LEAF-AREA DEVELOPMENT IN BARLEY-MODEL CONSTRUCTION AND RESPONSE TO SOIL-MOISTURE STATUS

A model to simulate leaf area development for barley at the whole plan t level was constructed. Data for leaf area development in the absence of soil water stress were collected from irrigated field trials grown at Hermitage Research Station, near Warwick, Queensland, in 1990. The response of leaf area expansion to soil water status was measured in a glasshouse trial. In the model, green leaf area per plant (GPLA) is derived as the difference between total leaf area produced per plant ( TPLA) and senesced leaf area (SPLA). TPLA and SPLA are described by lo gistic functions of thermal time. Two types of senescence are included : that due to ageing of the whole plant (ontogenetic senescence) and s enescence associated with the development of large canopies (light-ind uced senescence). The onset of ontogenetic senescence is linked to ant hesis, whereas light-induced senescence occurs if the leaf area index of the crop exceeds 55. Leaf expansion of plants in pots varying in th e fraction of transpirable soil water available (FTSW) was compared wi th leaf expansion of those in well-watered pots three times per week. The relationship between relative leaf expansion (RLE) and FTSW was de scribed by a logistic function (r(2) = 0.96). A 50% reduction in RLE o ccurred when FTSW = 0.34. Similarly, a logistic function described the relationship between relative transpiration (RT) and FTSW (r(2) = 0.9 6). A 50% reduction in RT occurred when FTSW = 0.17. Potential leaf ex pansion as predicted by the non-stressed model was reduced in response to moisture stress via a ramp function relating RLE to RT. The model gave an unbiased prediction of the leaf area dynamics for 21 rainfed a nd irrigated crops of barley grown in southern Queensland between 1986 and 1993 (RMSD = 1.09 m(2) m(-2) r(2) = 0.75, n = 76). Precision may have been reduced by the lack of information available on parameters f or soil water balance when barley is grown on a range of soil types.