Efecto del clorhidrato de dopamina sobre el consumo de oxígeno (QO 2 )

T. ambiguum(Kura clover or Caucasian clover) is a long lived perennial species having

rhizomes, thick deep roots and vigorous stems. Its large rhizomatous root mass provides drought tolerance, insect resistance and virus resistance. Its flowers produce a very pleasant scent and abundant nectar. It lacks nodal roots and is a slow to establish clover type. It exists in a series of ploidy levels i.e., diploid, tetraploid and hexaploid adapted to a continuum from high altitude to low altitude respectively (Williams and Nichols, 2011). This species has been hybridized with white clover to introgress its persistency, drought tolerance and biotic stress tolerance into white clover (Abberton, 2007).

Williams (1978) produced mature hybrids between white clover and T. ambiguum with an embryo rescue technique using transplanted nurse endosperm. Both parents were tetraploid but their hybrids were highly sterile. Although the hybrids showed potentially good agronomic trends, especially a stoloniferous nature and the leaflet texture of white clover, they were generally intermediate to their parents. In a further attempt (Williams

and Verry, 1981), a partly fertile hybrid (H-435) between these two species was produced. Using this hybrid, they also produced F2 and backcrossed generations with

white clover as recurrent male parent. One backcrossed progeny had normal (2n=32) chromosomes, while another appeared to be a pentaploid (2n=42). Both plants showed improved resemblance to their recurrent parent and irregular chromosome pairing during meiosis.

Anderson et al. (1991) assessed the possibility of introgression between white clover

and T. ambiguum by backcrossing 4x and 8xH-435 plants. The tetraploid hybrids were

self-incompatible, whereas selfing of the octaploid hybrid resulted in an F2 generation.

The backcrossing of 4x H-435 with T. ambiguum remained unsuccessful, but backcrosses to white clover resulted in BC1 populations consisting of 4x, 6x and

aneuploid plants. The backcrossing of 8x H-435 with white clover and T. ambiguum

produced hybrids with expected ploidy level of 6x. Both 4x and 8x levels of H-435 showed frequent existence of uni-, bi- and quadrivalent chromosomal associations. The researchers predicted the possibility of successful selection in advanced generations of 8xH-435 for improved fertility and persistence.

Pederson and Windham (1989) found 38% of T. ambiguum plants resistant to

Meloidogyne incognita. Difficulty in producing fertile hybrids with white clover was a

potential hurdle in transferring this resistance into the latter species. Meredith et al. (1995) produced interspecific hybrids between white clover and T. ambiguum (2n = 4x

= 32) and backcrossed generations with white clover as the recurrent male parent. Most of the BC1 plants had 48 chromosomes but some were normal i.e., 32 chromosomes,

indicating production of both normal and unreduced gametes from the hybrids. Frequent bivalent chromosomal association during meiosis of BC1 plant with 48 chromosomes

showed a stable chromosome balance. Morphology of the BC1F1 plants was generally

similar to F1 plants. They were stoloniferous with some evidence of rhizomatous

growth. Plants of BC2F1 showed more variation in expression of T. ambiguum traits

than BC1F1. Hussain and Williams (1997) overcame the difficulty of hybridizing white

clover with T. ambiguum by creating a ‘fertile bridge’ between them. The 8x H-435 produced backcrossed seeds with white clover. However, it failed to backcross with the

T. ambiguum parent. Cytological analysis of BC1F1 showed high frequencies of

multivalents at metaphase-1. This indicated occurrence of both autosyndetic and allosyndetic pairing and chances of recombination in this hybrid generation. Selfing of

these hybrids resulted in appearance of improved traits like increased fertility, nodal rooting, nodulation and appearance of combined stoloniferous and rhizomatous growth habit in BC1F2 plants. One of these plants was found to be cross compatible with T.

ambiguum resulting in a 6x BC2 generation. Being cross compatible with white clover

and T. ambiguum, the plants of the BC1F2 or BC2F2 generation could be an efficient

‘fertile bridge’ between these two species.

Marshall et al. (2001) investigated the performance of white clover, T. ambiguum and their hybrids under drought stress. The purpose of hybridization and backcrossing was to in-corporate the rhizomatous characteristics of T. ambiguum into white clover genotypes. They studied parents and backcrossed generations in monocultures as well as in mixed culture with perennial ryegrass. The monoculture experiment presented clear results with better drought tolerance indications for T. ambiguum in terms of relative water content (RWC), leaf water content (LWC) and leaf water potential (LWP), compared to those of white clover. The performances of BC1 and BC2, with white

clover as recurrent parent, were generally in-between the parents. However, dissimilarity between results of the monocultures and mixed cultures was observed. Mixed culture produced complex results. For example, BC1 and BC2 retained lower

LWC even than white clover in mixed culture. Differences were also apparent for LWP, with T. ambiguum and white clover having the highest and lowest values, respectively and BC1 and BC2in-between them in monoculture, however in mixed culture, the order

was BC2 > white clover >T. ambiguum> BC1. In particular the BC2 made a dramatic

shift from the lowest to highest rank during the third week of high water stress. The difference between the results of monoculture and mixed culture could be linked with the poor competitiveness of rhizomatous development in T. ambiguum. As the BC1 and

BC2 generations also had some rhizomes, along with stolon characteristics, they

behaved differently in mixed culture (competition with grass) as compared to monoculture. The dry matter yield of the BC1 followed the behavior of T. ambiguum,

and increased gradually with increase in water deficiency. However, the BC2 generation

showed similarity to white clover and did not show increase in dry matter production with increasing water stress. This response was only studied in monoculture. In general, BC1 and BC2 combined the rhizomatous trait with stolons in a white clover genetic

background. They showed improved RWC and LWP over white clover under water deficit conditions in monoculture. The production of dry matter by the BC1 was near to

that of white clover with improved drought tolerance. But this combination did not follow into the BC2 population. The researchers emphasized the need to improve dry

matter yield under water deficit conditions. This work showed the importance of study under mixed culture to have results with application to field conditions and to study seedling vigor for better competitiveness under natural mixed cultures. The T.

ambiguum parent used in this study was tetraploid (Meredith et al., 1995).

Williams and Hussain (2008) explored suitable breeding strategies to combine desirable traits of T. ambiguumand white clover. They employed a colchicine doubled 8x hybrid of these species to create four backcrossed (white clover as recurrent parent) and four segregating generations. The intercrossed BC1 generations i.e., BC1F1 – BC1F4,

showed predominantly T. ambiguum traits. They had stable chromosome numbers (6x). Their roots were better in thickness and nodulation than white clover but lacked rhizomes. They were inferior to white clover in production of dry matter, nodes, stolons and inflorescences. Fertility was lower in the BC2F1 generation due to the 5x

chromosome number which would lead to abnormal chromosome pairing in subsequent generations. Advanced generations backcrossed with white clover were comparable with white clover in terms of nodulation, stolon thickness, nodes and internodal length. However, they showed inferior dry weight, root thickness, numbers of stolons, nodal rooting and inflorescence production. Based on this experiment, the researchers recommended advancing generations of BC1 by intercrossing and selecting for

combinations of desired traits. They also suggested that selection, especially for vegetative traits, should be delayed for at least two years.