Introduction

One of the most effective methods of evaluating plant water use is by measuring water flow through the xylem of a plant. This technique, known as sap or stem flow measurement, can continuously monitor water flow without disturbing normal plant functions (Senock et al., 1996), and has been shown to give highly accurate measurements of plant water use (Lascano et al., 1992, Devitt et al., 1993, Dugas et al., 1993). Despite the use of sap flow measurement in several different crops (Dugas et al., 1994; Senock et al., 1996; Gordon et al., 1999; Lascano, 2000; Lee et al., 2002; Li et al., 2002; Lu et al., 2002), very few studies have directly examined water flow in peanut. Erickson et al. (1986) used the heat pulse method to measure apparent sap flow velocities in peanut genotypes but this was done in a greenhouse for very limited time periods (1100 to 1300 hour apparent solar time).

The quantification of stem water flow could provide very critical information towards solving a wide variety of problems in peanut production. These include: quantifying the effects of insect damage to plant function; determining how changes in peanut water use may affect pre-harvest aflatoxin production; or evaluating the efficacy of irrigation scheduling or irrigation methods in peanut production. To determine the actual differences in plant water-use among irrigation methods for peanut, it is important to quantify plant water use on a daily basis and under typical field conditions. Sap flow measurements provide a means to accomplish this. However, the lack of verification of this technique in peanut remains a barrier to the utilization of this technology.

Maintaining efficiency in irrigation remains a major problem in almost every U.S. peanut producing region. Irrigated peanut acreage comprises over 50% of all peanut production in the U.S. and can increase yields by up to 19% over non-irrigated production (Lamb et al., 1997). But in utilizing irrigation, it is important to maximize agronomic water-use efficiency while maintaining optimum production. The primary method of irrigation in peanut production is overhead sprinkler irrigation (OH) (Anon., 1996). An alternative irrigation method that promises to provide greater agronomic water-use efficiency over typical center pivot or lateral sprinkler systems is subsurface drip irrigation (SDI) (Phene et al., 1992; Sorensen et al., 2001; Assouline et al., 2002). However, despite the purported benefits of SDI over OH irrigation, few studies have quantified the actual differences in plant water use between the two systems.

For this study, peanut sap flow measurements were taken to evaluate the efficacy of the technique under field conditions and to determine if the technique could detect differences in plant water use between irrigation methods. In addition, if differences in peanut water use existed, crop biomass production and carbon isotope discrimination were measured to determine the differences in water allocation strategies between plants in different irrigation schemes. The specific objectives of this research were to: 1) determine if the technique of sap flow measurement could be successfully utilized in peanut; 2) determine if daily peanut stem water flow, biomass partitioning, and carbon isotope discrimination differed among irrigation treatments, and 3) determine what environmental parameters were most closely correlated with peanut stem water flow.

Materials and Methods

Results

Sap flow was successfully measured in peanut during a two week period in all irrigation treatments. Figure 1 illustrates the daily fluctuations of water use measured in the three irrigation treatments and their responses to irrigation and/or rainfall. Peanut water use characteristically peaked during midday with no flow or very low levels of water flow at night. Typical daily maximum flow rates ranged from 4–6 g/hr in irrigated treatments and from 1–2 g/hr in the NI treatment. Decreased water use in response to rainfall was recorded in all treatments during the day the water was received. However, in SDI plots, water use apparently was not dampened by the daily application of subsurface water (Fig. 1). The sap flow technique was also able to record minute changes in peanut water flow during the day presumably in response to changing edaphic and climatic factors (Fig. 1).

Significant differences in peanut water use as measured by sap flow gauges existed among NI, SDI, and OH treatments. Statistical analysis of two replications each in OH and SDI treatments and three replications in the NI treatment showed significant differences among irrigation treatments in both average daily sap flow rate (df  =  2, F  =  32.66, P-value  =  <0.0001; R-square  =  0.76) and cumulative sap flow over a 24 hour period (df  =  2, F  =  32.19, P-value  =  <0.0001; R-square  =  0.76). Plants in the SDI treatment had significantly higher average sap flow rates and total water use during a 24 hour period than plants in the OH or NI treatments (Fig. 2).

