Etapas del proceso de cicatrización corneal 27

In white clover, an axil at a node produces one bud which develops into either a stolon or an inflorescence. Each inflorescence occurs at the expense of a stolon and vice versa. This creates an inbuilt inverse relationship between vegetative and reproductive growth. Inhibitory effects of inflorescences on stolon production and growth, and competition for nutrients also contribute to a negative correlation between vegetative and reproductive development (Thomas, 1987). Woodfield and Caradus (1996) emphasized importance of persistence of white clover plant to improve the competitive edge of New Zealand farming. They suggested breeding for higher stolon growing points at a particular leaf size can improve plant persistence and yield. Gibson (1957) found a strong negative relationship between inflorescence production and persistence of Ladino clover. He used profuse, medium and non-flowering genotypes under both normal and long day length conditions. Long light periods were used to induce flowering to the genotypes that produced few flowers in Alabama, USA, conditions. Flowering, induced

by plant genetics or prolonged light, negatively affected persistence. Interestingly, extended light periods made profuse flowering genotypes to change their normal tendency of high flowering and low stolon production. They produced more stolons than non- flowering genotypes under those conditions. The researchers hypothesized that long days were sensed by plants as an occurrence of unusual hot weather and plant’s response of improved stolon production to ensure their survival was turned. Crowder and Craigmiles (1960) studied the effects of soil temperature, soil moisture and flowering on the persistence and forage production of Ladino (large type) and Louisiana (intermediate type) white clover varieties under Georgia, USA, conditions. They found that a decrease in stolon population did not reduce forage production during the spring season. Repeated defoliation, flowering, and to some extent, deterioration due to diseases, weakened the plants. The weakened plants, when exposed to high soil temperatures and moisture stress, lost large proportions of their stolons due to desiccation. Both studies (Gibson, 1957; Crowder and Craigmiles, 1960) showed that high flowering reduced stolon production during summer, making them vulnerable to drought and lowering their persistence.

In Italy, Piano and Annicchiarico (1995) observed agronomic traits of 16 landraces and 12 natural populations of Ladino white clover. They found differences between land races and natural populations for leaf size, stolon density and long and short term dry matter (DM) yield. The natural populations showed stolon densities typical of intermediate-leaved white clovers, while their leaf lamina size and petiole length were similar to Ladino white clovers. This combination of traits could lead into a variety having high DM yield, tolerance to competitive stress, better persistence and tolerance to grazing, if seed production of these natural populations could be improved. In Ladino clover, improvement in seed production and less emphasis on long term survival during the domestication process has decreased stolon density and its associated agronomic characteristics. The phenotypic observations showed a strong positive correlation (r = 0.74) between persistence and stolon density in the third year of production. However, there were negative correlations between persistence and seed yield components. In particular, seed head production showed a strong negative correlation (r = - 0.7) with persistence. Stolon density appeared to be negatively associated with production of seed heads. The researchers showed that very few ecotypes, for example ‘landrace 19’, had the combination of persistence and high seed yield. These two traits appeared to be

difficult to combine. However, high seed yield could be combined with high DM yield, at least in earlier years.

Cain et al. (1995) did correlation studies for biomass and reproductive growth components of white clover. They studied 30 possible trait pair combinations for number of inflorescences, number of flowers per inflorescence, number of fruits (pods) per inflorescence and number of seeds per pod in both late and early maturing pods. There were no significant negative correlations on a genetic, phenotypic or micro- environmental basis amongst the studied traits. There was a strong positive association between numbers of pods and florets per inflorescence, showing potential for improvement in seed yield. Number of florets per inflorescence was moderately correlated with biomass production on both genetic and phenotypic bases. Except for early seed weight, biomass appeared to have positive phenotypic and genetic correlations with all observed reproductive traits. There was weak evidence for trade- offs among reproductive components and between reproductive traits and biomass. However the environmental conditions of this experiment were not optimal and, according to the researchers, resources and (or) pollen might have been insufficient to express the maximum potential of the plants. Annicchiarico et al. (1999), while working on ladino ecotypes and their sibs, also found negative genetic correlations of persistence (as indicated by stolon density) with seed yield (rg= - 0.70) and DM yield (rg = - 0.6).

Their results also showed a slight negative genetic correlation (rg = - 0.45) between seed

yield and DM yield. The narrow sense heritabilities for seed yield and its components including peduncle thickness were moderate to high. Number of florets per head and seeds per floret showed high narrow sense heritabilities. There were positive genetic correlations between sizes of different vegetative and reproductive organs. Larger leaf size was positively associated with number of florets per head and had a negative relationship with number of inflorescence per plant. Seed weight showed low heritability but this might have been due to low variability for this trait in the parents. Medeiros and Steiner (2000) in Oregon, USA, also found that intermediate leaf size cultivars produced more flowers than Ladino type large leaved cultivars