Item

Phenological development of perennial ryegrass in response to temperature and photoperiod : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Chynoweth, Richard
Date
2021
Type
Thesis
Fields of Research
ANZSRC::3004 Crop and pasture production , ANZSRC::300406 Crop and pasture improvement (incl. selection and breeding)
Abstract
Following emergence, perennial ryegrass is a vegetative juvenile and goes through primary (PI) and secondary induction (SI) phases before becoming reproductive and exhibiting floral initiation (FI). Experimental and anecdotal evidence shows the duration of primary induction is influenced by temperature while photoperiod (Pp) may influence the duration of both primary and secondary induction. The range of flowering responses shown by perennial ryegrass genotypes may be characterized by different responses to Pp and temperature. The aim of this work is to understand and quantify the range of observed responses. The influence of temperature and Pp on the development of perennial ryegrass was investigated using genotypes from different centres of origin grown under controlled environment and field conditions. Initial screening showed many accessions that originate from ~33 to 46° latitude became reproductive in constant 18°C, 20 h Pp, while few flowered in constant 18°C, 14 h Pp. These results demonstrate that flowering occurred in response to a long Pp, without prior PI. Progress through PI was assessed at 4, 8, 12 and 18°C, in both 8 and 17 h Pp. Plants were subsequently transferred to non-primary inducing conditions, 18°C, 17 h Pp, that would fulfil the SI requirements, at two weekly intervals for up to 12 weeks. Flowering was determined as when 50% of plants produced seed heads following SI. FI was determined by tracking accumulated organ number (primordium plus leaves) relative to Haun Stage (HS). The expected pattern was represented by three straight lines. The first period represents vegetative growth with a rate of ~2 primordium/HS. The second stage began at FI when the rate of primordium production increased. The final period was when primordium production ceased, which was represented by no further increase in the number of primordium/HS. FI and terminal spikelet (TS) were calculated as the inflection points for this relationship. Maximum temperatures for completing PI in an 8 h Pp and 17 h Pp (respectively) were 18°C and 18°C for ‘Medea’, 12°C and 4°C for ‘Kleppe’, 12°C and 8°C for ‘Grasslands Nui’ and 12°C and 12°C for ‘Grasslands Impact’. Thus, a function was required to reduce the upper limit of effective temperatures as Pp increased. No treatment achieved FI prior to exposure to long photoperiods. Thus, optimum temperatures for progression through PI (Vsat) were defined from treatments exposed to short days (8h). Vsat was defined as when the number of leaves produced post transfer reduced to 4HS from the shortest treatment duration. For ‘Medea’, this was 18°C and Vsat occurred after 28 days exposure, ‘Grasslands Impact’ required 56 days at 12°C, while ‘Grasslands Nui’ and ‘Kleppe’ required 70 days between 4 and 12°C. Vbase, the minimum exposure required at the optimum temperature for 50% of plants to flower, was 0 days for Medea, 23 days for ‘Grasslands Impact’ and 46 days for ‘Kleppe’ and ‘Grasslands Nui’. After Vsat plants continued to produce primordium at the vegetative rate until they were transferred to 17 h Pp, when FI occurred. This confirmed exposure to a long Pp was an obligate requirement for flowering. Additionally, no plant achieved FI prior to obtaining HS4-5, which sets a base HS for photoperiod perception while the minimum number of main stem leaves was nine. In the field, all four genotypes achieved primary induction from all autumn sowing dates prior to the shortest day. Subsequently FI was triggered by lengthening Pp at a genotype specific base Pp (Ppbase) of ~10.5 h for ‘Medea’, Grasslands Nui’ and ‘Grasslands Impact’ and 12 h for ‘Kleppe’. When FI occurred at Ppbase, Grasslands Nui’, ‘Grasslands Impact’ and ‘Kleppe’, produced ~6.5 leaves compared with ‘Medea’ that produced 5.5 while all genotypes reduced towards four leaves when FI occurred at the saturating Pp (Ppsat). Ppsat were 14 h for ‘Medea’, 15.7 h for ‘Kleppe’, 15.6 h for ‘Grasslands Nui’ and 17 h for ‘Grasslands Impact’. Therefore, combinations of Ppbase and the slope of Pp response separated the genotypes and described the time from FI to final leaf emergence. Concurrently the relationship between the number of leaves to emerge post FI multiplied by the phyllochron determined the date of final leaf emergence. Since ~2HS remained to emerge at TS for all sowing dates, the mechanism reducing the duration from FI to final leaf emergence is a reduction in the HS duration from FI-TS as Pp increases. Two modelling techniques were calibrated to incorporate vernalisation and Pp responses. These predicted competence to flower and final main stem leaf emergence to within 6 days for four genotypes sown on five dates between early autumn and late spring.
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