Alleviating delayed field curing-induced deterioration in rice seeds by cathodic water invigoration: a comparison of seed and seedling responses in two upland rice species and their interspecific hybrid.

Doctoral Degree. University of KwaZulu-Natal, Durban.

Rice constitutes the major source of the world’s food supply. A number of varieties are grown in many parts of the world, all which produce orthodox seeds that are usually stored between growing seasons. As in other crop species, storage-induced loss of seed quality (viability and vigour) is inevitable but considerable research effort has been invested in optimising seed bank storage protocols for rice to ensure acceptably high levels of seed germinability and seedling emergence. However, poor post-harvest practices such as delayed field curing before threshing in developing countries in the humid tropics, such as Ghana, result in curing-induced seed deterioration in the field prior to ex situ storage. Given that many rice growing regions are likely to experience significant levels of climate change, seed processing and storage induced declines in rice seed quality could exacerbate the crop losses incurred by rice farmers in these regions in the future. This motivated the present study which was designed around three aims: 1) to investigate how environmental conditions and the duration of field curing influenced seed macro-structural integrity, susceptibility to microbial infection, and seed germinability and vigour; 2) to identify macro-structural and ultrastructural indicators/ biomarkers of field curing and associated storage-induced stress/ damage; 3) to assess whether the invigoration of field-cured seeds with cathodic water (CW), an established antioxidant-based seed invigoration medium, and deionized water (DW) can alleviate the deteriorative effects of delayed field curing on rice seed cellular integrity, germination and subsequent seedling growth and biomass. The specific objectives of this comparative study, which involved an Asian (Oryza sativa L.) and African (Oryza glaberrima Steud) upland rice species and their interspecific hybrid (O. sativa × O. glaberrima), were as follows: a) to compare the impacts of delayed field curing in wet and dry environments on seed physical, physiological and pathological quality; b) to identify potential ultrastructural biomarkers of seed sensitivity to delayed field curing-induced stress/ damage in embryonic root meristematic cells using transmission electron microscopy (TEM); c) to assess whether CW invigoration alleviates cellular stress/ damage induced by delayed field curing using selected ultrastructural biomarkers of seed sensitivity to such curing-induced stress/ damage; and d) to assess whether invigoration with CW and DW improves seed germinability and emergence, and subsequent plant growth in field cured seeds, relative to non-invigorated (NI) dry seeds.