Effect of heat stress on pollen development and seed set in field pea (Pisum sativum L.)
Date
2017-02-01
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0003-4260-6513
Type
Thesis
Degree Level
Doctoral
Abstract
Pea (Pisum sativum L.) is a major legume crop grown in a semi-arid climate in western Canada, where heat stress often causes flower abortion and reduces yield. Heat specifically affects pollination and seed set, the processes associated with pollen fertilizing an ovule to form an embryo, and seed development. The goals of this research were to investigate the effect of heat stress on pollen development, function, and seed set, and to identify single nucleotide polymorphism (SNP) markers associated with reproductive development traits via association mapping.
Heat stress reduced pollen viability, the proportion of ovules that received a pollen tube, seed number per pod, and the seed-ovule ratio in a pod when exposed to 35/18°C day/night temperatures for 4-7 days. Heat stress also reduced ovule viability with P = 0.09. High temperature reduced in vitro pollen germination by approximately 30% in CDC Sage and 55% in CDC Golden when the 10-h incubation temperature increased from 24 °C to 36 °C. Pollen wall (intine) thickness increased by 46% from 222 nm to 414 nm as a result of heat stress, and anther dehiscence failed to occur following exposure to 35/18°C day/night for 7 days. The lipid region of the pollen coat and exine of CDC Sage was more stable compared to CDC Golden, which may explain the greater robustness of CDC Sage pollen to elevated temperature.
Timing of flower appearance and age of the two flowers is sequential on a nodal raceme, and nodes and flowers along the main stem are also sequential. Heat stress impacted young, barely visible floral buds in the developing inflorescence apex more than more advanced buds and open flowers. The flower abortion rate was greater when plants were exposed to heat stress and when young flower buds were visible at the first reproductive node (T1) compared to the development stage when flowers at the second reproductive node were fully open (T2). Similarly, seed-set, pod development, and seed yield were more negatively affected when high temperature exposure started at T1 compared to T2.
Seed development was negatively impacted by the pollen interaction with pistil under heat stress, such as anther indehiscence and fewer ovules evidently fertilized. Heat stress accelerated seed abortion in all ovule positions within pods. In half of the cultivars tested, ovules at the pod’s medial and stylar-end positions were more likely to develop into seeds compared to basal ovules. Cultivars with small seed-size such as ‘40-10’ and ‘Naparnyk’ were able to retain the most ovules and seeds per pod compared to large seed size cultivars, and large-seeded cultivars like ‘MFR043’ aborted seeds when exposed to heat.
Population structure analysis was conducted on a panel of 92 diverse pea varieties, and they clustered into three subpopulations mainly consistent with their geographical origins. Association analyses identified 60 single nucleotide polymorphisms (SNPs) significantly associated with reproductive development related traits including days to flowering (DTF), duration of flowering (DOF), number of reproductive nodes, number of pods on the main stem, pod set potential, percentage of pod set, and pollen germination reduction due to heat stress. Among these 60 marker-trait associations, 33 SNPs were associated with the onset of flowering, 8 SNPs with pod development, and 19 SNPs with the number of reproductive nodes. Twelve SNPs associated with DTF and 2 SNPs associated with DOF overlapped with the SNP markers associated with the number of reproductive nodes. However, markers associated with in vitro pollen germination were not found due to the variability in sampling 92 genotypes. The findings obtained from this research will benefit plant physiologists and plant breeders for a better understanding of successful reproductive development in field pea and other legume crops experiencing increasing temperatures due to global climate change.
Description
Keywords
association mapping, heat stress, Pisum sativum, ovule fertilization, ovule position, ovule viability, pollen germination, pollen tube, pollen viability, pollen wall, population structure, reactive oxygen species, seed set, stigma
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Plant Sciences
Program
Plant Science