Ensayos con modelos de membrana

The first of the two sections of this discussion considers aspects of the experimental methodology that need to be accommodated in any interpretation of the results of the study. The second section discusses the main findings of the study which centre on the negative influence of stolon burial on branching which is caused by burial decreasing the survival of young developing branches rather than decreasing initiation of outgrowth of axillary buds.

4.4.1 EXPERIMENTAL METHODOLOGY

The experiments all used similar methodology and were performed to study the effects of different aspects of stolon burial on the branching of treated stolons in the field. These experiments involved imposing burial treatments on stolons consisting of the apex and up to ten nodes and then after a time interval assessing the effect of treatments on outgrowth of axillary buds present at the time of burial.

Branching was registered once the axillary bud had produced a leaf of development stage 0.2 on the Carlson ( 1 966a) scale. Although the node position at which axillary bud outgrowth commences varies with growth conditions (Davies & Evans 1 990a), in grazed pastures in Manawatu nodes are positioned three or more from the apex before branch initiation occurs (Table 4.2; Hay et al. 199 1 ) and under pastoral conditions most branch initiation ceases once node position exceeds eight (Chapman 1 983; Davies & Evans 1 990a; Hay et al. 1 99 1 ). Hence interpretation of results must take account of this ontogenetic development of axillary buds. The response of buds at positions 1 to 3 therefore chiefly centres on the initiation of branches whereas that for positions ;:::5 becomes increasingly centred on branch survival as with increasing node position the probability of initial branch presence increases (Table 4.2) and the potential for branch initiation decreases (Newton et al. 1 992).

The system used to classify the response of buds was insensitive in that the category of no outgrowth before or after treatment (0, 0) contained two types of buds. The first type were the inactive buds and the second type reflected the net result of development over the burial period. This latter component comprised those buds that initiated outgrowth during the burial period but which then died before harvest date. As 50-75% of axillary

buds of field populations of white clover may be inactive (Chapman 1 983; Davies & Dutta

1 988; Newton et al. 1 990, 1 992) the proportions of inactive buds after a burial period of

one grazing interval (29-68% ; Table 4.8) were not indicative of major distortion from this source. With an increase in the duration of burial the proportion of buds recorded as having initiated branches steadily decreased (Tables 4.5 and 4.8). Comparison of similarly treated stolons which had a range of durations of burial (i.e. along rows of Table 4.8) indicates that for longer durations of burial, branch survival following initiation is reduced and that this inflates the proportion of buds recorded in the inactive (0,0) category.

4.4.2 EFFECT OF BURIAL ON BRANCHING

The overall effect of burial on axillary bud outgrowth was to significantly reduce branching. This occurred regardless of whether the comparison was of burial of complete stolons (depth-of-burial and date-and-duration-of-burial experiments) or of portions of stolons (zone-of-stolon-buried experiment) . Although in all experiments the effect of burial (of nodes) on axillary bud outgrowth was significant for nodes 1 to 3, results for nodes positioned >3 at the time of burial were less conclusive. B urial significantly reduced branch survival in the depth-of-burial experiment (Table 4.5) but had no significant effect in the zone-of-burial experiment (Fig. 4.2). A small impact of burial on branching of axillary buds of older nodes would not be a surprising result. Buds at these nodes have reduced potential to initiate branches (Dutta 1 988; Davies & Evans 1 990a; Newton et al. 1 992) and the larger branches at these nodes have increased chances of survival (Chapman 1 983). Both these factors act to limit the potential for response to burial under the assessment procedure used. A non-significant effect of burial on axillary bud outgrowth was reported in a glasshouse trial (Grant et al. 1 99 1 ) although a stimulation of axillary branching was measured on the portion of stolon that resurfaced following burial. Such a comparison was not valid for this trial as resurfacing stolons grew into open conditions on the elevated soil surface in the metal containers, whereas unburied stolons were in the sward. On the other hand burial of nodes 1 to 3 had a significant depressing effect on initiation of branches from these buds (Table 4.6, Fig. 4.2). Work reported by Davies & Evans ( 1 990a) and in Chapter 6 indicates that as these nodes had leaf or petiole above the soil, and in light, a depression in branching due to exclusion of light at the axillary bud site could not be expected. However there is some conflicting evidence (Harvey 1 979; Newton & Hay 1992) that presence of a subtending leaf can act to depress axillary bud outgrowth. The depressive effect of burial on axillary bud outgrowth at nodes 1 and 2 (Fig. 4.2) suggests that buds at this ontogenetic stage are sensitive to an inhibitive effect of burial not alleviated by the presence of the subtending leaf in light. It is interesting to note that Grant et al. ( 1 99 1 ) found a depression with burial of axillary bud outgrowth at nodes on the secondary branch stolons. A higher proportion of the nodes on these secondary stolons would have been of node position 1 to 3 at the time of burial. If buds at these nodes are sensitive to burial then this may have contributed to the result they obtained. Clearly then the major effects of burial centre on its effects on axillary bud outgrowth of the most recently formed nodes (positions 1 , 2 and 3) and further work under more controlled conditions is required (see Chapter 6) to confirm these findings and possibly identify the processes responsible for the observed responses.

Duration of burial influenced survival of branches in both the depth-of-burial and the date-and-duration-of-burial experiments. A short duration of burial of one grazing cycle had no significant effects on axillary bud outgrowth (Table 4.5) whereas after two grazing cycles, trends were evident but not significant (Tables 4.6 and 4.7), and after three or more grazing cycles, responses were significant and of approximately maximal extent. Branches formed prior to and after the date of burial were both affected by burial (Tables 4.5, 4.8). In both cases burial decreased survival of branches. No evidence was obtained to suggest that burial treatments negatively affected the initiation of branches (see Table 4.5 ; also

compare response category values of unburied buds after one grazing cycle in Table 4.5 with those of the burial treatment of 1 6/6/86 after one grazing cycle in Table 4.8). Thus the response to burial developed over time as a result of burial reducing survival of branches, there being no effect on initiation of branches. Hence if the effects of burial on axillary bud outgrowth are to be observed the duration of burial should be such that the effects on branch survival can be detected. Under these trial conditions this corresponded with the elapse of three grazing cycles.

The effect of other factors such as depth and date of burial on outgrowth of axillary buds were generally small. During December 1 986 and January 1 987 very pronounced soil moisture deficits developed (see Chapter 3, Fig. 3 . 1 ). The normal response of white clover to moisture deficit includes the death of small daughter branches and inhibition of axillary bud outgrowth (Wang 1 99 1 ). This response pattern is strikingly represented by the comparison (Table 4.8) of the first four dates of burial with the fifth date (2 December 1 986) for frequencies of branch death and initiation of stolons buried for one grazing cycle. In the depth of burial trial (Section 4.3 .2) this effect is not likely to make a large contribution to the observed difference in response between short and long intervals of burial as nodes and branches were considerably older at the time the drought developed and therefore beyond the zone where plant response to moisture stress was most marked (Wang 1 99 1 ).

CHAPTER 5 EFFECT OF REDUCED INTRA-PLANT AVAILABILITY