After transferring my lemon plant to another sunny spot, hoping it would respond well with sunlight, the whole plant started to shed its leaves leaving only the flowering buds. What should I do?
Generally, citrus trees do not like being transplanted. Physically they shed their leaves quite quickly and often die as too many roots are cut or they have dried out at any time after being shifted. Always remember that in Christchurch, like many colder parts of NZ, it’s marginal to successfully grow citrus trees. Not only frost but also very cold, wet soils are quite foreign to citrus trees.
From your description, we would assume the orginal planting site was not perfect for a lemon tree. It was possibly a little shady or probably too wet in the winter months. Unfortunately, now with no leaves on the plant, it can no longer photosynthesise and therefore sadly, it will probably die over the next few months.
If you decide to plant another citrus tree and cannot find the perfect spot in your garden e.g well drained soil, full sun and protected from cold winter winds, consider growing it in a large container. This can be moved for protection in winter and to take advantage of sun during the summer months.