Peanut sap flow was significantly correlated on an hourly basis with most of the environmental conditions measured across all irrigation treatments: soil moisture, soil temperature, and infrared canopy temperature (Table 1). However, it was not possible to correlate sap flow with these environmental parameters for the NI treatment due to large gaps in the environmental data for this treatment. When both 100% SDI and 100% OH treatments were pooled, sap flow was significantly positively correlated with: soil moisture at 30–50 cm depth; the average of the shallow and deep soil moisture; soil temperature at the five cm depth; and infrared canopy temperature (Table 1). The most significant relationship was found between infrared canopy temperature and sap flow. Examining irrigation treatments individually revealed that sap flow was significantly correlated with shallow soil moisture, average soil moisture, soil temperature, and infrared canopy temperature in the SDI treatment. In the OH treatment, sap flow was not correlated with soil moisture at either measured depth, but was significantly negatively correlated with soil temperature and positively correlated with infrared canopy temperature (Table 1).

Differences in plant biomass among irrigation treatments revealed that overall plant biomass was greatest in the SDI treatments. There were significant differences among irrigation treatments for apex stem biomass traits, whole plant leaf area and biomass, and leaf area index (Table 2). On apex stems, average leaf area and internode length were significantly greater for both SDI levels than for either level of OH treatment or for the NI treatment (Fig. 3). Specific leaf area was significantly lower for the 100% SDI treatment than for any other treatment (Fig. 3), indicating that leaves in this treatment were significantly thicker. Further, whole plant biomass was generally larger in both levels of the SDI treatments than either level of OH irrigation or the NI treatment (Fig. 4). This included larger total leaf area, total leaf weight and total internode weight, with most comparisons showing significant differences between SDI levels and OH or NI treatments (Fig. 4). This increased aboveground biomass was also evident in significantly larger leaf area indices for both SDI levels than for either OH level or the NI treatment (Fig. 5).

There were no significant differences in seasonal water-use efficiency among irrigation treatments as measured by carbon isotope discrimination at 90 days after planting, although trends were apparent (Fig. 6). Carbon isotope discrimination indicated that plants in both SDI levels tended to have lower water-use efficiencies than either OH levels or the NI treatment, but these differences were not statistically significant. SPAD chlorophyll content (measured on the same leaves that were analyzed for isotope composition) did differ among irrigation treatments with slightly lower chlorophyll levels in the SDI treatments than in either OH or NI treatments (Fig. 6).

Although there was increased water use and increased aboveground biomass in the SDI vs. OH treatments, there was no concomitant increase in pod yield for SDI plants. Yields were not significantly different between OH and SDI treatments when compared across the three replications within the whole field (data not shown). Yields within the plots containing the sap flow collars were: 2180 and 2210 kg for the 100% and 66% OH respectively; 1997 and 2212 kg for the 100% and 62% SDI respectively; and 1441 kg for the NI treatment.

Discussion

This study has demonstrated that sap flow gauges can be used successfully to quantify stem water flow in peanuts in a field production setting. Historically, previous studies have had great success in quantifying sap flow and the effects of many environmental conditions and treatments on water use in: trees (Hatton et al., 1995; Sorensen et al., 1999); shrubs (Allen and Grime, 1995); rangeland plants (Dugas et al., 1992; Senock and Ham, 1995); and crops (Senock et al., 1996; Gordon et al., 1999; Lee et al., 2002; Lu et al., 2002), including corn (Li et al., 2002) and cotton (Dugas et al., 1994; Lascano, 2000). However, only one previous study has examined sap flow in peanuts under greenhouse conditions (Erickson et al., 1986).

The results in this study indicate that peanut plants in the SDI treatment used significantly more water during the late season sampling period than plants in either OH or NI treatments. This was likely due to the larger plant size, as evidenced in both leaf area and internode lengths, of the SDI plants. Because there were no significant differences in yield among irrigated treatments, and given the greater water use of SDI vs. OH plants, the SDI plants in this study appeared to have lower WUE than OH plants. However, this may not be due to the technique of SDI per se but in the management system used in this study to schedule irrigation.

