SUMMARY

This study was conducted to determine the biomass production of Common bermudagrass, 'Tifdwarf' bermudagrass, Seashore paspalum, and 'Z-3' zoysiagrass under different light conditions. Biomass production is one factor that can be used to help select a suitable turfgrass.

Potted turfgrasses were grown outdoors under shade cloths to provide 100% (full sun), 70%, and 50% light conditions. Common bermudagrass and Seashore paspalum had greater clipping biomass in 100% and 70% light. Seashore paspalum clipping biomass was greater in 50% light. Verdure biomass was not different among the four turfgrasses in the three light conditions. Root biomass of Common bermudagrass was greater than 'Tifdwarf' bermudagrass in 100% light. No differences in root biomass among the four turfgrasses were seen in 70% light. Seashore paspalum had greater root biomass in 50% light.

Our study showed that the different light conditions affected biomass production and biomass allocation of the four turfgrasses.

PROCEDURE

Turfgrass Planting and Data Collection

Common bermudagrass (Cynodon dactylon (L.) Pers.) was planted by seeds, and 'Tifdwarf' bermudagrass (Cynodon dactylon x transvaalensis), Seashore paspalum (Paspalum vaginatum Swartz), and 'Z-3' zoysiagrass (Zoysia japonica x matrelia) were transplanted by stolons or verdures. The potting media was a 4 silica sand : 1 peat moss (by volume) mixture.

Turfgrasses were grown in plastic pots (15 cm diameter x 15 cm tall) on an outdoor bench at the Magoon Greenhouse Complex (University of Hawaii at Manoa) in Manoa Valley (elevation 85 m) under three light conditions--100% (full sun), 70%, and 50%. Shade cloths, 50% and 30%, provided the 50% and 70% light conditions, respectively. The photoperiod ranged from 13:06 hours at the beginning of the experiment (August) to 10:50 hours at the end (December).

The turfgrasses were watered by an overhead irrigation system twice a day. A 15N­15P­15K granular fertilizer was added at the rate of 0.45 g/pot every two weeks. There were four replicates for each turfgrass. After being established in the pots for one month, the turfgrasses were clipped twice a month starting in August until December. Clippings were oven dried at 60°C for 24 hours, and dry weights were recorded.

Environmental Recording

Environmental conditions were recorded with a CR21X Micrologger weather station (Campbell Scientific, Logan, Utah). In each light condition, a solar radiometer (H190 SB-U LI-COR) quantum sensor was set at pot level to measure photosynthetic active radiation. An air temperature probe (model 107) was used to record air temperature. A water-soil temperature probe (model 107B) was buried in the pot at a 5-cm depth to record soil temperature. To record soil water matric potential, a soil matric potential block (model 227) was buried in the pot at a 5-cm depth. Data from these sensors were recorded hourly, transferred to a SM716 storage module, and downloaded to a computer.

Multiple regression analysis was used to develop equations for the relationship between turfgrass biomass production and the environmental factors. Dry weights were regressed with the environmental factors one at a time, and those found significant were regressed with turfgrass dry weight again.

RESULTS AND DISCUSSION

Environmental Conditions

Under all light conditions, average daily photosynthetic active radiation peaked in late August with average daily air temperature maximal in September. Photosynthetic active radiation and air temperature were greatest in 100% light (full sun), followed by 70%, and 50% light. Average daily soil temperature was greater in September through October and generally greater in 100% light, followed by 70%, and 50% light. Average daily soil water matric potential fluctuated during the experiment with 100% light showing lower values at times, and 50% light showing higher values periodically.

Our results showed that the shade cloths not only changed the amount of photosynthetic active radiation reaching the turfgrasses, but also affected the microenvironment of the turfgrasses‹air temperature, soil temperature, and soil moisture. The microenvironments, in all likelihood, helped influence turfgrass biomass production. The shade cloths could also have modified wind movement, relative humidity, carbon dioxide level, and light quality. However, we did not measure these factors so it is uncertain to what extent they influenced turfgrass biomass production in our experiment.

Clipping Biomass Production

There were differences in clipping biomass production among the four turfgrasses with regard to light condition and time of year. Monthly clipping biomass was generally greater for Common bermudagrass and Seashore paspalum than 'Tifdwarf' bermudagrass and 'Z-3' zoysiagrass in the 100% light condition (Table 1). There were no significant differences among the turfgrasses in December. Clipping biomass of Common bermudagrass and Seashore paspalum were, in general, greater than the other two turfgrasses in the 70% light condition. Common bermudagrass had significantly greater clipping biomass than the other turfgrasses in November and December. Seashore paspalum clipping biomass was, in most cases, greater than the other three turfgrasses in the 50% light condition (Table 1). There were no significant differences among the turfgrasses in December.

