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Transpiration-use efficiency coefficient (Kc) describes the amount of biomass produced per unit transpiration at a given vapor pressure deficit. A series of greenhouse experiments were conducted to determine the Kc values of seven weed species and to measure how Kc values were affected by fraction of transpirable soil water (FTSW) level and plant growth stage. Experiments were conducted using a factorial design with 4 levels of water stress (0.3, 0.4, 0.7, and 1.0 FTSW) and two harvest times (first bloom and seed maturity). After plants attained a predetermined size, each plant was sealed at the base using a polyethylene bag. Pots were weighed daily and maintained the required weight by watering through an inserted syringe. Pre-bagging transpiration was back-calculated from 0 d to the first 20 d of measured daily transpiration. One set of plants was harvested at first bloom and another set was harvested at seed maturity. Kc was calculated as the ratio of total biomass to the cumulative transpiration multiplied by the average daytime vapor pressure deficit.
FTSW level did not affect the Kc of henbit (vegetative growth) or shepherd’s purse. Kc values increased as FTSW levels declined for common lambsquarters, pinnate tansymustard, henbit (complete lifecycle) and field pennycress (vegetative growth). Kc values decreased as FTSW declined for dandelion, Carolina foxtail, and field pennycress (complete lifecycle) suggesting that these species were relatively sensitive to water stress. Plant growth stage did not affect Kc for pinnate tansymustard and dandelion. Kc values decreased between the first bloom and seed maturity for field pennycress, common lambsquarters, shepherd’s-purse and henbit. The decline in Kc may be attributed to high oil content in the seed of field pennycress, shepherd’s-purse and henbit, and the highprotein content of common lambsquarters. Carolina foxtail Kc values did not respond consistently to FTSW across harvest times – Kc values increased from first bloom to seed maturity at FTSW levels 0.3 and 0.4, but remained same at the 0.7 FTSW level.
Advisor: Mark L. Bernards