with maximum pasture production. Unnecessarily high soil P concentrations are associated with greater potential for P loss to the wider environment, and with higher rates of P accumulation in soils that have a high P-sorption capacity. Soil microorganisms already play a crucial role in P cycling and its availability for pasture growth, but are not amenable to management. Consequently, plants with lower critical P requirements, particularly because of better root foraging, will be an important avenue for improving the P-balance efficiency of fertilised pastures. Traits such as long fine roots, branching, root hairs, and mycorrhizal associations all contribute to improved root foraging by pasture plants; some of these traits are amenable to breeding. However, progress in breeding for improved P efficiency in pasture plants has been minimal. It is likely that traditional plant breeding, augmented by marker-assisted selection and interspecific hybridisation, will be necessary for progress. There are practical limits to the gains that can be made by root foraging alone; therefore, plants that can ‘mine’ sparingly available P in soils by producing organic anions and phosphatases are also needed, as are innovations in fertiliser technology.

Nutrient responses and macronutrient composition of some Trifolium repens×Trifolium uniflorum interspecific hybrids

Abstract. Interspecific hybridisation is being utilised in white clover (Trifolium repens L.) breeding programs to overcome factors currently restricting productivity and persistence. Valuable new traits that may be introduced from the wild relative T. uniflorum include root characteristics and other adaptations to its natural, Mediterranean habitat. This study examined the effect of hybridisation on growth and macronutrient composition of white clover compared with T. uniflorum and T. repens × T. uniflorum backcross 1 (BC1) hybrids in two glasshouse sand culture experiments. Shoot and root dry weights of BC1 hybrids were greater than of white clover in low-concentration nutrient treatments but not in a more concentrated treatment. Decreases in dry weight with decreasing nutrient treatment strength were also smaller for some BC1 hybrids compared with white clover and other hybrid families. Most foliar macronutrient levels were adequate for white clover growth, but mean shoot or leaf phosphorus (P) concentrations were below published critical levels. Higher dry matter production under these low internal P concentrations suggests that some T. repens × T. uniflorum BC1 hybrids may be more tolerant of lower soil P levels than white clover. Such adaptations are likely to have been inherited from T. uniflorum. However, transgressive segregation may also be occurring, as T. uniflorum was larger than white clover in some, but not all, cases of low nutrient supply.

Adventitious root mass distribution in progeny of four perennial ryegrass (Lolium perenne L.) groups selected for root shape

Abstract The effects of one cycle of selection for adventitious root system shape (i.e. percentage of total root mass in successive depth increments) were investigated in progeny of four perennial ryegrass pools with contrasting rooting patterns: (1) low surface (0–10 cm) root mass and roots to 1 m; (2) high surface root mass and roots to 1 m; (3) high root mass 10–20 cm and roots to 1 m; (4) high surface root mass, shallow rooting. Ten half-sib families were selected from each of the root type progenies, and five seeds from each family were sown and raised as stock plants. Tiller cuttings of each plant were planted into individual 1 m deep root screening tubes of sand and irrigated with nutrient solution After 115 days, the shoots were cut off, the sand/root column cut into 10 cm increments and the roots washed free of sand. The shoots and root samples were oven-dried and weighed. The four root shape progeny groups did not differ significantly for shoot or root dry weight (DW), or root/shoot DW ratio. Root type 2 (high surface root mass and roots to 1 m) progeny had a significantly higher percentage of total root mass between 0 and 10 cm than did root type 1 (low surface root mass and roots to 1 m) but none of the other differences were significant for percentage roots 0–10 cm. The root type 2 progeny had a lower percentage of total root DW between 10 and 20 cm than all the other root types, none of which differed and had a significantly lower percentage of root DW 20–30 cm than types 1 and 4 but did not differ at this depth from type 3 progeny. The probability of root type 4 (high surface root mass, shallow rooting) plants having roots at 1 m depth was 0.67; this was significantly lower than for the other selections, none of which varied significantly. These results indicate that it should be possible to change root system shape in perennial ryegrass using conventional breeding techniques.

Comparisons between wild populations and bred perennial ryegrasses for root growth and root/shoot partitioning.

Abstract Root and shoot dry weights (DW), and root DW depth distribution were determ ined for wild type perennial ryegrass, breeding lines and cultivars. The plants were grown in 1 m deep tubes of sand culture in a glasshouse experiment. There were significant differences in shoot DW among accessions in the wild types but not within the bred material, and the wild types had more accessions with low shoot DW. Root DW varied significantly among accessions in both the bred material and the wild types. There were significant differences in root/shoot DW ratios among accessions, and between pools in the breeding lines, but no differences within the wild types. Root/ shoot ratios in the wild types had a much narrower and generally higher range of values than those in the bred lines. The percentage of root DW in the top 10 cm of sand did not vary significantly in the bred lines but it did in the wild types. Variation was recorded in root DW depth distribution at both the accession and genotype levels. We concluded that selection for merit based on shoot performance does not necessarily result in large root systems, and better plant performance might result from an increase in root system size in perennial ryegrass.

