Plant development and sward characteristics

The tiller is the basic unit of growth of the perennial ryegrass plant. Its apical meristem is located below the soil surface, and consists of dividing cells that initiate new growth from which leaves develop in regular sequence on alternate sides of the apex. Each leaf attaches to the shoot apex at a point called the node, and the stem tissue which separates one node from the next is called the

internode (Langer, 1973; Parsons & Chapman, 2000). In the vegetative state, internodes generally do not elongate so the true stem of the tiller is only a few millimetres in length. However, in the

reproductive states, several younger internodes elongate rapidly to produce the flowering stem which supports the seedhead. The spikelets of the seedhead are formed by differentiation of bud primordia on the apex such that they are committed away from leaf production to reproductive development.

The leaf comprises two parts: the leaf blade, or lamina; and the sheath. The lamina is connected to the sheath, at its base. As each leaf grows inside the encircling sheaths of older leaves, a ‘pseudo- stem’ formed by the older sheaths develops, while the ‘true stem’ (apical meristem, nodes and internodes) remains located at the base (Langer, 1973; Parsons & Chapman, 2000). But the true stem also may branch forming ‘tillers’. When the apical meristem produces a leaf, an axillary meristem develops on the opposite side of the internode, in the axil of the previous leaf. If this axillary bud becomes active, its apex produces leaves, and the replication of this process permits the increase in tiller numbers. In perennial ryegrass, the number of live leaves per tillers remains constant at approximately 3, because the rate of formation of leaves is similar to the rate of death (A. Davies, 1978). Once the tillering process starts, and the plant becomes larger, competition for resources (mainly light) within the plant or with adjacent plants takes place, and the pattern of tillering changes, resulting in a lower rate of production of tillers in relation to the rate of leaf appearance (site filling) (A. Davies & Thomas, 1983).

While the development of the aerial parts of the plant occurs, adventitious roots grow from nodes close to the soil surface, and with time, each tiller is able to produce its own network of roots (Langer, 1973).

In 1993 Chapman and Lemaire (1993, p. 96) stated:

Plant morphogenesis can be defined as the dynamics of generation and expansion of the plant form in space. It can be described in terms of the rate of appearance of new organs (organogenesis), their rate of expansion (growth), and their rate of senescence and decomposition.

These authors mentioned that leaf appearance rate, leaf elongation rate and leaf life-span are the three main characteristics determining the morphogenesis of a vegetative grass sward, and indicated that although these characters are genetically determined, they could be modified by variation in factors such as temperature, N nutrition and water status amongst others (Chapman & Lemaire, 1993). The combination of the above mentioned three main characteristics determines the structural characteristics of the grass sward which are: leaf size, resulting from the leaf elongation rate and leaf appearance rate; tiller density related to leaf appearance rate by ‘site filling’; and the number of living leaves per tiller, which depends on leaf-life span and leaf appearance rate. As these authors

in the canopy divided by the area of ground below) of the sward, which is the key determinant of light interception and regrowth dynamics (Chapman & Lemaire, 1993).

Previous research has linked tiller density and tiller size, through the self-thinning rule or size-density compensation response, named the -3/2 boundary rule, due to the negative slope of the line relating the logarithm of unit mass to the logarithm of population density (Sackville Hamilton, Matthew, & Lemaire, 1995). Size-density compensation occurs in response to modifications in the management of pastures and exemplifies the phenotypic plasticity of this species (Chapman & Lemaire, 1993). Establishment and maintenance of tiller population is vital for pasture persistence (Edwards & Chapman, 2011). Despite tillers being formed continuously, spring is the time of the year when tiller appearance rate is high; however it is also a time of high tiller death rates. In New Zealand, peak tiller densities has been observed in late winter-early spring and increased tiller appearance rate has been reported before flowering in mid spring (Edwards & Chapman, 2011; Hunt & Field, 1979). Frequency, severity and timing of grazing are crucial factors in determining tiller population and consequently tiller size (Edwards & Chapman, 2011) but other environmental and endogenous factors also play important roles.

