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Tillering, Internode Development, and Dry Matter Partitioning in Creeping Bentgrass.

CREEPING BENTGRASS is an obligate outcrossing species (Bradshaw, 1958) which spreads vegetatively via extensive stolon growth (Kik et al., 1990). Vegetative growth predominates in areas where growing conditions are favorable (Kik et al., 1990). Creeping bentgrass is primarily grown on golf courses in temperate regions. As a creeping bentgrass turf develops, stolons provide surface structure and cushion the surface to help resist wear. Shorter internode length is related to high tiller density in creeping bentgrass turf (Cattani et al., 1996). High tiller density provides a dense surface that allows for smooth ball roll and resistance to ingress by weeds. Once a creeping bentgrass stand is established, very little subsequent growth of new seedlings occurs within a stand (Jonsdottir, 1990; Bullock et al., 1994), even where regular overseeding is practiced (Sweeney and Danneberger, 1998). Therefore, an understanding of vegetative growth in creeping bentgrass is essential to its culture and would help in selecting for improved turf characteristics in germplasm screening programs.

Robson (1973) describes an exponential phase, a linear phase and a static or decreasing phase of tiller appearance in grasses. Jonsdottir (1990) monitored tiller initiation and death in a naturally occurring stand of creeping bentgrass and found that they were offsetting. Tiller proliferation in young creeping bentgrass plants, under noncompetitive conditions was found to be in the exponential growth phase (Cattani, 1999).

Beard (1973) uses the emergence pattern of new vegetative growth in the classification of either a tiller (intervaginal emergence or emergence through the entire length of the sheath) or a stolon (extravaginal emergence or emergence through the sheath tissue). Turgeon (1999) defines tillers as growing upward at emergence in contrast to stolons. The main stem (stem) is the seminal vegetative axis arising from the seed. All tiller growth arises from axillary buds on stems.

Tiller development in grasses has been described as being orderly in theory (Neuteboom and Lantinga, 1989) and is related to leaf appearance (Davies and Thomas, 1983). Leaf morphological characteristics such as leaf length and width will affect leaf growth rate and thus leaf appearance rate and therefore ultimately tillering rates (Bos, 1999). Leaf appearance rates are drastically reduced in late September to late October in perennial ryegrass (Lolium perenne L.), thus reducing tillering potential (Vine, 1983).

Primary tillers appear in the axil of a leaf on the main stem or seminal vegetative axis. The first primary tiller should appear between the appearance of the second and third leaf on the main stem (Neuteboom and Lantinga, 1989). The first primary tiller generally appears in the axil of the first leaf in creeping bentgrass and is labeled 1-1 [degrees] (Cattani, 1999). The second primary tiller will theoretically appear in the axil of the second leaf on the main stem between the appearance of the third and fourth leaves (Neuteboom and Lantinga, 1989) and is labeled 2-1 [degrees], and all other primary tillers will follow this labeling system. Under good growing conditions, a secondary tiller should simultaneously appear from the axil of first leaf on the 1-1 [degrees] tiller (Cattani, 1999) and is labeled 1-1-1 [degrees]. This generally takes place as the second leaf is appearing on the primary tiller, and this apparent early appearance is due to the production of the prophyll, or rudimentary leaf (Neuteboom and Lantinga, 1989) which is usually not visible at early stages of growth in creeping bentgrass (Cattani, 2000). The appearance of the fifth leaf on the main stem will indicate the potential of four new tillers. The third primary tiller on the main stem from the axil of the third leaf is referred to as 3-1 [degrees]. The 2-1-1 [degrees] refers to the tiller arising from the axillary bud of the first leaf of the second primary tiller. The 1-1-2 [degrees], refers to the tiller arising from the axillary bud of the second leaf of the first primary tiller. A tertiary tiller (1-1-1-1 [degrees]) refers to the tiller arising from the axillary bud of the first leaf of the first secondary tiller of the first primary tiller. With the appearance of the next leaf on the main stem, there is the potential for eight new tillers. The potential for increase in tillers follows an exponential equation of [2.sup.n-3], where n = leaf number on the main stem greater than 3 (Neuteboom and Lantinga, 1989). Skinner and Nelson (1992), working with tall fescue (Festuca arundinacea Schreb.) and van Loo (1992), working with perennial ryegrass have used the proficiency (actual tillers/potential tillers) of a plant to meet this theoretical tiller production, called site usage, for modeling and selection purposes.

