THE WINTER HARDENING AND FOLIAR FROST-RESISTANCE OF SOME NEW-ZEALAND SPECIES OF PITTOSPORUM

The foliar frost resistance of Pittosporum eugenioides, P. obcordatum, P. tenuifolium, and P. crassifolium increased from -6 degrees C to -8 degrees C in March and April to between -9 degrees C and -10 degrees C in July. Leaves of P. obcordatum had the lowest frost resistance (-7 degrees C) in April, but showed the highest resistance (<--10 degrees C) in July. The foliar frost resistance of P. eugenioides is greater than that previously recorded, similar to that of P. crassifolium and P. tenuifolium, and is consistent with the widespread distribution of this species within New Zealand. The relatively high frost resistance of leaves of P. obcordatum is in accord with its occurrence in frost prone habitats. Seedlings of P. eugenioides subjected to short days and night-time temperatures around 0 degrees C became about 1 degrees C more frost resistant than untreated seedlings in autumn (April and May). The maximum frost resistance (-11 degrees C) attained by treated seedlings of this species was no greater than that of untreated seedlings, but was 2 degrees C greater than that of adult plants in the field. The youngest and oldest leaves of seedlings were less frost resistant than leaves of intermediate age.