Effect of Age, Size, and Weight of Witchweed Seeds on Host/Parasite Relations

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Abstract
An inherent problem of work with parasite weeds is variability of seed germination. A study was conducted to develop a procedure for segregating seeds of S. asiatica by age, size and weight to eliminate part of this variability. Seeds were classified into 7 age groups (1, 2, 3, 4, 6, 7 and 8 yr old) and 5 size and weight groups for testing. The latter were large (> 149 .mu.m in diameter) and heavy (specific gravity > 1.0) (LH), large and light (specific gravity < 1.0) (LL), small (< 125 .mu.m in diameter) and heavy (SH), small and light (SL), and random (mixture of seed sizes) (R). Significant reductions in seed viability were manifested with seeds > 2 yr old and in seed germination with seeds > 4 yr old. Average viability rate of seeds > 4 yr old was 20%. Witchweed total infestation and emergence significantly increased with 1-, 2-, and 3-yr-old seeds but significantly decreased with seeds > 4 yr old. Shoots of corn (Zea mays) plants grown in soil infested with 1-, 2-, 3- and 4-yr-old seeds were significantly lighter than those grown in soil infested with seeds > 6 yr old; roots of corn plants grown in soil infested with 1-, 2-, 3- and 4-yr-old witchweed seeds were significantly heavier than those grown in soil infested with seeds > 6 yr old. Maximum seed viability and germination occurred in LH seed. Seed viability and germination diminished with age fasted with LL, R, SH and SL than with LH. Maximum witchweed infestations and emergence in corn was obtained with LH seed. Shoots of corn plants grown in soil infested with LH, R and LL, seed were significantly lighter than those grown in soil infested with SH and SL seed; roots of corn plants grown in soil infested with LH, R and LL were significantly heavier than those grown in soil infested with SH, SL and the control. Witchweed seed viability and germination responses were less variable with uniform seed size (e.g., LH, LL, SH and SL) than with mixed seed (e.g., R).