Dry matter accumulation in tillers is important with respect to survival. Ong (1978) found tiller size (by weight) to be the important factor in tiller survival under whole plant stress. Dry matter partitioning is therefore important for plant development and persistence. Dry matter accumulation per tiller is important for wear stress resistance in Kentucky bluegrass (Poa pratensis L.) (Shildrick and Peel, 1984) and appears to be important in creeping bentgrass (Cattani and Clark, 1991). Lush (1990) used tiller density and dry weight per unit area to estimate potential wear stress resistance. Trenholm et al. (1999) reported higher tiller densities resulted in greater wear resistance for seaside paspalum (Paspalum vaginatum Swartz).

Time of seeding in Atlantic Canada is usually in the late spring or early autumn. Early autumn is preferred, in part as the summer season-of-play has been completed. Day length is rapidly decreasing at this time of year and seeding often takes place late into October. Hunt et al. (1987) demonstrated high shoot stress and reduced relative growth rate under low light intensity conditions. Smith (1982) suggested that the season of growth will have an effect on the red:far red light ratio, which has been shown to affect tillering (Casal et al., 1985).

The effect of seeding time on plant growth will affect the development of plants within the resulting turf. Understanding the developmental stages of creeping bentgrass will aid in making management decisions such as timing of vertical cutting and grooming and other decisions related to turf quality and playability. The objectives of this study were to: (i) describe early tiller development in creeping bentgrass; (ii) investigate the relationship between early main stem elongation and tillering; and (iii) investigate the dry matter partitioning in tillers and branching systems in developing creeping bentgrass plants under long- and short-day conditions.

MATERIALS AND METHODS

The design of the study was described in Cattani (1999). In brief, 25 pots each of two creeping bentgrass populations, Emerald and UM67-10, were planted with a single pregerminated seed. Seedlings were transplanted into 10-cm pots containing an 80:20 sand:sphagnum peat media. The particle analysis of the sand was a medium:fine sand and the pH was 7.2. Fertilizer was applied twice weekly starting at 3 DAT at the rates of 56 g N 100 [m.sup.-2] [application.sup.-1] in Week 1; 112 g N 100 [m.sup.-2] [application.sup.-1] in Week 2; and 225 g N 100 [m.sup.-2] per application thereafter. Two experiments were conducted in growth cabinets as follows: 16-h photoperiod, long day (LD), and 8-h photoperiod, short day (SD), at 20/15 [degrees] C day/night temperatures. Lighting was maintained at 150 [micro]mol [m.sup.-2] [s.sup.-1] supplied by a combination of incandescent and fluorescent bulbs. Plants were arranged in a completely randomized design and rerandomized twice weekly to remove position effects.

Any plant exhibiting damage or a reduction in growth that may have been due to injury during transplanting was removed from the study. There were 15 plants per population in each of the first runs in the LD and SD conditions and 20 (LD) and 19 (SD) plants per population for the second run. Tillering was monitored daily until 35 DAT. As each new tiller arose, the day and site of appearance were recorded. Color coded wire loops were used to identify each new tiller. Node appearance and main stem elongation were used to assign growth stage values based on West (1990). Once tillering was initiated (rating = 10), stem elongation was then used to assign a growth stage value. The appearance of the first node on the main stem was then rated as 11, the second node as 12, and so on. The monitoring was terminated at 35 DAT as the LD plants had elongated main stem and tillers outside of the pots and interplant competition for light was becoming a factor. At 35 DAT, plants within populations were paired on the basis of tiller number and growth stage. One plant from each pair was dissected to determine individual tiller dry weight.

Values reported in this paper for tiller age are from the plants that were dissected for tiller weights and values for all tillers were reported in Cattani (1999). Node appearance was derived from the growth stage measurements. Dry weight tiller order (main stem, primary, secondary, etc.) and dry weight per individual main stem branches [primary tillering system (PS)] were calculated per plant to examine dry matter partitioning within plants. Therefore, the first PS consisted of all tillers arising from and including the 1-1 [degrees] tiller; the second PS consisted of all tillers arising from and including the 2-1 [degrees] tiller; etc.. Tiller succession rates were calculated [time (days) between successive primary tillers (i.e. date of appearance for 2-1 [degrees] tiller minus the date of appearance for 1-1 [degrees] tiller] for each plant.

