A SIMULATION-MODEL OF KENAF FOR ASSISTING FIBER INDUSTRY PLANNING IN NORTHERN AUSTRALIA .2. LEAF-AREA DEVELOPMENT

Leaf area development is an important determinant of the proportion of the incident radiation intercepted and consequently of stem yield for paper pulp production in kenaf (Hibiscus cannabinus L.). Relationships were developed describing the component processes of node production, leaf area per node and leaf area senescence, to predict green leaf area of non-branching plants of kenaf during the period from sowing to flowering. Data were collected on cultivar Guatemala 4 grown under irrigation at a range of plant population densities at three sites in tropical Australia. The thermal time required for production of new nodes remained constant from sowing to flowering at 36.3-degrees-C days node-1 at a population of 50 plants m-2. Leaf area per node was described quantitatively in terms of three phases where, as node number increased, leaf area per node at first increased linearly, reached a plateau and finally declined linearly. The rate at which the leaf area at old nodes was senesced was found to be equivalent to the rate of production of new nodes, such that the number of nodes bearing green leaves was relatively constant; at a population of 50 plants m-2 there was green leaf area at 7 nodes plant-1 over time. Empirical functions describing the effects of plant population on rates of node production and senescence and on maximum leaf area per node were developed. In limited testing, these functions predicted leaf area development of crops of kenaf grown under optimal conditions. The functions developed here, when combined with others in this series of papers, provide algorithms to be used in the development of a crop simulation model of kenaf for use in the semi-arid tropics of northern Australia.