|Abstract or Summary
- Under adverse soil moisture conditions a critical water stress
or water potential develops in the plant and productivity is reduced,
or in extreme cases, terminated. Research to determine the soil
moisture status for maximum production of specific crops has been
conducted for decades. The results have done much to increase the
productivity of the irrigable acreage. Yet, even with irrigation
water available, under adverse atmospheric conditions water stress
will still develop in the agronomic crops. The degree of stress
that develops and the duration, however, is generally unknown.
Limited research in controlled environments has been conducted,
but actual plant water stress levels under field conditions have
not been studied.
The purpose of this research was to determine the levels of
leaf water potential (water stress) that develop in specific horticultural
crops under conditions of optimum soil moisture. Further studies were made to relate the leaf water potential to atmospheric
parameters and leaf temperature.
Three horticultural crops, sweet corn (Zea mays), tomatoes
(Lycopersicon esculentum), and bush beans (Phaseolus vulgaris)
were grown on a Cloquato silt loam soil under high soil moisture
conditions and exposed to the natural atmospheric conditions common
to the Willamette Valley of Oregon. A pressure chamber was used to
monitor, in the field, the leaf water potential that developed during
each of five days throughout the summer months. An infrared radiometer
was utilized to monitor the temperatures of the leaves during
the same periods.
Sweet corn consistently developed the highest leaf water potential,
reaching as high as 13.0 atmospheres. The tomatoes and beans
developed leaf water potentials above 10.0 atmospheres. On all five
days the corn plants developed leaf water potentials above the critical
8.0 to 9.0 atmospheres suction levels reported in the literature
by other investigators. On four of the five days, the Ψl of the tomato
and bean plants equalled or exceeded these critical levels although
not to the same degree as the corn plants. The length of time
that the leaf water potential remained above 8.0 atmospheres suction
varied, but lasted up to 9 1/4 hours in the corn and up to 7 hours
in the beans and tomatoes. During these periods photosynthesis formation,
soluble nitrogenous and phosphorous compounds formation, RNA
accumulation, carbohydrate synthesis, and cell growth are reportedly
impaired, limiting productivity.
Leaf temperature under these stress conditions followed closely
the same general trends as ambient air temperature. High levels of leaf water potential did not necessarily cause the temperature
of the leaf to rise significantly. Leaf temperatures exceeded ambient
air temperature by a maximum of 3.8 °C, but were generally in close
agreement with or below ambient air temperature.
The results of this study indicated that many horticulture
crops may be limited in productivity by fairly frequent periods of
critical leaf water potential even though soil moisture levels are
adequate. On none of the five days studies were made did the atmospheric
conditions exceed those normally experienced in the Willamette
Valley. The maximum air temperature was 32.5 °C and the maximum solar
radiation level was 1.16 cal/cm²/min.
Further investigation is required to determine the effects
on yield, quantity and quality,of detrimental plant water potentials
and to develop means of alleviating Ψl before it reaches detrimental
levels. Further investigation could also lead to a new outlook in