Analysis of variance was performed by PROC GLM in SAS (SAS Institute, Cary, NC). There were no run x population interactions and therefore means reported were combined over the two successive runs. Significantly different means were separated by the Student t-test at P = 0.05. Regression analysis was performed by SAS, with mean values for tiller age on dry weight per tiller for LD plants. Tillers were analyzed within the following categories: main stem and primary tillers, secondary tillers, and in the case of UM67-10 LD, tertiary tillers.

RESULTS

Tiller Age

Significant differences were found for tiller age within the two populations for both LD and SD (data not shown). Main stem age comparisons are not included because of the setting of the transplanting date as Day 1. The 1-1 [degrees] tiller appeared before all other tillers (18.8 and 20.6 d old, for Emerald and UM67-10 under LD, respectively), followed by the 2-1 [degrees] (11.7 and 16.8 d old for Emerald and UM67-10, respectively). The tillering order, in general, followed the expected pattern (Neuteboom and Lantinga 1989). The 3-1 [degrees] tiller appealed after the 1-1-1 [degrees] except for Emerald in SD (Table 1), the first tertiary tiller (1-1-1-1 [degrees]), appeared after the 4-1 [degrees] tiller. The length of the study did not allow for the in depth investigation of high tiller orders, with quaternary tillers just appearing at 35 DAT.

[TABULAR DATA 1 NOT REPRODUCIBLE IN ASCII]

Comparisons between the populations showed that appearance of tillers in UM67-10 took place at an earlier date than Emerald, especially for primary tillers (Table 1). For example, tiller age of the 1-1 [degrees] tiller was significantly greater for UM67-10 at 20.6 and 17.6 d, LD and SD respectively, compared with 18.8 and 13.7 d for Emerald. The exception to this trend was with the 4-1 [degrees] and coleoptile tillers (tillers arising at the coleoptile node on the main stem) in the LD (Table 1).

Effect of Tiller Age on Dry Weight per Tiller

The regression line of best fit for the individual tiller classes within the two populations for LD are found in Fig. 1a and 1b. In general, linear regression equations gave the best fit for the data with [R.sup.2] values ranging from 0.93 to 0.99. The major exception was for UM67-10, where no age and dry weight relationship for tertiary tillers was found (data not shown). Slopes indicate that main stem and primary tillers are stronger sinks, compared to later arising tiller orders, which was most likely due to internode development. The regression lines for Emerald had a greater slope for both the main stem and primary tillers (Fig. 1a) and for secondary tillers (Fig. 1b) than UM67-10. This again is indicative of the smaller tiller and leaf size noted in UM67-10 (Cattani, 1999).

[Figure 1 ILLUSTRATION OMITTED]

Dry Weight per Day

Dry weight per day accumulation (DWD) was analyzed to evaluate relative dry matter partitioning. Significant differences were found within both populations for DWD under both LD and SD conditions (Table 1). With the exception of the 2-1-1 [degrees] tiller in SD, between population comparisons for DWD indicated that regardless of whether significant differences were found, Emerald had a higher DWD value than UM67-10 (Table 1). The main stem of Emerald under LD had a 50% higher DWD (1.51 mg [d.sup.-1]) than the main stem of UM67-10 under LD (1.02 mg [d.sup.-1]). Comparisons between populations for the 1-1 [degrees], 2-1 [degrees], 3-1 [degrees], 1-1-1 [degrees], and 1-1-2 [degrees] tillers all showed Emerald with a significantly higher DWD than UM67-10 for both SD and LD conditions (Table 1).

Tiller Order Comparisons

Tiller number per order in LD, on a plant basis, was found to approximate a normal distribution across the orders (Table 2). Dry weight per order in LD was skewed towards the lower orders with primary tillers making a significantly greater contribution to total above ground dry weight per plant than the main stem (Table 2). For Emerald, the primary tillers accounted for significantly more dry weight per plant (77.9 mg) than the main stem (53.1 mg), which was significantly higher than the secondary tillers (29.6 mg) and significantly higher than the tertiary tillers (4.1 mg). The germplasm UM67-10 was also significantly highest in dry weight for the primary tillers (64.96 mg); however, the main stem and secondary tillers were not significantly different (36.1 and 43.3 mg, respectively). The DWT was in all cases significantly highest for the main stem, followed by primary tillers, and then by the other tiller orders in sequence (Table 2).

