Some aspects of the ecophysiology of Paulownia spp.

Abstract

Members of the Paulownia genus have recently received widespread attention in New Zealand for their potential as special purpose timber trees and application in agroforestry systems. There has been only limited prior investigation of the ecophysiology of Paulownia. This study aimed to quantify aspects of gas exchange; biomass, nutrient and carbohydrate partitioning; and propagation and seedling establishment techniques in field grown and pot grown Paulownia trees, in Hamilton, New Zealand. Gas exchange measurements were undertaken using either a closed system LICOR 6200 unit or an open system Walz CMS 400 unit. Rates of gas exchange of Paulownia leaves under saturating light were high, with Aₘₐₓ in fully mature, non-senescent leaves ranging from 12 μmol m⁻² s⁻¹ to 25 μmol m⁻² s⁻¹, with gₛ between 0.2 mol m⁻² s and 0.8 mol m⁻² s⁻¹. The relationship between A and gₛ was variable which resulted in Cᵢ ranging from 190 μl 1⁻¹ to 280 μl 1⁻¹ under ambient CO₂ concentrations. High rates of transpiration were maintained in the large leaves to facilitate evaporative cooling under high radiation load. Leaf water potentials remained high throughout the day and, on the occasions when a midday depression of gas exchange rates was found, there was little evidence that it was a response to water stress. Circumstantial evidence suggested some form of negative feedback was involved in decreasing gas exchange rates in during the latter part of the day. Measurement of the ACᵢ response found low levels of stomatal limitation of photosynthesis, with l typically between 12% and 30%. The plants exhibited a number of characteristics of obligate sun plants, with the PPFDₛₐₜ of the leaves generally above 1000 μmol m⁻² s⁻¹. The pattern of biomass allocation, with a large proportion of the perennial tissues present as roots and the large investment in foliage, particularly in the establishment phase, was another feature typical of sun plants and enabled the plant to maintain a high growth rate. Growth rates were enhanced by the regular application of liquid fertilizer and frequent watering, and were higher in the warmer conditions of a glasshouse environment than outside. However, the temperature regime during leaf development had no effect on values of Aₘₐₓ. The temperature regime during growth did not affect the allocation of nutrients. In the field grown trees, the highest concentrations of all nutrients, except zinc, were found in the annual tissues, with foliar levels of nutrients in Paulownia similar to those reported for other timber species. Nitrogen levels in fully mature Paulownia leaves were generally between 2% and 2.5%, and within this range there was little correlation between nitrogen content and Aₘₐₓ. Nutrient levels in perennial tissues showed little change throughout each season, although nitrogen levels did increase in woody tissues at the end of each season. The levels of soluble sugars were high in all tissues of the Paulownia trees studied, in comparison with those found in other trees. The perennial root material stored large amounts of soluble sugars over the dormant season, with an average of 30% of the dry weight of the lateral roots found as soluble sugars at the last harvest of each season. The high soluble sugar content of these roots probably facilitated the growth of new root and shoot material when root cuttings were taken. The position of the root cutting in the parent root system had no effect on the regeneration capacity of the cutting. Cuttings with diameters less than 5 mm were less likely to regenerate than larger cuttings. Paulownia seedlings were found to suffer from transplant shock and generally grew more vigorously after transplanting if they were coppiced, rather than planted intact. Thus it was concluded that high rates of gas exchange and growth could be achieved by Paulownia trees if grown under conditions of high light, consistently warm temperatures, with frequent watering and the application of high rates of nutrients.