Browsing by Author "Matthew, Cory"
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- ItemA study of seasonal root and tiller dynamics in swards of perennial ryegrass (Lolium perenne L.) : a thesis presented in partial fulfilment of the requirements for the degree of Ph D in Agronomy at Massey University(Massey University, 1992) Matthew, CoryObjectives of this study were (i) to provide data on seasonal variation in root mass and root replacement in perennial ryegrass dominant swards, (ii) to simultaneously collect parallel data for above-ground parameters tiller population density, tiller natality, tiller mortality, herbage mass and herbage production, and (iii) to determine if such information on the behaviour of root and shoot systems and the inter-relation between the two could identify ways in which grazing management manipulation favouring root system development might subsequently result in pasture production increases. Perennial ryegrass was chosen for study because it is the species most commonly used in new pasture sowings in New Zealand. Four field experiments and two glasshouse experiments are reported. In the first field experiment, techniques for making measurements of root mass and root production in field swards were evaluated. Over 80 days from November 1985 to February 1986, total root mass measured by washing roots from "intact" soil cores did not change, but root mass in core-holes bored out and "refilled" with sand was 53% of that in intact cores. The refilled core technique was therefore adopted as a measure of "apparent" root production, and a later calibration study showed that measurements using the refilled core technique underestimate actual root growth. Using the refilled core technique, differences in root production were detected between six mowing treatments designed to allow varying degrees of reproductive development. Root growth was greater where mowing of swards was delayed sufficiently to allow reproductive growth until head emergence or anthesis than where seedheads were either removed before head emergence or left un-mown until seed-set. There was also evidence of increased tillering on treatments with the highest root growth. In the second experiment (December 1986 to May 1988) plots were subjected to lax (LL) or severe (HH) grazing management or to cross-over LH or HL grazing managements. The cross-over date, December 7 1987, was timed to coincide with peak reproductive development. Swards in this study had approximately 100 m m-2 underground stolon, with a seasonal increase in late winter and higher stolon formation on LL plots than on HH plots. Apparent root growth rates exhibited marked seasonal variation, and were typically about 15% of above-ground net production. For 12 months from January 1987 to January 1988 apparent root growth averaged 8.4 and 7.3 kg DM ha-1 day-1 for LL and HH plots, respectively for 0 - 600 mm soil depth. Because of these relatively small differences in root growth, it was concluded that manipulation of root growth would not enable herbage production advantages to be achieved. However, after introduction of crossover grazing managements, high herbage production was observed on LH plots and tissue turnover and herbage dissection measurements showed that this high herbage production was associated with high daughter tiller formation, probably from stubs of decapitated flowering tillers. Experiment 3 (November 1988 to January 1989) comprised 3 plots under common grazing management, and was designed to provide detailed information on the location on the tiller axis of actively elongating roots, and to confirm seasonal patterns of root and tiller growth observed in Experiment 2. Root initiation normally occurred at the same node as leaf senescence, normally two roots formed at each node, and few active roots were found more than 10 nodes below the last leaf. Seasonal timing of peak root growth and tiller appearance was different from that in Experiment 2, however. This is believed to reflect genetic differences between the cultivars 'Ellett' used in Experiment 2 and 'Grasslands Ruanui' used in Experiment 3, but specifically designed controlled comparisons would be needed to confirm this. Experiments 4, 5, and 6 were designed to provide more information on the reasons for high tillering on LH plots in Experiment 2, and investigated the number of daughter tillers formed by flowering tillers subjected to differing cutting treatments. In all three experiments the number and weight of daughter tillers formed was greatest where a degree of reproductive growth occurred, and was reduced where seedheads were cut closer to the ground or earlier, and where seedheads remained uncut to act as a competing sink. These observations indicate that assimilate from parent flowering tillers is important for daughter tiller formation and, in Experiment 6, a cutting treatment which increased translocation of carbon-14 tracer from labelled flowering tillers to daughter tillers also increased the number and weight of daughter tillers formed. It is concluded that grazing management which exploits the potential for high tillering rates from stubs of flowering tillers could increase herbage production on many New Zealand farms by more than 0.5 t DM ha-1 over the summer/autumn period, and implications for farm practice are briefly discussed.
