Tissue biomechanical strength, wear resistance and recovery in C4 turfgrass species: physiological and morphological factors and innovative evaluation techniques.

Turfgrass wear effects are known to be the sum of soil compaction and plant tissue injury. As such, tissue intrinsic resistance to several mechanical factors, including traction stress, is a decisive in determining the wear resistance of a turfgrass species. Wear simulation in the field can suffer from climate, soil and machinery operator error, and is always inclusive of the soil compaction factor that is one of the origins of turfgrass wear effects in the field. Lignin, dry matter, starch, sugars and silica are some of the tissue constituents and characteristics that have been associated with leaf and stem mechanical resistance, while little information is to be found concerning stolons and rhizomes. These organs not only enable C4 turfgrass species lateral growth, soil colonization and injury recovery, but are also key constituents of mature swards. A firsts study consisted in an extensive investigation on the effective leaf, rhizome traction resistance of Cynodon dactylon L. Pers. var. dactylon x C. transvaalensis Burt-Davy cv. Tifway 419 (Cdxt), Zoysia matrella (L.) Merr. Cv. Zeon (Zm) and Paspalum vaginatum Swartz. cv. Salam (Pv), as measured with a FIFA-approved dynamometer, and correlating these results with laboratory investigations on key tissue constituents. Several aspects emerged from the present work that can be summarized as follows: 1. Tensile strength tests on leaf, rhizome and stolon tissues of Cdxt, Zm and Pv can supply useful information regarding these species’ starch, sugars, dry matter, lignin and silica content. 2. Tensile strength was more influenced by tissue constituents than by tissue dimension. 3. The results of tensile strength tests are in accordance with these species’ wear resistance as tested in previous work, with Zm stronger than Cdxt and Pv. 3. In rhizomes and stolons, tissue breakage usually occurs in the area at the intercalary meristem at the apical zone in the immediate proximity of a node. 4. Older tissues have higher tensile strength thanks to their higher lignification. 5. Starch and sugars content found in tissues is in accordance with the species’ previously observed linear growth rate, with Cdxt faster than Pv and Zm. 6. Starch content is generally inversely proportional to lignin content. 7. Stolon TSS content, and glucose in general, is a clear marker of tissue mechanical strength. 8. Lignin is the principle constituent in determining tissue tensile strength, and as such it could be used as a turfgrass wear resistance predictor in the cultivar breeding stages. 9. Silica is a constituent undermining tissue tensile strength. 10. Leaves are the plant organs with the highest silicization and the lowest lignification of tissues. A second study consisted of testing slabs of mature canopies of the same species for wear resistance in laboratory with a Lisport machine, as adopted by FIFA for artificial turf testing. Worn slabs of turfgrass were then allowed to recover in greenhouse to fathom out percent recovery of shoots. The results of these investigations were once again plotted again laboratory investigations on key tissue constituents. Several aspects emerged from this second work that can be summarized as follows: 1. The Lisport machine can be successfully used in an effective and reproducible way to fathom out natural turfgrass wear resistance, devoid of soil compaction effects. 2. Wear resistance for C4 species as observed in the field does not necessarily coincide with the relative tissue intrinsic resistance, but rather with the initial canopy density. 3. C4 species show a wear resistance that is much higher than C3 species. 4. C3 species show a virtually nil recuperative capability (mainly due to the lack of vegetative propagation organs). 5. The species with a very high intrinsic (tissue) wear resistance are also the species with the slowest recuperation potential. This seems to be due to lower levels of starch and TSS available for recovery. 6. Starch was a clear marker of wear resistance (negatively correlated) and recovery (positively correlated). 7. Silica was a marker positively correlated with wear resistance. 8. Lignin was the clearest marker found to be positively correlated with wear resistance. 9. A more severe wear induces a higher percentage of shoot recovery, and this particular aspects deserves further investigation.