The physiological and genetic control of vegetative phase change in Pisum sativum L

Plants develop through at least three distinct phases after germination: a juvenile vegetative phase; an adult vegetative phase; and a reproductive phase. Therefore, there exist two main ontogenetic transitions, the juvenile-adult vegetative transition or vegetative phase change, and the adult vegetative-reproductive phase transition or reproductive phase change. The genetic and physiological control of the transition to flowering has been studied intensively over the past four decades but the other major transition has received much less attention. This study investigates the genetic and physiological regulation of vegetative phase change in the garden pea (Pisum sativum L) using a novel heterochronic mutant, accelerated phase change (apc), which expresses a consistent difference in the rate of vegetative phase change when compared to the wild type plants. Genetic analyses, using morphological and enzyme markers, revealed that the apc locus is on linkage group 131 (between a and aat-p genes) of the garden pea. When the apc and the WT plants were grown under 8 h and 18 h photoperiods to determine the effect of environmental cues on vegetative phase change, the results demonstrated that not only is the timing of vegetative phase change affected by environment but the vegetative and reproductive phase changes are regulated independently. Using isogenic lines differing only in the apc mutation, the nature of changes in the morphology of the shoot apical meristem (SAM) associated with the vegetative phase change was examined. The dimensions of the SAM (width, height and volume) were measured without fixation using an environmental scanning electron microscope. The materials were harvested at a critical time (11 days of age) when the vegetative phase change was being initiated in the SAM of the apc mutant but not the WT. The study found that the dimensions of SAM are larger in the apc mutant than in the WT plant suggesting that the vegetative phase change to the more complex leaf type is controlled genetically via changes in SAM morphology. Reciprocal grafting techniques showed that vegetative phase change was not significantly altered in the apc scion by the WT stocks and vice versa. This indicated that the site of action of the apc mutation is in the shoot system. To determine whether vegetative phase change involved changes in endogenous hormones in the shoot apex, the levels of GA\(_1\), GA\(_{20}\), IAA and ABA were quantified using GC-MS-SIM. The plant materials were harvested at 11 days after sowing, at the critical time that vegetative phase change was being initiated in the apc SAM, but not in the WT plants. Differential changes in the hormone levels between the apc and WT SAM were sampled by excising only the 2mm long shoot apices. The results showed no significant difference in GA\(_1\), GA\(_{20}\) and IAA levels between the apc mutant and the WT, which indicates that these hormones are not involved in the vegetative phase change in the garden pea. Although, a significant difference in ABA levels was detected in the SAM of the WT and the apc plants, ABA does not appear to be a crucial hormone in vegetative phase change. Further work showed no significant differences in the rate of vegetative phase change between a prominent ABA-deficient mutant (wi/) and the WT (Wit). The ABA-deficiency of the apc mutant does, however, appear to have a small effect on drought response, with the apc and the WT plants showing differences in transpiration rate and stomatal conductance