TRANSPIRATION-USE EFFICIENCY COEFFICIENT OF SEVEN WEED SPECIES AS AFFECTED BY FRACTION OF TRANSPIRABLE SOIL WATER AND GROWTH STAGE

Authors

Venkatarao Mannam, University of Nebraska-LincolnFollow

Date of this Version

Summer 7-27-2011

Comments

A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Agronomy, Under the Supervision of Professor Mark L. Bernards. Lincoln, Nebraska: August, 2011

Copyright 2011 Venkatarao Mannam

Abstract

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 K_c values of seven weed species and to measure how K_c 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. K_c 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 K_c of henbit (vegetative growth) or shepherd’s purse. K_c values increased as FTSW levels declined for common lambsquarters, pinnate tansymustard, henbit (complete lifecycle) and field pennycress (vegetative growth). K_c 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 K_c for pinnate tansymustard and dandelion. K_c values decreased between the first bloom and seed maturity for field pennycress, common lambsquarters, shepherd’s-purse and henbit. The decline in K_c 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 K_c values did not respond consistently to FTSW across harvest times – K_c 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