No abstract

Rates of photosynthesis at five light intensities and rates of respiration at fot:r temperatures, 9°, 16°, 24° and 30°e, were measured in three P. radiata seedling~ from each of the growing conditions IS°/10°, 24°/l9°, and 33°/28°c dly/night temperatures. Net photosynthesis at 30° and 9°c was reduced in seedlings grown u!1der the cold and the two warmer. preconditioning treatments respectively. ~Ixteen ~egrees appeared to be the optImum temnerature for net photosynthesis m seedlmgs grown at Iso/lO· and 24°/l9°e, with 24°c being the optimum fol' the plants raised at 33·/28·e.Photosynt~esis per seedling .in~reased by almost 35% l!1'!d rates. of respimtion were app!oxI~ately halved wlthm two days of the seedhngs bel\';g transferred from 15 /10 to 33 ° /28°c. In that transfer (and also the reverse of it i e 33°/28°. to ISO/10°c in which photosynthesis dropped by some 25%) most of th~ adaptation of each seedling occurred within two days, and within a few days more the ~eedlings were similar in behaviour to seedlings which had becn raised entirely under the changed temperature conditions.

Juvenile seedlings, tnicropropagated plantlets, and adult rooted cuttings of Pinus radiata, together with seedlings of Agathis australis and Dacrvdium cupres.sitium, were grown under either high (670/nnoltn's') or low (200/nnol tn's ') photosynthetic photon flux density (PPFD) and, at low PPFD, under three red:far-red (R/FR) conditions, approxitnately simulating canopy shadelight, daylight and one intertnediate value. In all types of P. radiata, a shade-intolerant species, stetn height and diatneter, stetn and needle dry weight, and apical dotninance wete tnarkedly increased by a reduction in R:FR ratio while fascicle density was decreased. In eonttast, these increases were considerably less for the shade-tolerant D. eupres.simitn and tnoderately tolerant A. australis. With the exception of the P. radiata seedlings, height growth was gteatest in all speeies under high compared to low PPFD for daylight R:FR tatio conditions. Total shoot (or plant) dty weight was increased approximately two-fold in all species and types by the higher PPFD. Shoot extension tates were negatively cortelated with calculated phytochrotne photoequilibriutn with P. radiata, the n-iost shade-intoletant species, having the highest change in tate per unit change in photoequilibriutn (i.e. very responsive), and D. eupressinum, the tnost shade toletant species evaluated, having the stnallest change (i.e. largely untesponsive). Within the specttal quality treattnents at low PPFD, it is suggested that higher rates of dry tnatter aecumulation under the low R:FR ratio were the result of reduced mutual shading of adjacent leaves as a consequence of pholotnorphogenically-conttoiled internode lengths rather than of enhanced photosynthesis per .sc. The significance of the tesults is diseussed in relation to planting stock tnanagetnent in nurseries, the management of forest canopies for understotey seedling growth, and to the construction of representative growth sitnulation tnodels. Consequences for controlled environment lighting are also cotisidered.

No abstract

The gaseous exchange characteristics of two clones of P. radiata were compared. Clone 457, which had a low survival rate in the field, showed a transpiration rate up to twice that of clone 456, which had a high survival rate. Transpiration in both clones was not markedly affected by leaf-air vapour pressure differences varying between 0.4 and 1.4 kPa. The stomatal resistance of clone 457 was almost half that measured for clone 456 under all experimental conditions; however, increasing the leaf-air vapour pressure difference resulted in increased stomatal resistances in both clones. The photosynthetic response to CO2 concentration was also determined. The measured rates for clone 457 were twice those of clone 456 and both clones gave maximum rates of apparent photosynthesis above about 700 ng cm-3 internal CO2 concentration (corresponding to about 1400 ng cm-3 external concentration). The differences in photosynthetic rate could be accounted for by the greater than twofold difference in mesophyll resistance between the clones. Mesophyll resistance was not affected by altering the leaf-air vapour pressure difference. It is suggested that a factor in determining the relative survival rate of the two clones is the greater ability of clone 456 to limit transpiration water loss and that this more than compensates for the reduced rate of photosynthesis of this clone compared with clone 457.