[TABULAR DATA 2 NOT REPRODUCIBLE IN ASCII]

A separate analysis was carried out to determine the contribution of individual main stem branches (PS) to above-ground dry matter accumulation. The first PS refers to tillers arising from and including the 1-1 [degrees] tiller, second PS to 2-1 [degrees] tiller and its descendants, and so on. Tiller number distributions between PS branches were similar, where developed, for the two populations (Table 3). The first PS had significantly more tillers than second PS and the following tiller orders. Emerald had a higher proportion of coleoptilar tillers (C-PS) than UM67-10. Dry weight distribution among the main stem and PSs were different between the populations (Table 3). For Emerald, main stem and first PS had the highest dry weight, followed by C-PS, second PS, third PS, and fourth PS (Table 3). The germplasm UM67-10 had the highest dry weight for first PS, followed by second PS, main stem, third PS, C-PS, fourth PS, and lower orders (Table 3). Within a PS, the primary tiller had the highest mean dry weight (Table 3). The germplasm UM67-10 showed a higher mean DWT, although not significant, for second, third, and fourth PS as compared with first PS, whereas, in Emerald, first PS had significantly higher dry weight than third and fourth PS.

[TABULAR DATA 3 NOT REPRODUCIBLE IN ASCII]

Tillering and Internode Appearance

Tillers appeared first in UM67-10 at 15.0 DAT, while for Emerald a first tiller appeared (TAD) at 17.3 DAT in the LD (Table 4). Internode development under SD was not seen at 35 DAT, therefore, the following data are from the LD only.

Table 4. Mean comparison for tiller appearance date (TAD), date of internode elongation (ITAD), difference between TAD and ITAD, main stem growth rate and pre- and post-internode development tiller appearance rate for the long day growth cabinet.
                                              Pre-elon-    Post-
                                               gation    elongation

                                     TAD to          Tiller
Population        TAD         ITAD    ITAD       appearance rate

                                     d           tillers [d.sup.-1]
Emerald      17.3a([dagger])   27.4a    10.4b     0.35b      0.65b
UM67-10      15.0b             26.5b    11.5a     0.48a      1.37a


([dagger]) Means followed by the same letter within a column are not significantly different using the Student t-test (P = 0.05).

There was less than a day difference between populations for mean internode appearance dates (ITAD) (Table 4). Differences between TAD and ITAD, (ITAD --TAD), showed that UM67-10 had significantly longer interval (11.5 d) between tiller appearance and internode appearance than Emerald (10.4 d) (Table 4).

Internode development, measured as the appearance rate in days of successive nodes on the main stem was not significantly different between populations (data not shown). Mean tiller appearance rate (tillers [day.sup.-1]), was calculated for pre- and post-internode development. The germplasm UM67-10 had a significantly higher tiller appearance rate than did Emerald (Table 4).

No significant differences were found between populations for the internode succession rate with the exception of the time interval between the second and third node in LD (Table 5). Time interval between the appearance of the second and third nodes was longest (Table 5). Once internode growth was initiated (identifiable nodes), gross tillering rate increased in both populations, although the relative tillering rate remained stable (Table 5). There was no significant difference in relative tiller appearance rate (tillers [tiller.sup.-1] [d.sup.-1])(data not shown).

[TABULAR DATA 5 NOT REPRODUCIBLE IN ASCII]

Correlation coefficients were calculated for TAD, ITAD, nodes elongating per main stem, number of elongating main stems and tillers per plant, tillers per plant, main stem length, and dry weight of the main stem for LD (Table 6). First appearing tillers and tillers per plant data were not correlated with ITAD, with the exception of UM67-10 (r = 0.36, P = 0.05). These low values indicate that internode development and tillering may be independent. Nodes per main stem number of elongating stems per plant, main stem length, and main stem dry weight all were significantly correlated with ITAD, and ranged from -0.448 to -0.685 (P = 0.01), with the exception of main stem dry weight for Emerald (r = -0.347) (Table 6).

