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Boronia is a native plant with potential as a new essential oil crop. Various growth and metabolic responses of boronia to N were investigated to improve N nutrition of boronia. When N was supplied to boronia at 50 or 100 kg ha-1 , two fast release N sources (ammonium sulfate and calcium nitrate) caused toxicity while a slow-release N source (IBDU) did not cause toxicity. When N was supplied in a single dose in October during the active vegetative growth phase, highest flower yield was obtained and when the same amount of N was supplied in split doses at different phases of plant growth, the yield decreased. Increasing N levels from 0 to 25 mM in the nutrient solution increased the plant's production of nodes, lateral shoots from these nodes and further nodes on these lateral shoots. With the same level of N, production of nodes and lateral shoots was in the order: NH4+ +NO3- > NH4+ > NO3. The increase in the number of nodes subsequently translated into increased number of axils initiating flower buds and then into fully developed flowers. However, increasing N levels decreased the percentage of total flower buds that developed to anthesis and the individual flower weight. Increasing N levels increased the leaf N concentration, with the concentration in the order: NH4+ +NO3- > NH4+ > NO3- . N form did not affect the leaf tissue concentrations of P, K, Ca and Mg. At lower N levels, the concentrations of these nutrients in the leaf tissue were higher and may have reached toxic levels and caused the toxicity symptoms on the leaves. At higher N levels, the concentrations of these nutrients were diluted due to increased growth and no toxicity symptoms were produced. At decontrolled as well as controlled pH (4.5 and 6.5) of the nutrient solution, uptake of NH4+ -N by the plant was higher than that of NO3- -N. With NH4+ nutrition, concentrations of amides (namely, asparagine and glutamine) in plant tissues were higher than with NO3- nutrition. When a low NO3- level (15 mmol per plant) was given, NO 3- entering the plant was reduced without any accumulation and without nitrate reductase (NR) activity reaching its maximum capacity. When higher NO3- levels (>-25 mmol per plant) were given, NR activity increased to a maximum of only ca. 500 nmol NO2 - g-1 fresh weight h-1 both in the roots and leaves irrespective of 6-fold difference in the NO3- supply while NO3- continued to accumulate in proportion to the level of NO3- supplied. Consequently, high levels of NO3- accumulated in the plant tissue and at ca. 32 umol NO3- g-1 fresh weight, toxicity symptoms appeared on the leaves. The low level of NR in boronia was not due to limited NO3- or electron donor availability, but it seems to be genetically tuned to slow growth in low NO3- producing native soils. These responses of boronia to N are discussed in terms of the plant's adaptations to survive in native soils and their manipulation in commercial cultivation of boronia. |
