FROM FIELD TRIALS TO MOLECULAR STUDIES: GIBBERELLIN-MEDIATED SEED YIELD ENHANCEMENT, VERNALIZATION RESPONSIVE GENES AND RAPID SPRING GREEN UP IN POA PRATENSIS

Xin Xin

doi:10.7273/000007885

Kentucky bluegrass (Poa pratensis) is an economically important cool-season turfgrass species that supports multimillion-dollar seed and turf industries in the Inland Pacific Northwest. However, rising winter temperatures have shortened cold exposure and vernalization, resulting in reduced seed yield. This dissertation integrates agronomic and molecular approaches to address seed yield loss and insufficient vernalization in Kentucky bluegrass, as well as to explore the use of plant hormone foliar applications for turf management. A two-year seed-production study tested RyzUp Smartgrass® at four dose–timing combinations on newly established Kentucky bluegrass production field of the commercial elite cultivar ‘Midnight’. A single fall spray of 35 g ha⁻¹ doubled first-year seed yield and increased harvested biomass by up to 144 % without altering thousand-grain weight, standard germination, or accelerated-aging vigor. Spring-only applications were not significant, whereas repeated 140 g ha⁻¹ spring sprays suppressed yield by 63 %. These results demonstrate that low-dose GA₃ delivered prior to winter dormancy promotes reproductive tillering under field conditions and can partially compensate for inadequate natural cold exposure, whereas excessive post-dormancy doses risk yield loss. To uncover genetic limits on vernalization responsiveness, full-length VERNALIZATION 1 (PpVRN1) and VERNALIZATION 2 (PpVRN2) were cloned, providing the first vernalization-pathway sequences reported for P. pratensis. Phylogenetic and motif analyses confirmed their placement within the canonical grass VRN1 and ZCCT clades, respectively. Quantitative PCR across six different cold-treatment regimes showed that PpVRN1 expression rises in parallel with acquisition of flowering competence, while constitutively high pre-vernalization PpVRN2 levels in ‘Midnight’ are associated with its vernalization requirement. Cross-species expression of PpVRN1 in both spring- and winter-type Arabidopsis thaliana lines lower their cold requirement, accelerated flowering, and up-regulated FLOWERING LOCUS T, establishing PpVRN1 as a conserved floral promoter across monocot-dicot boundaries. Together, these data position the PpVRN1–PpVRN2 module as a tractable target for breeding cultivars with customized flowering windows suited to milder winters. Finally, a split-plot, two-year turf trial evaluated the interaction of GA₃ and spring nitrogen on mature Kentucky bluegrass stands. A single early-spring GA₃ application of 0.5–1.0 oz acre⁻¹ improved National Turfgrass Evaluation Program quality ratings by up to 14 % and advanced visual green-up without perceptible color loss. In contrast, for dosage over 1 oz acre⁻¹ repeated sprays caused color rating decreased and no additional turf quality benefit. Spring nitrogen fertilizer at 49 kg ha⁻¹ independently boosted turf quality and color and partially masked GA-induced bleaching effects; however, GA × N interactions were not statistically significant, indicating additive rather than synergistic effects. The optimal fast green up strategy emerging from these experiments, which is one moderate GA₃ spray coordinated with standard nitrogen fertilization, offers a cost-effective tool for superintendents seeking rapid spring recovery while maintaining aesthetic standards.

FROM FIELD TRIALS TO MOLECULAR STUDIES: GIBBERELLIN-MEDIATED SEED YIELD ENHANCEMENT, VERNALIZATION RESPONSIVE GENES AND RAPID SPRING GREEN UP IN POA PRATENSIS

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