Studies in the promotion of precocity in 'Doyenne du Comice' pear : a thesis presented in partial fulfilment of the requirements for the degree of Master of Horticultural Science at Massey University, New Zealand
Flower evocation is the process, generally hormonally controlled, that occurs before the vegetative bud apex changes from the differentiation of leaves to the differentiation of floral structures, and at that stage the bud has begun the reproductive cycle.
Four treatments were chosen that had been shown in the literature to stimulate flower evocation and differentiation in pipfruit, and were applied in an experiment on 'Doyenne du Cornice' pear trees. These treatments were applied to experimental potted trees during the 1990 - 1991 growing season, and consisted of the application of one of the following: nitrogen fertiliser in the form of ammonium sulphate, subtoxic levels of simazine herbicide, plant growth regulator paclobutrazol or a period of regulated deficit irrigation (R.D.I.). A Control treatment was also monitored
The flower clusters were monitored in the spring of 1991 and all treatments had increased flower clusters per centimetre of wood compared to the Control, however only the Paclobutrazol and the
R.D.I. treatments increased flower clusters significantly (P <
0.01). Trunk diameter and shoot extension growth were both reduced although only the former was significantly reduced by the Paclobutrazol treatment (P < 0.01).
The total free nitrogen levels were monitored in the leaves and the buds of the experimental 'Doyenne du Cornice' at various harvest dates during the season, which included assessment of ammonium, nitrate, arginine and total amino acids. There were no clear seasonal trends among the treatments in the levels of any individual nitrogenous components or the total free nitrogen levels.
The R.D.I. treatment reduced the photosynthetic rate during its application period to a maximum significance of P < 0.01 just prior to the reinstatement of full irrigation. The water deficit imposed significantly reduced the xylem water potential for a period of 50 days although no statistically significant differences in water content of the growing medium was demonstrated.
Examination of the bud apex with a Scanning Electron Microscope (SEM) during the development of floral structures was made during the season. This linked with a defoliation study done on spur buds on mature trees in an orchard near Wanganui which showed that defoliation before 4.12.90 significantly reduced bloom and defoliation after 11.2.91 had no effect. Evocation occurred between 14.12.90 and 8.1.91 as shown by the SEM. Fruitlet retention was also significantly affected by defoliation and the presence or absence of the bourse shoot.
The ability of the spur bud to produce flowers depends on its position in the canopy of a mature 'Doyenne du Cornice' tree and a separate study showed that positions in the tree that had low PPFD had reduced flower numbers. A further study showed tying branch angle down from the vertical during winter is beneficial in terms of increased flower formation and reduced vegetative growth.