[TABULAR DATA 6 NOT REPRODUCIBLE IN ASCII]

DISCUSSION

These studies investigated the development of young creeping bentgrass plants in the exponential growth phase under LD and SD conditions. Stolons, according to the accepted description of plant growth for creeping bentgrass being of extravaginal emergence in origin (Beard, 1973), were rarely seen. This may have been due to the low light intensity in the growth cabinets. In fact, the main stem evolved into an elongated stem with all other characteristics of a stolon with the exception of its emergence. The development of primary, secondary, and in some cases, tertiary tillers into creeping stems via intravaginal emergence also may be indicative of the arbitrary nature of the accepted plant development descriptions for stolons (Beard, 1973; Turgeon, 1999).

Interplant competition may lead to an increase of tillers in the higher tiller orders (tertiary or higher), which tend to be smaller and at least initially are spatially in greater competition for light. This has implications with respect to tiller development within a turfgrass community. The present management methods for high tillering creeping bentgrass cultivars grown on putting greens include mowing to less than 3-mm height and fertilizing at lower annual nitrogen rates; light, frequent topdressing; and core aeration six to eight times a year (E.K. Nelson, 2000, personal communication). High tillering bentgrasses, such as UM67-10, have a greater number of tillers at the higher tiller orders (Cattani, 1999). Once full competition between plants is reached, the ability of the plant to spread laterally is severely reduced and the replacement of leaf area lost during mowing will take place closer to the crown of the plant and at higher tiller orders. As noted above, internode development takes place at an earlier stage and the upward development of these tillers is dictated by competition (Cattani, 1999). If internode elongation begins to take place, leaf tissue will be lifted higher into the canopy and this will result in a "puffy" turf. Live leaf number per tiller under established golf green conditions have been reported to be between 2.7 and 3.1 (Cattani and Clark, 1991), regardless of tiller density. This is similar to that reported by Robson (1973) in perennial ryegrass. The number of leaves above the last tiller was not measured; however, if a turf allows for a maximum of three live leaves, tillering at lower nodes may be foregone for nodes of more advantageously placed leaves Frequent topdressing will help alleviate this problem by raising the surface. Core aeration, however, should allow for the spread of plants laterally by increasing the penetration of light into the canopy on the periphery of the aeration holes, therefore allowing for tiller initiation at the lower tiller orders and lower on the individual tillers. The physical opening of areas in the turf also will allow for stolon elongation. New tiller development will therefore take place at the orders of the stolon and higher. Once a frequency of core aeration is found that maintains a relatively consistent tiller weight, provided it is above the level require for survival (Ong, 1979), a uniformly dense turf should be achieved.

The germplasm UM67-10 demonstrated greater dry matter partitioning between tillers than did Emerald (data not shown). That is, there was less variability between tillers of the same order, due in part to the greater number of elongating stems per plant (Cattani, 1999). Provided that the stress survival threshold value for tiller dry weight has been surpassed, this should confer greater overall tiller survival to UM67-10 (Ong, 1978).

Tiller size decreased as tiller order increased for both populations studied. The sixth PS was similar in weight to the fifth PS for UM67-10 (Table 3). Both consisted of a single tiller, and as the fifth PS was older, this indicated an increasing tiller size with successive tillers to this stage of development. Lower productivity (DWT-and DWD) of the first tiller of each order and branch with UM67-10 also was indicative of tiller size.

The efficiency of bud conversion to tillers (site usage) decreased in the higher tiller orders (Cattani, 1999). A positive relationship between DWT and tiller age was not found for tertiary tillers in UM67-10, but was observed for primary and secondary tillers. This may indicate that potential tiller size within a plant is important with respect to development. Other possible explanations for this include consistently lower DWT for the first tiller of new branches for UM67-10, insufficient data for tertiary tillers because of to their recent development, and the possibility that these tillers are still in the elongation phase of growth.

