DATOS IMPORTANCIA.

The effects of treatments on the selected plant characteristics of plants at harvest are presented in Table 5.5. S ignificant genotypic differences were recorded for the leaf characteristics of petiole length and area per leaf, stolon diameter with greater diameter associated with the larger leaved genotype 2, number of branches on the main stolon and total number of nodes per plant. Dry matter yields, mean internode length and P content of leaf and stolon did not differ between genotypes. All characteristics except stolon

diameter increased with increasing P supply.

The genotype by P supply interaction was significant only for two characteristics, area per leaf and total node number per plant. The significance of the interaction for area per leaf was due to differential increases of 50 and 1 00% in values at 0.2 mM and 1 .0 mM P supply for genotypes 1 and 2, respectively. The significance of the interaction for total nodes per plant reflected the larger percentage increase in values at 0.2 mM P supply for genotype 1 than for genotype 2.

5.4 DISCUSSION

The objective of the reported experiments was to further understanding of the effects of deprivation of resources, in particular photosynthate and P supply, on branch initiation in white clover. This has relevance as regards the effect of stolon burial on branching as at the time of grazing, defoliation and burial can occur simultaneously and act to substantially reduce the availability of photosynthates within plants. Also as supply of P is suboptimal in many New Zealand pastures the effects of deprivation of P supply were studied to establish if the directions of the branching responses to both resources were similar.

Table 5.5 Mean values of specified plant characteristics of two genotypes of Grasslands Tahora white clover grown at three levels of phosphorus supply as measured at harvest. F test values and significance are given for the main effects for genotype (G) and

phosphorus supply (P) and for the genotype by phosphorus supply interaction (GxP) for

each characteristic.

Phosphorus Supply Treatment

Plant 0.01 mM 0.2 mM l .O mM Significance levels

Characteristic

Genotype Genotype Genotype G p GxP

1 2 1 2 1 2 First unfolded leaf Petiole length (cm) 4.3 4.6 7.0 8.3 9.3 1 1 .3 F 9.3 68. 1 1 .9 p < _{0.006 0.0001 ns} Area leaf1 (cm2) 1 .08 1 .43 2.08 3 . 1 6 2.95 6.31 F 14. 1 1 6.2 4.5·· p < _{0.00 1 0.0001 0.03} Stolon diameter proximal to 1 .65 1 .85 1 .67 1 .80 1 .67 2.07 F 2 1 .2 2.8 2.2 first unfolded p < _{0.0001 ns ns} leaf (mm) Internode length main 1 .25 1 . 1 3 1 .47 1 .59 1 .69 2.14 F 3.3 23.5 1 .4 stolon (cm) P < _{ns 0.0001 ns} No. branches on main 6.8 3.4 1 2.0 7.2 1 3 .2 1 0.8 F 6 1 .7 80.9 2.4 stolon p < _{0.0001 0.000 1 ns} No. nodes 29.0 1 7.6 66.6 33.8 9 1 .6 67.0 F 63.0 1 25.3 4.7 planr1 p < 0.000 1 0.000 1 0.02 Dry Weight planr1 (g) 0. 1 39 0. 175 0.555 0.438 1 .496 1 .975 F 1 .2 62.5 2.2 Leaf P ns 0.0001 ns 0.088 0.085 0.264 0.202 0.72 1 0.992 F 1 .0 44.8 2. 1 Stolon P < _{ns 0.0001 ns} Total 0.226 0.260 0.8 1 9 0.640 2.217 2.9 F 1 . 1 56.0 2.2 p < _{ns 0.0001 ns} P content (% of DW) . 1 4 . 1 3 . 17 . 1 9 .32 .33 F 0.6 62. 1 0.4 Leaf P < _{ns 0.0001 ns} Stolon . 1 4 . 1 1 . 15 . 1 6 .26 .27 F 1 .8 47.3 0.9 P ns 0.0001 ns

This discussion consists of four sections followed by a conclusion. Firstly, aspects relating to the scope of interpretation of the study are considered before the second section discusses the effects of photosynthate and P supply on the plant attributes that determine branching of white clover. The third section discusses the effects of deprivation of photosynthate and P on the allocation of resources to metamers and to organs within metamers. Prior to the conclusion, the fourth section comments on the integration of clonal growth in relation to branching.