Common bermudagrass and Seashore paspalum had greater clipping biomass production in the 100% and 70% light conditions. Differences between the two bermudagrasses, Common and 'Tifdwarf', indicated that there are differences among bermudagrass species and hybrids. In the 50% light condition, in which Seashore paspalum had the greater clipping biomass, we expected the 'Z-3' zoysiagrass to perform better.

A seasonal effect on clipping biomass production was observed (Table 1). There were no significant differences among the turfgrasses in the 100% light condition in December. During the experiment, photosynthetic active radiation under all light conditions was lowest in December. Air temperature and soil temperature were similarly lowest in December. The seasonal effect may be due to less than optimum environmental conditions during the winter, which reduced turfgrass clipping production. There were also no significant differences among the turfgrasses in the 50% light condition in December when the average daily photosynthetic active radiation was 157 µmol/m²/s. The lack of any differences in clipping biomass production among the turfgrasses at that time may be due, in part, to the photosynthetic active radiation being too low during December. Since our experiment was only from August to December, it is unclear what the seasonal effect would be at other times of the year.

Biomass Allocation

Clipping biomass. Clipping dry weights of Common bermudagrass and Seashore paspalum were greater than 'Tifdwarf' bermudagrass and 'Z-3' zoysiagrass in the 100% and 70% light conditions (Table 2). Seashore paspalum had greater clipping dry weight than the other turfgrasses in the 50% light condition. Compared to the 100% light condition, clipping biomass of Seashore paspalum increased in the 70% and 50% light conditions, whereas the 'Z-3' zoysiagrass clipping biomass decreased (Fig. 1A).

The differences seen in clipping biomass are not only due to differences in photosynthetic active ration, but also in the microenvironments. One major factor is air temperature. In our study, air temperature decreased with decreasing light level. Light level had a modifying effect on the microenvironment (air temperature), which then seemed to have affected turfgrass biomass production.

Verdure biomass. Verdure biomass was not significantly different among the four turfgrasses in the three light conditions (Table 2). Compared to the 100% light condition, verdure biomass of all four turfgrasses decreased in the 70% and 50% light conditions (Fig. 1B). With no significant differences among the four turfgrasses, the allocation of dry matter to verdure was similar. This indicates that the differences in biomass production among the turfgrass were primarily due to differences in clipping and root biomass production. Although verdure biomass production was reduced with decreasing light level, it does not appear to be important as a criterion for turfgrass selection with respect to light level.

Root biomass. Root biomass of Common bermudagrass was greater than 'Tifdwarf' bermudagrass and 'Z-3' zoysiagrass in the 100% light condition (Table 2). There were no significant differences among the four turfgrasses in the 70% light condition. Seashore paspalum had greater root dry weight than the other turfgrasses in the 50% light condition. Compared to the 100% light condition, root biomass of all four turfgrasses decreased in the 70% and 50% light conditions (Fig. 1C). Root biomass production is not only important for its contribution to plant biomass, but also for its involvement in rooting, sod strength, rooting depth, erosion control, pest susceptibility, and recuperative potential.

Biomass Equations

The multiple regression equations for the four turfgrasses were:

R2 and r2 were 0.67, 0.31, 0.32, and 0.37 (P ¾ 0.01) for Equations [1], [2], [3], and [4], respectively. With R² and r² ranging from 0.31 to 0.37 for three equations, other factors besides the environmental factors measured in this experiment may have had an effect on turfgrass biomass production. The specific environmental factors in each equation were different, suggesting that each turfgrass has different environmental requirements for biomass production. It should be noted that equations [1] through [4] are empirical or statistical models and not mechanistic models, which are process oriented and deal more with the physiology of the plant.

In our study, soil moisture did not show any significant effect on turfgrass biomass production. With the overhead irrigation system, soil moisture was apparently sufficient in the growing media during the experiment. Although average daily soil water matric potential fluctuated during the experimental period, it remain relative constant, averaging -2.24 kPa for all light conditions.

This study investigated the biomass production and allocation of four turfgrasses commonly grown in Hawaii. Shading modified the microenvironment of the turfgrasses, which has the potential to affect turfgrass biomass production. Each turfgrass has a characteristic biomass production potential which accounts in part for the differences in biomass. Because of this, the percent reduction in biomass production in comparison to full sun also needs to be considered in selecting an appropriate turfgrass.

Table 1. Monthly clipping dry weights (g) of four turfgrasses grown outdoors in three light conditions at the Magoon Greenhouse Complex (University of Hawaii at Manoa) in Manoa Valley (elevation 85 m).

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