Aplicaciones de los circuitos de circulaci´on natural

Cool season grasses have an optimal growth temperature ranging from 16°C to 24°C, temperatures, and above this range results in decreased metabolism (Du et al., 2009). With extremely high temperatures and drought conditions they undergo dormancy, cease to grow and turn brown in colour (Beard, 1994).

The C4 pathway naturally occurs mainly close to the equator under warm and sunny conditions. It involves complex metabolic and anatomical differences, compared to C3 plants, which allow efficient water use and retain productive capacity under high light intensity and temperatures (Beard, 1998). Warm season or C4 turfgrasses are widely grown in tropical regions of the world and even sometimes in temperate regions, where hot summers are experienced (Geren et al., 2009). The C4 grasses are able to tolerate high temperatures (i.e. warmer maximum summer temperatures ranging from 25 – 38°C) and water deficiency very well. Tolerance to low temperature is one major limiting factor for most of the warm season turf species (Thomas et al., 2009). Cold temperature damage in warm season turfgrasses lowers their aesthetic appeal. Their growth usually stops below 16°C,while foliage starts turning brown below 10°C (Anderson et al., 2002).

Since playing fields and parks are key amenities in urban infrastructure, the need for a sustainable turf surface always keeps managers under pressure. The tension between the desire for scheduled events to go ahead without cancellation, and the need to avoid damage and maintain visual appearance, is a central consideration nowadays when developing maintenance practices (Cisar, 2004). Damaged or dormant grasses lose their aesthetic and environmental values and are considered undesirable in sports and athletic fields. The costs of redevelopment of injured or dead turf can be a significant burden on municipal maintenance budgets (Anderson et al., 2007). There is, therefore, an increasing interest in superior varieties capable of survival and growth under both extremes of temperature. Turf breeders have made substantial progress with increasing stress tolerance in warm season grasses (Anderson et al., 2007). Research work with warm season turfgrasses has been carried out in many countries but mainly concentrated in USA, Australia and China.

Lack of tolerance to chilling temperatures is the most often investigated performance trait for selection and usage of warm season species (Bermuda grass, Zoysia

and seashore paspalum) under cooler climates (Stefaniak et al., 2009; Kopec et al., 2007; Casler, 2006; Anderson et al., 2003; Beard, 1982). Warm season grasses are usually considered to be intolerant to both sudden and prolonged exposure to low temperatures. Low temperature exposure caused by rapid temperature drop (frosting) has been referred to as ‘direct’ cold exposure, and exposure to low temperatures for a prolonged duration is termed ‘indirect’ cold exposure (Fry, 1990). Cold temperature exposure results in chilling injury leading to dormancy, chlorosis and necrosis, and such damage is followed by death in more sensitive species. Frost kill occurs because of disruption of cell membranes and other components due to expansion on freezing of solutes. Overall reduction in chlorophyll accumulation, photosynthesis, metabolite translocation, enzyme activity and protein synthesis can be observed (Sanghera et al., 2011; Samala et al., 1998; Salisbury and Ross, 1992).

Winter field performance is a typical assessment method for cold hardiness in grasses. One measure of cold hardiness is the LT50 (LT denotes lethal temperature), which refers to the low temperature exposure that will kill 50% of plants in a grass population (Zhang et al., 2009). Uncertainty over what kinds of fluctuations in temperature may be most damaging to plants limits the interpretive power of field experiments. In vitro

experiments using controlled environment cabinets and freeze chambers provide better information and understanding of cold stress mechanisms (Anderson et al., 2005).

Cynodon (Bermuda grass, couch grass) is the most common and widespread genus among the warm season group of grasses (Taliaferro, 2003) and is extensively used for golf courses and athletic surfaces. However Cynodon varieties often lack winter hardiness and so are vulnerable to frost damage or winter kill (Munshaw et al., 2006). The cold tolerance level of different Bermuda grass cultivars was evaluated by Anderson et al. (2002) using a laboratory freeze method and found a great variation in tolerance levels based on cultivar type and usage. Varieties particularly recommended for fairways were more tolerant compared with those used for greens.

Anderson et al. (2007) observed that vegetatively propagated Bermuda grass had a lower LT50 (–6.2°C to –11.3°C) than seeded varieties (–5.3°C to –8.7°C). Similarly Zhang et al., 2006 compared two seeded cultivars of Cynodon (Riviera and Princess-77) for their level of cold tolerance. LT50 was lower (–8.3°C) for Riviera compared to Princess–77 (–6.3°C). Tolerance to cold, while partly mediated by physiological factors, is also influenced by grass morphology. A positive correlation can be found with stolon length, stolon size and rhizome length (Ahring et al., 1975). Bermuda grass cultivars with increased cold tolerance are mostly selections of clonal (vegetatively propagating) material from cooler regions which were further hybridized with superior mat forming cultivars (Samudio and Brede, 2002).

As noted above, Zoysia is one of the genera of C4 grsses used in the turf industry. Species, with the three species most commonly used by the industry being (Z. japonica, Z. matrella, and Z. pacifica. Z. japonica is considered most tolerant to low temperature stress and to have a superior shade tolerance compared to Cynodon (Zhang et al., 2009; Engelke and Anderson, 2003). Exposure to air temperatures below 10°C is problematic for common Z. japonica and can cause leaf discoloration and increase in carbohydrate and proline contents (Wei et al., 2008). Patton and Reicher, 2007 evaluated in vitro freeze tolerance of different Zoysia varieties, and in that work a cultivar Diamond displayed inferior performance (LT50 = –8.4°C) compared with Meyer and Zenith (LT50 = –11.5°C).