5.4.1 DELIMITATION OF INTERPRETATION

The study was not undertaken with the purpose of assessing the extent of genetic variation in response but included known genetic variation within experiments so as to ensure that the principles of response were more general to white clover than that specific to one genotype cloned for glasshouse experimentation. Hence experiments were inclusive of elements of genetic variation only for the purpose of monitoring for the possibility of large genetic influences on either the direction or the scale of responses to treatments. Within these constraints genotype was not found to significantly alter the direction or scale of branching responses to shade or defoliation but to influence the mode of response to limitation in P supply. Although genotypic variation in node appearance rate (Burdon & Harper 1 980; Caradus & Chapman 199 1 ) and position of first branching node (Erith 1 924; Caradus & Chapman 199 1 ; Turkington et al. 1 99 1 ) has been established, the absolute values of these characteristics are dominated by environmental conditions in any given situation (Sackville Hamilton & Harper 1 989) and this proved to be the case within the shade and defoliation experiment. However under the conditions of the P supply experiment significant and consistent genotypic differences were obtained for node appearance rate and position of first branching node (Table 5.4, Fig 5.5) although the direction response was the same for both genotypes.

Understanding of branching responses of plants requires information at the individual axillary bud level, rather than at the developed meristem level (Harper 1 977; Sackville Hamilton & Harper 1 989; Hay et al. 1 99 1 ; Newton et al. 1 992). Thus as there is an axillary bud at each node, the demography of nodes becomes an issue that is involved in the assessment of branching responses of plants to treatments. As, in these short-term glasshouse experiments, there was no plant fragmentation the response to imposed treatments can be described by the simple model proposed by Turkington et al. ( 199 1 ) ; the

number of nodes per plant is a function of node appearance rate, position of first branching node (relative to the stolon apex) and frequency of branching at nodes proximal to the first branched node. These parameters define the recruitment and activity of axillary buds on a stolon basis which when summed over stolons provides whole plant data. However, the short-term nature of the experiments reported means that analysis has necessarily been restricted to the main stolon of plants. It is recognised that this may underrepresent the longer-term branching response of the plant to treatments as branching performance of secondary branches can be affected to a greater extent than the main stolon under some

situations (King et al. 1978; Grant et al. 199 1 ; see also Chapter 6). An advantage of use

of the three parameters cited by Turkington et al. ( 1 99 1 ) to assess branching response is

that assessment is non-destructive which means that assessments can be repeated throughout an experiment and so allow identification of when a particular response occurs.

Defoliation treatments were applied as a single event at commencement of the experimental period. Thus the response to defoliation during the course of the experiment was one of recovery and so differed fundamentally from the response to shade treatments which imposed a regime of increasing intensity of deprivation of photosynthate. Hence the previously observed depressive effects of defoliation on node appearance rate (Mitchell 1 956; Carlson 1 966b; Sanderson 1 966; King et al. 1 978; Sackville Hamilton & Harper 1 989; Grant et al. 1 99 1 ) were most evident early in the experiment (Fig. 5.2). The major effect was an immediate halving of node appearance rate in the growth interval following defoliation; an effect which, for unshaded plants, had disappeared by growth period three (Fig. 5 .2) . A probable mechanistic explanation is that defoliation reduces photosynthate production and hence supply of carbon for new growth. The plant adapts by remobilizing photosynthate stored as starch in root and stolon (Moran et al. 1 953; Vez 1 96 1 ; Murphy 1 982; see Sections 7.3.4 and 7.4 for description of starch depletion and replenishment in stolon following defoliation). However the rate of remobilization is insufficient to prevent a penalisation of new growth through reductions in leaf (Carlson 1 966b) and internode (Thomas 1987b) dry weight and node appearance rate. However defoliation had a quantitatively small effect on position of first branching node which did not alter significantly with growth period, and indicated that large changes in this characteristic were associated with variation in the light rather than the defoliation environment (Fig. 5. �}.