Water-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
Knowledge 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.