The current irrigation scheduling systems used in both the OH and SDI treatments were based on principles defined as deficit irrigation. Deficit irrigation is also known as the engineering approach and involves irrigating at a level below what the crop demands. Such deficits are generally determined by taking reference evapotranspiration (ET_o) and multiplying it by a crop coefficient (Allen et al., 1998; Lascano, 2000). In this study, this was done directly in the SDI treatment using a modified Jensen-Haise equation and indirectly in the OH treatment using soil temperatures and equations that relate crop coefficients to irrigation required in the Irrigator Pro program. For cotton grown on the Texas High Plains, the engineering approach was found to inadequately describe the daily ET of the crop and was less sensitive to the dynamic nature of soil evaporation (Lascano, 2000). In the current study, it appears that the engineering approach also failed to describe the actual water use of peanuts but in the direction of overestimating crop evapotranspiration. Consequently, both irrigation treatments received more water than was needed, especially in the SDI plants. The overestimation of deficits may have been due primarily to outdated crop coefficients which were developed for varieties no longer grown in peanut production (Stansell et al., 1976).

Sap flow in peanuts was significantly correlated with several environmental parameters. Such significant correlations are extremely important for possible applications in irrigation scheduling and crop stress monitoring. Soil temperature and peanut canopy temperature are more easily measured than sap flow, and are often assumed to be indicative of plant available water. Nevertheless, validation of such assumptions are rarely confirmed by direct measurements of plant water use (e.g., Fortin and Moon, 1999; Hochman et al., 2001; Morgan et al., 2003). However, validations are essential before assuming that soil or externally measured plant traits correspond with plant water use. This study has directly correlated actual peanut sap water flow with two soil parameters (moisture, temperature) and with peanut canopy temperature and has illustrated that some traits are better than others as predictive tools for plant water use or drought stress.

The most significant correlations of peanut sap flow were with canopy temperature regardless of irrigation treatment. This is not surprising because leaf or canopy temperature has been used as an indicator of plant water stress under many environments for years (Tanner, 1963; Idso et al. 1981; Jackson et al., 1981; Jones 1999). The significant correlation of sap flow with soil temperature provides supportive evidence for the use of Irrigator Pro, an irrigation scheduling program used extensively in peanut production, which relies primarily on soil temperature as a trigger for irrigation (Lamb et al., 1993; Davidson, et al. 1998). It was surprising that soil moisture was not a good indicator of plant water status in the OH treatment during this experiment. Surface application interface may have been a factor, i.e. a wet surface soil in the OH treatment did not necessarily reflect lower moisture levels of deeper soil layers which may have impeded plant water use. However, surface soil moisture was a good indicator of peanut sap flow in the SDI treatment, and is more likely to indicate soil moisture status as a whole in this treatment since water was applied below the surface and would reach deeper soil depths quickly.

The differences in plant biomass among irrigation treatments indicate that the additional water flow measured in peanut stems in the SDI treatment may be allocated more to aboveground plant biomass than final yield. Plants in the SDI treatment were larger and had more leaf mass, longer internodes, and overall larger canopies than either the OH or NI plants (Figs. 3, 4, 5). Interestingly, for all biomass traits, NI plants were very similar in size and biomass to the OH treatment plants (Fig. 4). This may have been due to NI plants receiving adequate early season precipitation (data not shown) to attain large size in mid-season. However, measured water flow in the OH plants was substantially higher than in the NI plants (Figs. 1 and 2) during the active pod fill stage, thus indicating that these NI plants experienced some late-season drought stress.

The high water-use of the SDI treatment plants without increased yield indicates SDI plants may have had lower water-use efficiency than OH plants in this study. Carbon isotope discrimination, though not statistically significant, indicated that plants in both the 100% and 62% irrigation levels in the SDI tended to exhibit lower WUE than either OH or NI treatments. The inability to detect significant differences may be due to inadequate sample sizes because significant effects of irrigation amount on carbon isotope discrimination have been quantified by using larger sample sizes at this experimental site (data unpublished). However, the apparent lower WUE of SDI plants in comparison to OH plants was most likely due to the effect of irrigation management and not on an inherent characteristic of SDI in general.

This study has successfully demonstrated the technique of sap flow measurement in peanut under field conditions which had not been previously demonstrated to date. The technique was further able to quantify water use differences between irrigation treatments. While this study found a higher plant water use in SDI treatments as compared to OH treatments, further studies need to be conducted before conclusions about differential water use patterns between irrigation treatments can be made. The successful application of the sap flow technique in this study shows the efficacy of applying this technique in future studies addressing this question.

We thank Kathy Gray for her excellent aid in design, implementation, and analysis throughout this experiment. We thank Larry Powell and John Gardner for invaluable field and technical assistance. We thank Lori Riles for superb statistical analysis consultation. We dedicate the manuscript to the excellent mentoring, advisement, and overall support of Paul Blankenship.

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