An estimated 25% of the turfgrass is grown under shade. Bermudagrass performs poorly in shade in compared to other warm-season species, although there is some variation among the bermudagrass cultivars in this regard. Turfgrass evapotranspiration (ET) rates are sensitive to solar radiation and therefore can be much lower in shaded environments. In southern California, ET of lawns under trees was lower than without trees by 0.9 to 3.9 mm d-1. A different study found that turfgrass shaded either by trees or by shade mesh reduced total water use rate by at least 50%.
In a turfgrass system, shade stress often co-exists with drought stress. For some instances, shade can be beneficial in that it reduces the evaporative demand and heat load on turf. However, shaded plants that have weak root systems may suffer from soil moisture stress more quickly than their deep-rooted and full sun versions. Similarly, it is often thought that a little bit of shade is good but too much can lead to big problems—particularly if soil moisture becomes limiting.
To better understand how drought and shade stresses interact, we conducted a greenhouse study on three bermudagrass cultivars. The objectives of this research were 1) to measure the effect of drought stress on shaded and non-shaded bermudagrass turf quality and water use rate, and 2) to determine if the response to shade, drought, or combined shade and drought differ among bermudagrass cultivars.
Materials and methods
The experiment was conducted at the Oklahoma State University Horticulture Research Greenhouse located in Stillwater, OK. Grasses were established in 18-inch long, 4-in diameter polyvinyl chloride pipe (PVC) with a flat bottom cap. Pots were well watered to avoid drought stress during establishment and before starting treatments. Fertilizer was applied at 0.25 lb N 1000 sq ft-1 every week using 20-20-20 water-soluble fertilizer (JR Peters Inc., Allentown, PA) throughout the study. Grasses were clipped at a height of 2 inches once a week and clippings were removed.
Once the study began, pots were placed in either non-shaded or shaded areas of the bench. Shade treatment was applied using a black shade fabric nominally rated to reduce incoming light by ~50%. Three bermudagrass cultivars and two irrigation levels were randomly assigned to each light treatment block. The three cultivars were ‘Celebration’ common bermudagrass, ‘Latitude 36’ hybrid bermudagrass, and ‘Patriot’ hybrid bermudagrass. After 4 weeks of acclimation to shade treatment, irrigation treatments were applied once per week for a 6-week period to replace either 100% actual ET or 50% actual ET.
Visual turf quality was assessed weekly following National Turfgrass Evaluation Program (NTEP) guidelines. Ratings were taken on a scale of 1 to 9 (9=ideal turf, 6=minimum acceptable turf, and 1=brown dead turf). Normalized difference vegetation index (NDVI) was measured every week using a Spectral Reflectance Sensor (Decagon Devices Inc., Pullman, WA). Leaf relative water content was measured at 0, 4, and 6 weeks after treatment (WAT) to estimate the hydration level of leaves, thus indicating the amount of drought stress within the plant. Plant cell membrane injury due to drought stress was estimated using the electrolyte leakage method.
Evapotranspiration rates were measured gravimetrically by weighing the pots weekly. Cumulative water use for each treatment was determined as the sum of all water applied during the entire study.
The study was arranged in 2 x 2 completely randomized factorial design. All data were analyzed using analysis of variance (ANOVA) with the general linear model procedure (GLM) in SAS software version 9.4. Treatment means were separated by Fisher’s protected least significant difference (LSD) test at p < 0.05.
Results
The non-shade block received an average of 26.8 mol m-2 d-1 PAR, while the Shade block received an average of 9.7 mol m-2 d-1 (64% of non-shaded) PAR during the treatment period.
In just 4 weeks of treatment, shade reduced turf quality for both the well-watered and deficit irrigated pots, most notably in the cultivar Patriot. Deficit irrigation treatments reduced turf quality similarly for each cultivar, although not as rapidly when combined with shade. At the conclusion of the study, deficit irrigation resulted in very poor turf regardless of cultivar or shade treatment. Similar results were seen for NDVI measurements.
Either shade or drought alone reduced the RWC of leaves, most notably for the cultivar Patriot. Differences in RWC between well-watered and deficit irrigation pots were diminished under shade, presumably due to the effect of cooler temperatures on evaporative demand.
Electrolyte leakage is a standard method of quantifying the stress a plant feels at the cellular level. In this study, deficit irrigation resulted in higher electrolyte leakage (more stress) at both the 4 and 6 weeks after treatment. Interestingly, shade also increased electrolyte leakage over time, most notably for the cultivar Patriot. By the end of the study, deficit irrigation resulted in an electrolyte leakage value of 59% for shaded pots and 72% for non-shaded pots. However, for a given leaf hydration level (relative water content), shaded pots were under more stress as indicated by greater leakage values. Cumulative water use varied by cultivar and shade treatments. Specifically, Patriot used the most water in non-shaded conditions but the least under shaded conditions. This general pattern was evident for both deficit and well-watered irrigation treatments.
What does this all mean?
Reduction of turf quality and NDVI due to shade alone or drought alone has been commonly reported in previous research. The apparent delay in drought stress under shaded conditions is related to a similar delay in soil water depletion due to the lower evaporative demand. Whether these findings would reproducible under longer-term shade pressure, wherein rooting depth has been compromised, is unclear. Nevertheless, shade does have some short-term benefit to the plant under diminishing water availability—but only if the shade is not too severe.
Patriot is known to have poor shade tolerance even in comparison to other bermudagrasses. In contrast, Celebration has typically been considered a relatively shade tolerant bermudagrass cultivar. More recently, Latitude 36 has been reported to have good shade tolerance, although not as good as Celebration. Our study is in agreement with much of these previous reports indicating Patriot as being inferior to Celebration or Latitude 36 in regards to shade tolerance. Our findings that the least shade tolerant cultivar (Patriot) incurred the lowest leaf RWC under shade support the theory of a linkage between leaf hydration level and photosynthetic efficiency. The relatively high water use of Patriot in the non-shaded block and the relatively low water use in the shaded block may be related to this as well. Higher demand for water in Patriot could be one of the reasons for poor performance when subjected to individual or combined stress. More research is needed to further refine these relationships in the hopes that we can identify key properties of bermudagrasses that do well under both shade and drought environments.
Shade reduced the severity of drought stress to some extent by reducing the evaporative demand. There is evidence that shade tolerance of the turfgrass can have a role in tolerance of the combined shade and drought stress as well. This study was among the first to directly investigate the combined effects of shade and drought stress on bermudagrass turf. Results will contribute toward improving irrigation management of shaded turf sites and the long-term sustainability of turfgrass management.
Charles Fontanier, PhD, is Assistant Professor, Turfgrass, in the Department of Horticulture and Landscape Architecture, Oklahoma State University. Manoj Chhetri is a graduate research assistant at Oklahoma State.