Data analysis determined the importance of grass, soil, stress, sand layering and time and their interactions (
Table 2) on clipping yield (CY), CI and VQ. For all three variables, most of the factors and their combinations showed some level of significance. Grass, stress and time were dominant, as evidenced by the greater variance ratios and consequentially higher level of significance. Greater attention was directed at these factors throughout the results and discussion. Correlations among the variables measured were highly significant. The highest
r2 value was between the CI and VQ rating (0.745), indicating great similarity between an apparently subjective indicator and an objective measurement. However, both indices provide a measure of quality dependent on chlorophyll. Since both measurements are instantaneous, the CI can be seen as an objective substitute for the older VQ rating system. Hoffman
et al. [
23] used reflectance as a quantitative measurement of injury as an alternative to qualitative (visual) ratings. The number of significant factors and combinations, as well as the level of significance was greater for VQ, indicative of greater variation for this subjective measurement.
3.1. Clipping Yield
Inclusion of a sand layer increased CY for all soils except Piarco; however, the increase was significant (
p < 0.001) only for the two clay soils (Princes Town and Talparo) (
Table 3). The opposite effect was observed for Piarco, where CY was significantly lower. Engelsford and Singh [
24] reported the positive effects of sand-based root zones on turf vegetative growth. In most instances, sand layers are superimposed on gravel beds [
25] facilitating drainage; however, in this study, the sand layer was placed directly above the soil. Topdressing with sand is a common local practice on soil-established turf fields and has been reported by Baker and Canaway [
26] to produce better turfgrass playing quality for perennial ryegrass relative to the soil-only control. Increased vegetative growth was stimulated by a better rooting environment provided by the sand layer. The effect was absent for Piarco, as this soil is coarse textured (
Table 1). The significant decrease in CY may be attributed to lower water and nutrient availability associated with increased macroporosity. The relative difference in yield for the clay soils with the sand layer was also important, as these values were similar when compared to Piarco. Sand layering was capable of improving rootzone properties, resulting in greater shoot growth.
Table 3.
Sand layering influence on turfgrass clipping yield (CY) across four soils.
Table 3.
Sand layering influence on turfgrass clipping yield (CY) across four soils.
Soil | Sand | |
---|
| No Sand | Sand | Soil Means † |
---|
| mg/pot |
Piarco | 1,225.3 a ‡ | 1,089.2 b | 1,157.2 a |
Princes Town | 822.2 d | 1,001.7 bc | 911.9 b |
River Estate | 863.6 cd | 959.3 c | 911.4 b |
Talparo | 765.1 d | 1,006.2 bc | 885.7 b |
Sand means § | 919 b | 1,014. a | |
Turfgrasses varied significantly (
p < 0.05) in their yield responses to stress conditions. All grasses performed better under waterlogged conditions compared to drought, with SG showing similar yields under the former stress, as it did for the compactive stresses (
Table 4). The superior response of SG under WL is related to physiological and morphological adaptations [
27]. This grass produced a comparatively high number of stolons, which increased clipping yield. The redox potentials (Eh) were −64, −25 and 13 mV for ZG, BG and SG, respectively, indicating that SG was capable of maintaining better aeration in the rootzone, probably via aerenchyma. The data implies that SG has tremendous potential for use in high rainfall areas, especially when established on soils with poor internal drainage. Fry [
28] reported higher shoot survival after submergence for BG compared to ZG. In our study, CY did not differ between these grasses, under WL conditions.
Table 4.
Turfgrass CY affected by applied stress.
Table 4.
Turfgrass CY affected by applied stress.
Grass | Stress | |
---|
| Drought | Waterlogging | High Compaction | Low Compaction | Grass Means † |
---|
| mg/pot | |
Bermuda | 483.7f ‡ | 876.4 e | 1,205.2 c | 1,300.0 bc | 966.3 b |
Savannah | 467 f | 1,374.7 ab | 1,349.3 b | 1,492.7 a | 1,170.9 a |
Zoysia | 116.9 g | 904.3 de | 1,005.5 de | 1,023.2 d | 762.5 c |
Stress means § | 355.8 d | 1,051.8 c | 1,186.6 b | 1,272 a | |
Contrastingly, CY for all turfgrasses was lowest for D stress, with the effect being particularly pronounced for ZG (
Table 4). D stress is known to affect turfgrass physiological and biochemical processes [
29], and ZG has been reported to exhibit low D tolerance/resistance [
7], which is partially associated with its slow growth rate [
30]. Soil compaction has been reported to affect shoot growth [
14]. This was evident from the lower CY across grasses for the HC compared to LC treatments (
Table 4). Comparatively, the compaction treatments did not affect turfgrass shoot growth as much as the water-related stresses. SG showed the greatest CY under the compactive treatments. It was evident that the compaction treatment did not affect the rootzone bulk condition, as the bulk density across treatments was below 1.6 g/cm
3.
