Table 6.3 The probability of axillary bud outgrowth from nodes that emerged on the primary stolon in each of the two treatment periods on stolons subjected to
CHAPTER 9 SUMMARY OF MAJOR RESULTS AND CONCLUSIONS
1 ) In the field trial, time of year (season) was the factor associated with major changes in the proportion of stolon buried, total stolon density, clover dry matter production and the branching structure of the white clover population. Although there were significant differences among the Tahora, Pitau and Kopu cultivars for seasonal growth rate, dry matter allocation to stolon and leaf, growing point density and plant density there were no differences in the mean node number per plant or branching structures of the populations.
2) Sequential sampling of undefoliated stolons either artificially buried or growing on the soil surface found probability of initiation of axillary bud outgrowth to be unaffected by burial but probability of survival of the outgrowing axillary bud to be significantly reduced which resulted in a three-fold lowering of probability of branch establishment at buried nodes. Axillary buds most affected by burial were at node positions 1 and 2 at the time of burial. Most of the negative effects of burial were evident after a burial period of three grazing cycles (approximately three months). Date and depth of burial of stolon had minor effects on the extent of the response.
3) Imposed treatments of neutral shading, defoliation and limitation in phosphorus supply significantly reduced node appearance rate (by 67%, 30% and 25% respectively) , could increase node position of fust branch (two- to three-fold), but had no significant effect on the proportion of nodes initiating branching. In general, where leaflets only were defoliated (petioles remained intact) values of assessed plant attributes were intermediate between undefoliated and defoliated stolons. Limitations of light and phosphorus supply influenced dry matter allocation to leaf and root in opposite directions.
4) Burial of already formed stolons had no influence on initiation of axillary bud outgrowth at nodes on the primary stolon but decreased outgrowth of buds on secondary stolons. This depressive effect in secondary stolons could be explained by the pattern of apical dominance in small branches and the ontogeny of axillary buds. Where axillary buds were on stolon tissue that emerged in soil
but was then exposed to light (after burial for a maximum of seven plastochrons) initiation of axillary bud outgrowth was significantly reduced (by 17%) although if burial continued for a further five plastochrons the reduction was 64%. Excision of the stolon apex, after burial for a period of seven plastochrons induced a probability of axillary bud outgrowth similar to that obtained by exposure to light at this time even though buds remained buried and received no light stimulus. These results indicate that burial of stolon decreased outgrowth of axillary buds by inhibition rather than by decreasing the viability of buds. Responses of stolon systems to burial were consistent with intraplant allocation patterns of carbon based on source-sink relationships.
5) Diurnal variation in contents of starch and hexose and sucrose sugars in stolons was not significant. Contents of hexose sugars and sucrose varied little with season or in response to burial or defoliation treatments. Starch contents varied five-fold with season (minimal and maximal contents in late spring and late summer respectively), ten-fold in response to defoliation but were unaffected by burial. Following defoliation starch contents in stolon were depleted preferentially from internodes immediately proximal to the apex and depletion progressed basipetally along the stolon. Starch contents were minimal about 1 5 days after defoliation. Subsequent repletion also occurred initially at internodes immediately proximal to the apex and progressed basipetally.
Starch, sucrose and hexose contents of stolon internode all varied significantly with position within the stolon. No significant correlations were found between any of the carbohydrate fractions in the internode basipetal to an axillary bud and initiation of outgrowth of the bud at any node position.
6) Results of this study were consistent with an hypothesis that in a stolon with a functioning apex there is a 'window of opportunity' for initiation of outgrowth of axillary buds which spans the interval of bud ontogeny from its release from inhibition by apical dominance until the bud is positioned more than eight nodes from the apex.
7) Expression of apical dominance (number of nodes from apex to first node position with axillary bud outgrowth) is determined by abundance of resource at the stolon apex. This is a function of the microenvironments of all metamers
contributing resource to the apex and this may include metamers from other stolons. Apical dominance is therefore a characteristic of a stolon but is influenced by all metamers that supply resource to the stolon. It is independent of other attributes such as node appearance rate and probability of successful branch establishment at metamers within the stolon.
8) It was hypothesised that in white clover initiation of axillary bud outgrowth occurred unless inhibitory processes (apical dominance, phytochrome mediated processes or spring dormancy) were activated to prevent initiation. All reported responses of initiation of branching to treatments could be interpreted within the confines of this model which indicated that initiation of bud outgrowth has no prerequisite for light stimulation.
9)
It was suggested that in spring increased demand for carbohydrate initiated by greater growth and nitrogen fixation rates in roots and increased production of leaf tissue initiated changes in intraplant allocation patterns of carbon which resulted in the observed rapid decrease in starch content in stolon in October. This was accompanied by an acceleration in death of older, basal stolon in October which fragmented larger plants, so altering the plant population by increasing the proportion of plants of less complex branching structure and decreasing mean plant dry weight.1 0) Following plant fragmentation in October, the physiological status of the
population (decreased mean plant DW and carbohydrate reserves) was
considered to have agronomic consequences in that it reduced ability to tolerate stress (environmental or managerial) and so increased the probability of large decreases in population density upon occurrence of adverse conditions. This period of increased vulnerability was considered to extend until January by
which time mean plant DW and carbohydrate content had usually recovered to
pre-spring values.
1 1 ) In regard to the population dynamics of white clover in grazed pastures the consequences of the effects of stolon burial on branch establishment in winter were not considered great because;
(a) several physiological and biotic factors act to significantly limit the branching potential of white clover in spring which means that the effect of burial of stolon is just one (physical) factor acting in the same direction as others.
(b) low node appearance rates over winter, when rates of stolon burial are greatest, acts to limit the formation of new nodes at this time and so reduces the number of axillary buds that are most susceptible to negative effects of burial. The most susceptible buds are those that emerge in and remain buried with soil.
(c) in the field, burial of stolon induces a negatively geotropic response that results in stolon curvature and a rapid return of the apex to the soil surface which in turn limits numbers of nodes that emerge in and remain buried with soil.
(d) other environmental and managerial regimes in spring usually have a major influence in determining the productive potential of the white clover population for the growing season; but burial in summer could have greater impact as node appearance rates are high which means that the reduction in branch survival on primary stolons coupled with a decrease in outgrowth of axillary buds on secondary stolons will lower total numbers of branches and nodes produced.