Time for cotton to uptake water of a known isotopic signature as measured in leaf petioles

Authors: Goebel, Tim; Lascano, Robert

Submitted to: Agricultural Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/2014
Publication Date: 2/17/2014
Citation: Goebel, T.S., Lascano, R.J. 2014. Time for cotton to uptake water of a known isotopic signature as measured in leaf petioles. Agricultural Sciences. 5(2): 170-177.

Interpretive Summary: Water management for crop production in the Southern High Plains has to consider methods to make better use of rainfall, particularly during the growing season. Further, the depletion of irrigation water from the Ogallala Aquifer means that the production level from dryland farming will continue to increase and deficit-irrigation is now the common practice to irrigate crops. A detailed study of the long-term rainfall patterns in the High Plains showed that most of the rain occurs during the growing season and that about 50% of individual rain events are less than 6 mm (¼ inch). The question remains on what amount of these small and frequent rainfall events does the plant use for transpiration and how much is lost due to soil water evaporation. Further, it is normally assumed that all rainfall, regardless of amount and intensity, enters the soil and is available for the plant. In reality what is needed is the effective rainfall amount, i.e., need an estimate of runoff, which is a function of many variables including rain amount and intensity. Thus the objective of this study was to develop and test a simple method to quantify what proportion of a known rain amount the plant would transpire. We selected cotton as our test plants, under greenhouse conditions and used stable isotopes of water as a simple way to study the use of rainwater by cotton plants. This is possible because rainwater has a different isotopic signature than the irrigation water from the Ogallala Aquifer. In our initial studies we determined how quickly simulated rainwater with a different isotopic signature than the irrigation water would show up in the meristematic petioles of the cotton plant. This information will then be used to measure the time, i.e., window, needed for rainwater from a rain event end in the petioles of cotton under field conditions.

Technical Abstract: While stable isotopes of water have been used to study nutrient cycling, nitrogen fixation and other environmental studies they generally have not been used to examine shorter, more transient events, such as disposition of rainfall into soil water evaporation (E) and transpiration (T). With the development of robust methods that use isotope ratio infrared spectrometry, evaluating samples has become faster and simpler, allowing more soil and plant samples to be collected and analyzed. The use of larger sampling rates increases the frequency of measuring changes in stable isotopes at shorter time-intervals and quantifies the rate at which rainwater that enters the soil by infiltration is transpired by plants via root-water uptake. This information is essential in dryland agriculture and in areas where irrigation water is declining, such as the Southern High Plains of Texas. A detailed study of the long-term rainfall patterns in this area showed that most of the rain occurs during the growing season and that about 50% of individual rain events are < 6 mm. The question remains on what amount of these small and frequent rainfall events does the plant use for T and how much is lost due to E. Thus, the objective of this greenhouse study was to establish a protocol to measure the time required by cotton (Gossypium hirsutum, (L.)) plants to transpire water from an enriched water source, simulating rainwater, and measuring the time for this water to show up in the meristematic petioles of cotton leaves. We showed that a simple and inexpensive method of obtaining a water source that was ~ -4.5‰ d18' more enriched than the irrigation water was possible to simulate rainwater. Results suggest that the uptake of rainwater by cotton plants could be observed in as little as a few hours after the rain and that is unnecessary to obtain samples for more than 24 hours (one diurnal cycle). Instead it would be more desirable to obtain samples more frequently over a single diurnal cycle after a rain event. Our findings need to be tested under field conditions where at any given time plants have access to stored soil water that is a mixture of water from different rain events and irrigation.