The D treatment comprising periodic D and recovery phases showed the greatest variability in CY across grasses and stresses for the 16-week trial period (
Figure 1). BG showed the best tolerance to D, recording clippings up to week seven, spanning two drought periods. The other grasses succumbed to the initial one-week drought period and, by week three, showed no vegetative growth. Longer recovery periods were associated with greater CY, the magnitude of recovery being greater for BG and SG.
Figure 1.
The effects of the applied stresses, drought (D), high compaction (HC), low compaction (LC) and waterlogging (WL), on clipping yield (CY) for Bermudagrass (A), Savannahgrass (B) and Zoysiagrass (C) over a 16-week growth period. Recovery from drought occurred at weeks 2, 5 and 6 and 10–12.
Figure 1.
The effects of the applied stresses, drought (D), high compaction (HC), low compaction (LC) and waterlogging (WL), on clipping yield (CY) for Bermudagrass (A), Savannahgrass (B) and Zoysiagrass (C) over a 16-week growth period. Recovery from drought occurred at weeks 2, 5 and 6 and 10–12.
ZG showed little tolerance to the D treatment, with CY remaining below 200 mg/pot after the second week. Huang
et al. [
7] in evaluating D resistance ranked BG and ZG similarly, which conflicts with the findings in this study, noting that the cultivars differed between the two studies. Qian and Fry [
31] also noted that Meyer ZG possessed a high level of osmotic adjustment that may aid its recovery after drought, although its susceptibility to drought is high, due to a relatively shallow root system. This study showed that BG and SG had superior recovery from D compared to ZG, which may be related to the nature of the drought regime used in this study, where grasses were exposed to prolonged periods of water deficit. All other treatments showed fluctuating trends, probably associated with fertilization and watering. Notably, SG showed higher, though statistically similar, CY under WL conditions for the first five weeks compared to other grass × stress treatments. This effect was explained previously.
3.2. Chlorophyll Index
Comparatively, SG showed a significantly (
p < 0.001) higher CI among stress treatments, except under D (
Table 5). Across turfgrasses, D conditions resulted in significantly lower CI compared to the other stress treatments. Dry conditions affect physiological and biochemical processes in turfgrasses [
6], including nutrient uptake, partitioning and assimilation [
32]. In reviewing drought stress on plant nutrition, Silva
et al. [
33] indicated that nutrient transport from the roots to the shoots is limited by a drought-induced decrease in the transpiration rate, as well as an imbalance in active transport and membrane permeability. This would affect plant physiological and metabolic processes, including chlorophyll formation. CI within grass type was also significantly (
p < 0.01) lower under WL compared to the compaction stresses. Malik
et al. [
34] in their study reported that WL decreased chlorophyll in wheat. Higher reflectance for SG (250.04) may be linked to the greater CY (
Table 5) and ability to tolerate applied stresses.
Table 5.
Turfgrass chlorophyll index (CI) affected by applied stress.
Table 5.
Turfgrass chlorophyll index (CI) affected by applied stress.
Grass | Stress | |
---|
| Drought | Waterlogging | High Compaction | Low Compaction | Grass Means † |
---|
| Reading | |
Bermuda | 139.46 f ‡ | 176.18 e | 195.75 c | 194.86 cd | 176.56 b |
Savannah | 153.33 f | 261.66 b | 286.87 a | 298.28 a | 250.04 a |
Zoysia | 105.91 g | 180.37 de | 198.74 c | 199.79 c | 171.21 b |
Stress means § | 132.90 c | 206.07 b | 227.12 a | 230.98 a | |
The single compaction event resulted in minimal effects on vegetative quality. The nominal effects were reflected in the small variation in CI over time for the compactive treatments across all grasses (
Figure 2). Springer
et al. [
35] reported minimal changes in rootzone bulk density after compaction for these treatments. Bulk density remained within the optimal range. A steady decline in CI was observed for the compactive treatments together with WL for all grasses, except ZG. There was a sharp decline under these treatments after the eighth week attributed to pest and disease incidences, which explains why the decrease was observed across all grass × stress treatments. The decline was not as pronounced in ZG. The fluctuating trend under D is reflective of the D regime with higher CI values during recovery weeks. CI for BG was least affected by the D treatment. Comparatively, this treatment only showed significant differences after the fifth week. After three weeks of drought, BG was able to recover with CI values not significantly different from the other stress treatments for the following two weeks. That was not observed for the other two grasses. Beard [
36] ranked warm season turfgrasses for D resistance according to the following series: BG > ZG > SG. Our data showed that SG also had greater tolerance than ZG, with less variation in CI between D and recovery periods.
Figure 2.
The effects of the applied stresses, drought (D), high compaction (HC), low compaction (LC) and waterlogging (WL), on the chlorophyll index (CI) for Bermudagrass (A), Savannahgrass (B) and Zoysiagrass (C) over a 16-week growth period. Recovery from drought occurred at weeks 2, 5 and 6 and 10–12.
