|Abstract or Summary
- Dryland winter wheat in eastern Oregon is usually subjected to
water stress several times during the growing period. Moreover, the
last three months of growth period depend strongly on the available
soil water. The fertility level, stage of growth, availability of
soil water and climatic conditions all interact to determine the
severity of crop water stress. The level of nitrogen and phosphorus
fertility in the growing wheat crop can affect plant growth and
development, water uptake and the incidence and severity of water
stress. In order to gain a better understanding of the complex
interactions leading to water stress in the wheat crop, a means of
determining when and how long the stress occurs is needed. The Crop
Water Stress Index (CWSI) developed by Idso et al in 1981 utilizing
the infrared thermometer was used to determine the crop water stress
level during the critical spring growth period.
The objectives of this work were: (1) to study the effect of N
and P fertilization levels on crop water stress and water uptake by
the crop; (2) to describe the crop water stress phenomenon in order
to help explain when, and why water stress occurs; (3) to analyze the
dry matter production and partitioning and yield components as related to fertilization, crop water stress and date of planting; and
(4) to attempt to develop an equation to predict grain yield of soft
white winter wheat in Oregon, given a certain level of water stress
assessed by the CWSI.
Two types of field fertilizer experiments were conducted using
a soft white winter wheat cv. Stephens at the Sherman Experiment
Station, Moro, Oregon during the 1982 and 1983 seasons.
Atypical climatic conditions with precipitation and relative
humidity levels greater than, and maximum temperatures less than the
long-term means combined to produce a relatively low level of crop
water stress. There were two relatively short periods in 1982 in
which moderate to severe crop water stress occurred. The CWSI
proved capable of detecting the severity and duration of these
stress periods with a good level of reliability.
Nitrogen fertilization increased the total crop water uptake.
Coincidentally, CWSI level was always reduced with the addition of
N. The only exception was in one N experiment in 1982, in which
water uptake was not increased with N fertilization.
The total dry matter production and yield relationship was
indicative of the climatic conditions which produced nearly optimum
soil water conditions in the 1982 and 1983 seasons.
Nitrogen increased total dry matter production during both
seasons, with a higher level being evident in 1983.
The yield increase from the added nitrogen was mainly due to an
increase in spike number and to a lesser extent an increase in the
number of kernels per spike. Late plantings produced larger individual spikes with a greater
number of kernels than earlier seeding , but these differences were
not great enough to overcome the drastic reduction in spike number.
A logarithmic relationship between grain yield and the CWSI
averaged on a daily basis was developed. Although somewhat
inconsistent, the need to account for other factors such as N
availability and the differences in vegetative growth produced before
the period of CWSI study, was recognized. The assumption that CWSI
alone could predict grain yield was originally based on limited soil
water conditions. If that condition is not present , the other
variables that may limit yield potential must be considered.
The use of infrared thermometry technology and the CWSI system
appear to be feasible tools to determine crop water stress at the
field level. However, one can expect more consistent and reliable
results under the more normal stress conditions.