- ItemWater-use efficiency in perennial ryegrass (Lolium perenne L.) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Science at Massey University, Manawatu, New Zealand(Massey University, 2021) Weerarathne, Lekamalage Visna YahaniKnowledge of genetic and physiological bases of drought responses and stress tolerance properties of pasture plants is an integral part of designing efficient pasture improvement programs to combat the consequences of climate change. However, experimental evidence or theoretical analyses on that aspect is sparse in the literature. Therefore, considering wider applications, high economic importance, and acknowledged poor tolerance of (Lolium perenne L.; PRG) to drought, the main aim of this research was to evaluate morpho-physiological trait responses that are linked to water-use efficiency (WUE) in different PRG populations from different sources in simulated summer drought cycles under a controlled environment. This study also included a quantitative genetic analysis conducted on the key traits to ascertain which traits are under genetic control for future breeding purposes. Experiment 1 screened single potted PRG genotypes from three commercial cultivars with industry reputation for persistence for natural differences in morpho-physiological traits related to water use (WU) under simulated drought. Large within-population variation was observed for the measured traits which included, among others, WUE (g WU/g plant dry matter); shoot dry weight, SDW; leaf osmotic potential, OP; leaf relative water content; predawn leaf water potential; root dry weight at 20–50 cm depth, RDWD; gravimetric soil moisture at 30–40 cm depth, SMCD; post-cutting regrowth. Principal component analysis (PCA) identified important trait associations contributing to high WUE (i.e. WUE-OP-RDWD trait association) and one related to higher SDW together with ‘SMCD conservation’ indicative of ‘true WUE’ was used to make a divergent selection of 20 high- and 15 low-WUE genotypes (HWUE and LWUE, respectively). Experiments 2 and 3 were conducted simultaneously using the same methodology as Experiment 1 and inter-randomised in the same glasshouse space. Experiment 2 retested clones of HWUE and LWUE plants selected in Experiment 1 for consistency of trait expression across the two consecutive growing seasons, and also collected data for additional traits. Results confirmed that the key trait associations identified in Experiment 1 were almost identically expressed in Experiment 2. From data on additional traits, it was established that the accumulation of high molecular weight sugars in the shoots significantly contribute to ‘true WUE’ of a subset of PRG genotypes but, with the less involvement of gas exchange data under the conditions tested. It is speculated that enhanced mesophyll conductance of CO2 might underlie this important trait association. However, the large majority of genotypes exhibited a ‘SMCD-depleting’ trait association of WUE with improved gas exchange and maximum quantum efficiency of PSII, demonstrating late A.R. Blum’s theory of WUE. Thus, selection of PRG for drought tolerance should consider yield and soil moisture data together to establish the most appropriate category of WUE trait association in improved cultivars. Experiment 3 investigated drought response trait associations in two further populations: a group of elite plants from a commercial plant breeder’s breeding program (CBL) and a PRG germplasm line derived from crossing Mediterranean and Middle Eastern seed accessions (MMEL) compared with those from the HWUE selection. PCA results showed that the major trait associations found in the yield and water relations data of elite subsets of CBL closely followed those of the HWUE selection, but elite MMEL plants exhibited typical summer dormancy characteristics where the average SDW of MMEL was 40% of lower than that of the CBL plants. Results further suggested that the company field evaluation system could benefit from the consideration of water relations traits, including WUE and associated traits like OP, as externally-measured selection traits for PRG drought tolerance. Experiment 4 evaluated quantitative genetic parameters of the key traits using the same methodology from Experiment 1 in a breeding population of 36 ‘half-sib (HS) families’ under both stressed and non-stressed conditions. There were significant estimates of among- and within-HS family genetic variances, narrow-sense heritability, and predicted genetic gain estimates for the key traits, indicating high genetic potential of each trait for breeding purposes under the conditions tested. However, the correlated response to selection of each trait pair comprising highly genetically correlated morpho-physiological traits with WUE was lower than that gain from the single-trait selection, accommodating further research questions on the efficacy of indirect and multi-trait selection of key traits. Based on the current results, it was found that the direct selection of PRG for WUE or concurrent selection for OP, RDWD, SMCD, and SDW or RGS traits under drought is advisable. Furthermore, significant quantitative genetic parameters estimated for WUE under non-stressed conditions together with high genetic correlation observed for WUE between stressed and non-stressed conditions suggested that a PRG population can potentially be selected for this trait at early growth stages or before imposition of water deficit.