David W. Still. Assistant Research Scientist, Department of Plant Sciences. Ph.D., Texas A&M University. (75% Research; 25% Extension)

• Office: Yuma Valley Agricultural Center, 6425 W. 8th St., Yuma, AZ 85384
• Telephone: (520) 782-3836
• e-mail: dstill@ag.arizona.edu

Seed physiology.

My research is primarily concerned with physiological aspects of seed quality. Despite the fact that the oldest known seeds are from the Devonian period 360 million years ago, and are the cornerstone from which our modern civilizations have been built, there are fundamental questions which remain unanswered. Questions such as why are some (orthodox) seeds able to withstand desiccation to 2-3% moisture content, while others (recalcitrant) cannot desiccate below 30% and remain viable? Why do seeds from certain species have greater longevity than others? What are the signals which induce germination?

With each generation of seed planted we witness the integration of millions of years of evolution as well as the influence of the immediate environment in which the seeds developed. The combined effects have an enormous impact on the way agriculture is practiced. The goal of every seed company which produces seed, and ultimately, the basis of natural selection, is to produce a high quality seed. Seed quality may be recognized by high germination percentages, rapid and uniform germination, the ability to withstand abiotic and biotic stress, and longevity.

We have applied a population-based threshold model to assess seed quality during seed development in Brassicas (rape, cabbage, Brussels sprouts and broccoli). We are in the process of studying various components such as sugars and proteins which may be associated with seed quality.

Vegetable producers in the lower desert regions of Arizona and California are faced with germinating seeds of cool season crops while soil temperatures are above 55°C. The optimum temperature for these seeds is generally below 30°C. We are beginning to investigate the physiological consequences of high temperatures stress on seed germination of cool season crops.

Still DW, Bradford KJ (1996) Seed quality during development in Brassica species using a population-based threshold model. (in preparation).

Still DW, Bradford KJ (1996) Population variation in mannanase activity and germination rates. I. Development of single seed assay. (in preparation).

Still DW, Bradford KJ (1996) Population variation in mannanase activity and germination rates. II. Relationship between frequency distribution of mannanase activity and germination rates. (in preparation).

Still DW, Kovach DA, Bradford KJ (1994) Development of dessiccation tolerance during embryogenesis in rice (Orza sativa) and wild rice (Zizania palustris): dehydrin expression, abscisic acid levels, and sucrose accumulation. Plant Physiol. 104:431-438.