Drought resistance of Trifolium repens × Trifolium uniflorum interspecific hybrids

Abstract. White clover (Trifolium repens L.) is a widely used and highly valued temperate legume; however, its productivity and survival are restricted under dryland and drought conditions. This study investigated whether drought resistance of white clover could be improved by interspecific hybridisation with Trifolium uniflorum L. After almost 4 months without irrigation in a rain-shelter facility, shoot dry weight (DW) decreased significantly less in first-generation backcross (BC1) hybrids (–47%) than second-generation backcross (BC2) hybrids (–68%) and white clover (–69%). Stolon morphological parameters such as internode length and leaf lamina area also decreased less under water stress in the BC1 hybrids than in BC2 and white clover. There was also lower senescence in BC1 under water stress than in the other clover types. Genotypes with smaller changes in leaf lamina area, internode length, senescence and lateral spread had smaller changes in shoot DW, and there were significant correlations between constitutive levels of some characteristics and the effect of water stress on shoot DW. Under water stress, the growth form of the BC1 hybrids was compact, dense and prostrate, whereas white clover was more spreading and open. Increased allocation of dry matter to roots under drought, and greater root diameter, may also have influenced the ability of BC1 hybrids to maintain water uptake and key physiological processes. Overall, the data confirm that the drought resistance of white clover can be improved through hybridisation with T. uniflorum.

Phosphate responses of some Trifolium repens × T. uniflorum interspecific hybrids grown in soil

Abstract. Previous studies in sand culture suggested that some white clover (Trifolium repens) × T. uniflorum interspecific hybrids were more tolerant than white clover of low external phosphate (P) supply. Here, P acquisition and growth responses were determined in soil for two T. repens × T. uniflorum backcross hybrids and their parental white clover cultivar, grown in a glasshouse pot experiment at Olsen P of 6, 7, 9, 14, or 20 mg P kg–1 soil. Growth of all of the clover entries responded strongly to increasing soil P levels, and one hybrid clover grew, on average, 17% better than the white clover control cultivar at Olsen soil P 9–20 mg kg–1. Internal P concentrations and shoot growth per unit P absorbed did not differ among the clovers. Instead, improved growth of the hybrid resulted from a greater ability to acquire soil P. This hybrid had the longest, most frequently branched roots. Frequent branching and growth of root tips into fresh soil would reduce the limitations to P uptake imposed by slow diffusion of P to the root surface. The results confirm previous observations that interspecific hybridisation is a useful strategy for increasing the range of P responsiveness in breeding populations for white clover.

Effects of inbreeding on nodal root system morphology and architecture of white clover ( Trifolium repens L.)

Plants of white clover (Trifolium repens L.) cultivar Crau, a self-fertile Crau genotype, and nine generations of inbred progeny were raised in sand culture in a glasshouse experiment. Digital images of the root systems were made and root morphological characteristics were determined on all the plants. Root architectural parameters were measured on the Crau parent and the S1, S4, S6, and S9 inbred lines. The clover roots became shorter and thicker with inbreeding but the number of root tips per plant was unchanged. Root architecture (branching pattern) was largely unaffected by inbreeding. It is concluded that inbreeding white clover will lead to shorter, thicker roots, and reduced nutrient uptake efficiency compared with the parent clover. The degree to which these deleterious traits are overcome during the development of F1 hybrids needs to be determined.

Loss of weight in ryegrass and clover roots preserved in ethanol prior to image analysis for root traits.

The dry weight content of root samples from perennial ryegrass and white clover decreased significantly over a week’s storage in 70% ethanol, but did not change further with longer storage times. Ryegrass roots lost on average 22.4% of the original dry weight, and clover roots lost 29.2%. Storage of roots in ethanol prior to image analysis of root traits could introduce significant error in the calculation of parameters involving root dry weight. Determination of root fresh weights, and dry weight for a subsample prior to preservation would allow calculation of a correction factor for dry weights obtained from preserved samples.

Effect of hybridisation with Trifolium uniflorum on tap root survival in white clover

A field study was conducted to determine whether tap root survival of white clover could be improved by hybridisation with Trifolium uniflorum. Tap root fragmentation and percentage of surviving tap roots were measured in 13, 16 and 19–20 month old plants. There were no intact healthy tap roots in white clover or second backcross (BC2) hybrids (12.5% T. uniflorum genes) post 13 months, but these were still present in T. uniflorum and first backcross (BC1) hybrids (25% T. uniflorum genes). Survival of T. uniflorum tap roots was higher than BC1, BC2 and white clover—30% of plants had intact, healthy tap roots at 19–20 months. The BC1 generation (31%) also had higher tap root survival than BC2 (13%) and white clover (11%) at 13 months. Although improved survival was not expressed as strongly in older BC1 plants, tap root deterioration was slower than in white clover and BC2. There is potential for targeted selection of specific genotypes and traits to further increase tap root lifespan in BC1 hybrids as there has been no previous selection for root traits in this material. The relationships between root diameter, leaf size and persistence in T. uniflorum and hybrids may differ from those expected for white clover cultivars. Characteristics of nodal rooting would also be expected to play a part in longer-term productivity and persistence.

Root morphology and architecture, and internal phosphate use efficiency, in related white clover cultivars of different ages

Root morphology and architecture were determined for four related white clover (Trifolium repens L.) cultivars—Huia, Pitau, Kopu II and Mainstay, released 57, 36, 15 years and 1 year ago, respectively. The plants were grown in soil maintained at a range of available phosphate (P) levels in a glasshouse pot experiment. Root samples for analysis were obtained from plants growing in the 17 and 62 mg P kg−1 soil treatments. There were no statistically significant (P < 0.05) effects of cultivar age or soil P level on root morphology or architecture. The inherently efficient herringbone branching pattern of white clover root systems has apparently been able to support ongoing improvements in shoot system performance during cultivar development, without changing. There was a significant increase in shoot dry weight yields per unit plant P absorbed, as cultivar age decreased. This is evidence that repeated selection for shoot traits has resulted in an increase in internal P-use efficiency.