The impact of light intensity and temperature on the pattern of growth and quantity of tissue produced by plants was studied since the 1950s. Under controlled conditions Mitchell (1953a); (1953b) observed that when one or both of these factors increased, the number of days between the appearance of successive leaves decreased. The axillary buds in these new leaves could develop to visible tillers or remain dormant, depending on the quantity of light energy available; raising light intensity increased rate of tillering, but the same effect was obtained by lowering temperature, or applying these two conditions. However, Mitchell also highlighted that the effect of changes in light quantity, temperature or defoliation on bud development or inhibition is conditioned by the level of the other environmental factors and by genotype (Mitchell, 1953a, 1953b). Later work in the field by A. Davies and Thomas (1983) showed that the rate of leaf appearance increased linearly with mean soil temperature up to approximately 14°C, but the rate of production of tillers in relation to rate of leaf appearance (site filling) appeared to be independent of weather conditions (A. Davies & Thomas, 1983).

Other factors affecting tillering were added to the analysis later by other studies: water supply, mineral nutrition, photoperiod, endogenous factors such as genotype, flowering, growth regulators, and management factors such as cutting and grazing. However the common ground of the effect of light quality on site filling is present in many of these studies. For example in A. Davies and Thomas (1983) study, site filling was less complete in larger plants, indicating within-plant competition for light and the effect of shading at the base of the plant. Similar conclusions were reached by

higher red/far-red ratios, without significantly modifying the photosynthetically active radiation, and concluded that branching of grasses was controlled by phytochrome activity (Deregibus et al., 1983). With increasing canopy growth, the capacity to produce new tillers and the light available per tiller decreased (Casal, Deregibus, & Sanchez, 1985). In later studies (using Lolium multiflorum Lam), Casal, Sanchez, and Deregibus (1987) found that adding low flux rates of red light at the base of the shoots increased tillering of plants that were exposed to low red/far-red ratios, irrespective of the ratios received by the rest of the plant. They suggest that these changes in the red/far-red ratio provide the signal that drives the plant response to competition for light (Casal et al., 1987).

Analysing the impact of grazing management on perennial ryegrass and white clover pastures, Korte, Watkin, and Harris (1984) also refer to the effect of shading at the base of the plant, when they explain greater tillering under the hard grazing treatment compared with lax grazing treatment. However, they also ascribe this higher tillering to greater assimilate availability, an argument that had been ruled out by A. Davies and Thomas (1983) as a reason for a cessation of tillering.

Simon and Lemaire (1987) studying the relationship between tillering of a vegetative grass stand and LAI, found that as soon as the LAI reached a value of 3, tillering rate slowed down, and then

terminated rapidly at higher LAI. The increase in LAI and decrease of tillering is associated with an increase in the rate of leaf elongation, a phenomenon that can be interpreted as an adaptation to competition for light, where carbohydrate is preferentially allocated to elongation of leaves. However, despite genotypes with high leaf elongation rate and long laminae being associated with reduced site filling, this does not necessarily mean low tiller number per plant (Bahmani, 1999). When analysing the effect of N, Simon and Lemaire (1987) found that this nutrient increased the number of tillers per plant at the beginning of the sward establishment, and as the LAI increased this effect disappeared. They concluded that in the absence of N deficiency the cessation of tillering was determined by the degree of self-shading of tiller buds (Simon & Lemaire, 1987). Langer (1963) however found that N affects the duration of tillering: plants inadequately supplied with N appear to stop producing new tillers at an early stage.

The need for a unifying theoretical synthesis of known effects of genetic and physiological factors and their interactions with the environment on control of tillering in grasses was recognized by Assuero and Tognetti (2010). Among the endogenous factors they cited biochemical changes, genetic control of tiller initiation and outgrowth, plant hormones (auxins, cytokinins, gibberellins),

compounds such as strigolactone and ethylene, as well as assimilate availability. Among the environmental factors they cited light intensity and quality, photoperiod, temperature, water availability, and mineral nutrition. They also cited biotic factors such as mycorrhizae, endophyte and plant growth-promoting rhizobacteria as well as management factors such as grazing (Assuero & Tognetti, 2010).