Does irrigation influence the growth, yield and water use efficiency of the protea hybrid 'Sylvia' (Protea susannae X Protea eximia)?

The response of the protea hybrid 'Sylvia' (Protea susannae X Protea eximia) to irrigation was investigated to determine the water requirements of this commercial floriculture crop. Both field and glasshouse trials were conducted in which plants were exposed to different irrigation levels. The effects of the irrigation regimes were determined by measuring the delta(13)C, delta(15)N, gas exchange characteristics, biomass accumulation, yield and root development. The plants in the glasshouse were exposed to watering regimes maintaining water at 20%, 40% and 60% of field capacity in sand-filled pots. Higher water supply resulted in increased growth of the roots in particular, but also of the shoots, reaching a maximum at 40% of field capacity. There were no significant changes in the gas exchange characteristics of the plants associated with this increased growth. In the field trial the intensity of irrigation (dry-land, normal and double-irrigation) did not influence the vegetative or reproductive biomass accumulation. Although the water content of the upper 30cm of soil was significantly different between treatments, there were no differences in delta(13)C, relative water contents or in elemental compositions of the leaves. The gas exchange characteristics of the plants were only slightly influenced by the irrigation intensity maintaining very similar water use efficiencies between treatments, concurring with 813 C results. However, the development of cluster roots was strongly increased by irrigation. Dry-land plants had extensive sinker roots penetrating the deeper (>1m) soil layers. It was concluded that the dimorphic and deep roots of 'Sylvia' allow access to water deep in the soil profile once the root system is established. Although maintenance of cluster roots for nutrient uptake is important, management practices should take cognisance of the fact that surface soil water contents only partially determine the availability of water for these deep-rooted perennial shrubs, if at all.