Ear: Stem ratio in old and modern wheat varieties; relationship with improvement in number of grains per ear and yield

Abstract

Three field experiments were conducted over two seasons at three sites in Western Australia, to investigate the physiological basis of increased grain number per ear in some old and modern wheat varieties and lines isogenic for Rht dwarfing genes. Growth rates and relative growth rates of the stem and ear varied between varieties, but there were no discernible trends with date of introduction or maturity grouping. The ratio of ear: stem growth rates was generally higher in modern varieties than in old. Among the Rht lines, the dwarf lines had a higher ratio of ear: stem growth rate than the tall lines. The ratio of ear: stem dry-matter increased from terminal spikelet stage to values ranging from about 20 to 50% at anthesis. The ratio of ear: stem dry-matter at anthesis was higher in modern than old varieties and in dwarf lines compared with tall. There was an allometric relationship between ear and stem dry-matter, indicating a constant ratio between relative growth rates of the ear and stem. This analysis showed that a difference between old and modern varieties was evident soon after the terminal spikelet stage, and that a greater ear: stem ratio was due mainly to a bigger intercept of the regression of In ear dry-matter versus In stem dry-matter. The allometric relationship showed that when ear weight was 1 mg there was a difference in stem dry-matter which ranged from about 50 mg in a modern variety to 200 mg in an old variety. There were from 6.7 to 9 florets initiated in the mid-ear spikelets, but only a proportion survived to form grain. Floret death started when the flag leaf was fully emerged, at the beginning of the rapid ear-growth and extension of the peduncle and was completed by ear-peep stage. Modern varieties initiated more florets and had a higher number of viable florets present at anthesis than did old varieties. The isogenic lines initiated similar numbers of florets, but the proportion of florets which survived was greater in the dwarf lines. The grain yield and harvest index were generally higher in modern than in old varieties. Among the Isogenic lines, the dwarf lines had a greater harvest index than the tall lines. Grains per spikelet and per ear increased from old to modern varieties and from tall to dwarf lines. There was positive correlation between ear: stem ratio at anthesis and harvest index with different responses between sites and years. The results indicate that improvements in grain number have come about because the stem competed less strongly than the ear for dry-matter. This reduced competition resulted in either the initiation of more florets and/or greater survival of florets to form grains. Selection for high ear: stem ratio at anthesis may lead to further improvement in grain yield.