Figure 2.
The effects of the applied stresses, drought (D), high compaction (HC), low compaction (LC) and waterlogging (WL), on the chlorophyll index (CI) for Bermudagrass (A), Savannahgrass (B) and Zoysiagrass (C) over a 16-week growth period. Recovery from drought occurred at weeks 2, 5 and 6 and 10–12.
3.3. Visual Quality
VQ was significantly lower for BG compared to the other grasses irrespective of sand treatment (
Figure 3). However, the inclusion of a sand layer resulted in a significantly lower VQ rating for BG. Contrastingly, the sand layer improved the VQ for ZG and had a non-significant influence on SG. Both SG and ZG had greater root densities [
35] than BG under sand treatment, which may have influenced nutrient uptake. A greater volume of roots may have penetrated the underlying soil layer for theses grasses. This result is interesting in that the common practice is to use BG on a sand layer, which has generally resulted in poor root growth and VQ under tropical conditions and present management. It would seem that consideration for the use of other species is warranted or at least the revision of the management protocol for BG.
Figure 3.
Sand layering effect on visual quality (VQ) for three grasses. Columns with similar lower case letters are not significantly different.
Figure 3.
Sand layering effect on visual quality (VQ) for three grasses. Columns with similar lower case letters are not significantly different.
Visual quality responded similarly to CI for the interaction between grass type and stress (
Table 6). ZG under D showed the lowest VQ, significantly different from the other grasses. The low VQ is associated with low chlorophyll content reflective of the grass × stress treatment with the lowest CI. The inhibition of photosynthetic activities is one of the major detrimental effects of drought [
37,
38]. Greater variation between grasses was present across the other stresses, with the best quality shown by ZG. This contrasted the CI results, where ZG showed significantly (
p < 0.05) lower values than SG. This may demonstrate the limitations of both quality measurements with a better assessment possible by combination. Additionally, other qualities are intrinsically included during visual quality assessment, such as density, texture, uniformity, color and growth habit [
39]. There was a clear effect of grass type on the method of quality measurement. Both indices showed similar comparative values for BG, but differed in their response to SG and ZG. It is notable that only SG and ZG under LC and HC produced acceptable VQ ratings > 6. The significantly higher VQ rating for ZG under these stresses may be attributed to the stiffness of its leaves, due to the high silica content [
7], which could reduce the effect of compaction. Results reported by Springer
et al. [
35] showed significantly lower rootzone bulk densities for ZG under compactive treatments compared to SG and BG. Waterlogging produced varied responses among the grasses, but all ratings were lower than acceptable. Liao and Lin [
40] noted that waterlogging has been shown to significantly inhibit photosynthetic capacity in intolerant plants.
Table 6.
Turfgrass VQ affected by applied stress.
Table 6.
Turfgrass VQ affected by applied stress.
Grass | Stress | |
---|
| Drought | Waterlogging | High Compaction | Low Compaction | Grass Means † |
---|
| Reading | |
Bermuda | 139.46 f ‡ | 176.18 e | 195.75 c | 194.86 cd | 4.188 c |
Savannah | 153.33 f | 261.66 b | 286.87 a | 298.28 a | 5.174 b |
Zoysia | 105.91 g | 180.37 de | 198.74 c | 199.79 c | 5.359 a |
Stress means § | 2.898 d | 5.029 c | 5.779 b | 5.922 a | |
Visual quality decreased gradually for all grasses across all stresses, except drought (
Figure 4). The pattern mimics that of CY and CI in that the growth and performance of all grasses decreased with time under all stresses. Prolonged stress of any nature decreases turfgrass VQ. The lesser effect of the compactive stresses resides in the single application of these stresses at the start of the experiment. The fluctuating response for the drought treatment has been previously explained. The ability to recover from drought was related to the extent of drought stress; the longer the drought period, the longer the recovery period. Jiang and Haung [
29] stated that, initially, drought reduced the relative water content, but prolonged drought was associated with chlorophyll loss and cell death. ZG retained minimal quality standards under the compactive stresses. Contrastingly, BG under these stresses showed low VQ from the start of the trial. This further supports the earlier inference that the stress tolerance or resistance differs between these grasses.
Figure 4.
The effects of the applied stresses, drought (D), high compaction (HC), low compaction (LC), and waterlogging (WL), on the visual quality (VQ) for Bermudagrass (A), Savannahgrass (B) and Zoysiagrass (C) over a 16-week growth period. Recovery from drought occurred at weeks 2, 5 and 6 and 10–12.
Figure 4.
The effects of the applied stresses, drought (D), high compaction (HC), low compaction (LC), and waterlogging (WL), on the visual quality (VQ) for Bermudagrass (A), Savannahgrass (B) and Zoysiagrass (C) over a 16-week growth period. Recovery from drought occurred at weeks 2, 5 and 6 and 10–12.