Emerald demonstrated greater DWD gains than UM67-10. This is indicative of the larger leaves, tillers, and elongating stems, found in Emerald. The effect of mowing on tillering and dry matter partitioning will determine the persistence of the turf under use. Emerald has been found to possess low tiller densities and higher DWT than high tillering cultivars such as 18th Green, which was selected out of UM67-10 (Cattani et al., 1992). If higher turf tiller density confers greater wear stress tolerance as predicted by Lush (1990), and found for seashore paspalum (Trenholm et al., 1999), then selection for tiller density via tillers per plant may provide a simple tool for wear stress resistance selection. High tillering creeping bentgrasses, however, possess shorter stolons, which may reduce spread into open areas (Cattani et al., 1996). Therefore, if plant attribute requirements differ between area of usage on a golf course (e.g., golf green versus tee), the desirable attributes of the grass to be selected for also may differ.

The node appearance on elongating stems was not related to the onset of tillering. Both populations showed a node appearance rate of approximately one node every 3 d, regardless of the environment in which they were grown (Table 5). Emerald initiated tillers later, but the elongating internodes appeared at a similar date to UM67-10. Therefore, Emerald had a shorter interval between first tiller and first internode appearance. Population differences were not found with respect to the rate of node appearance. Emerald had longer internodes than UM67-10 (Cattani, 1999), and given the same rate of internode appearance on the main stem, reinforces the concern regarding reduced plant spread characteristics in high tillering populations.

Population differences were found with respect to the effect of elongating stem development on tillering. Once stem elongation became evident, UM67-10 demonstrated a greater gross tiller appearance rate than Emerald, 1.37 versus 0.65 tillers per day in LD (Table 4). Fewer and larger elongated main stem and tillers in Emerald (Cattani, 1999) may represent a greater sink capacity than the more numerous, smaller elongated main stem and tillers of UM67-10, leading to a reduction in tillering.

The shorter period and/or amount of light available limited growth under the SD conditions. Total dry weight per plant was similar between populations within growing environments; however, the LD was approximately 500% higher than the SD for this measurement (Cattani, 1999). The light duration and amount in SD compared with LD restricted tillering and impeded internode elongation initiation (Cattani, 1999). Deregibus et al. (1983) suggested that phytochrome regulates tiller development. Casal et al. (1990) showed the importance of a high red/far-red light (R/FR) ratio with respect to tiller initiation. Shade, both within a grass community (Casal et al., 1990) and by trees (Bell et al., 2000) reduce the R/FR ratio and tiller density. Season also alters the R/FR ratio, with the duration of time the sun is below 10 [degrees] of the horizon being the controlling factor (Smith, 1982). Day length is most likely the factor controlling seasonal response with tillering in creeping bentgrass (Cattani et al., 1991).

Anslow (1966) states that reduced photoperiod generally increases leaf appearance rate, while lower light intensity decreases leaf appearance rate. Therefore, it is most likely that reduced light quantity and not photoperiod, which is reflected in our results showing reduced tillering, internode elongation, and greatly reduced dry matter accumulation under SD conditions.

Internode development is most likely a response to internal competition for an external resource, such as light. The decreasing stage of tiller growth at which internode development was initiated indicates greater competition for light. Internode development should therefore be seen as a plant response to place its developing leaves into a more favorable growing environment. The large increase in DWT in plants under LD conditions was primarily due to internode growth. Because of pot size and duration of these studies, the impact of rooting at nodes on the elongating main stems development was not ascertained.

Abbreviations: C-PS, coleoptilar tillers; DAT, days after transplanting; DWD, dry weight per day; DWT, dry weight per tiller; ITAD, internode appearance dates; LD and SD, long and short day, respectively; PS, primary tillering system; TAD, first tiller appearance.

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D. J. Cattani(*) and P. C. Struik

D.J. Cattani, Nova Scotia Agricultural College, Truro, Nova Scotia, Canada, B2N 5E3, P.C. Struik, Theoretical Production Ecology, Wageningen Univ., Wageningen, the Netherlands. The data contained herein partially fulfills requirements for a Ph.D. program at Wageningen Univ for D.J. Cattani. Received 8 Feb. 2000. D.J. Cattani (*) Corresponding author (Dcattani@cox.nsac.ns.ca).
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Author:Cattani, D. J.; Struik, P. C.
Publication:Crop Science
Geographic Code:1USA
Date:Jan 1, 2001
Words:5362
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