Thresholds for chickpea leaf expansion and transpiration response to soil water de®cit A. Soltani a,* , F.R. Khooie b , K. Ghassemi-Golezani b , M. Moghaddam b a Department of Agronomy, Gorgan University of Agricultural Sciences, Gorgan, Iran b Department of Agronomy, Tabriz University, Tabriz, Iran Received 15 May 2000; received in revised form 10 August 2000; accepted 11 August 2000 Abstract This study determined the thresholds (the onset of decline) for leaf expansion (C L ) and transpiration (C T ) responses to soil water de®cit in chickpea (Cicer arietinum L.) and evaluated their importance to crop performance under rainfed conditions using a chickpea simulation model in two contrasting locations. Two pot experiments were conducted to determine the thresholds during adrying cycle, when plant responses were monitored as the soil dried progressively. Leaf area expansion and transpiration did not change until the fraction of transpirable soil water (FTSW) reached 0.48 (C L ) and 0.34 (C T ), respectively, and then decreased linearly to FTSW of 0.03 for leaf expansion and 0 for transpiration. Simulation results of sensivity test showed that for long and intensive terminal droughts, high C L and C T and for short and intermittent droughts, low C L and C T favor grain yield but the yield increases with best combinations of C L and C T do not exceed 1% (20 kg ha 1 ) and 5% (40 kg ha 1 ), respectively. Thus, the contribution of C L and C T differences in crop performance under water de®cit conditions seems limited. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Leaf expansion; Transpiration; Water de®cit; Chickpea; Simulation 1. Introduction Water de®cit is a major constraint to crop produc- tion and crops are usually exposed to drought periods of varying duration and intensity (Sadras and Milroy, 1996). Plant processes that depend on increases in cell volume are particularly sensitive to water de®cits. Leaf expansion and leaf gas exchange rates are two such sensitive processes that depend on guard cell volume and cell expansion, respectively (Lecoeur and Sinclair, 1996). Modeling plant responses to water de®cit requires not only an understanding of, but also quantitative relationships for the effects of water de®cits on leaf growth expansion and gas exchange rates (Sadras and Milroy, 1996). Ritchie (1981) suggested that physiological pro- cesses such as photosynthesis, transpiration or leaf growth would show similar responses across a wide range of environmental conditions when compared on the basis of the fraction of total extractable water in the root zone. The proposal of Ritchie and its re®nement by Sinclair and Ludlow (1986) has provided consistent descriptions of responses of various physiological processes to soil water de®cit. In this scheme, soil water status is characterized by the fraction of tran- spirable soil water (FTSW) remaining in the soil. Total transpirable soil water is de®ned as the difference between the soil water content at ®eld or pot capacity, and the soil water content when transpiration of the Field Crops Research 68 (2000) 205±210 * Corresponding author. Fax: 98-171-2220981/438. E-mail address: asoltani_99@yahoo.com (A. Soltani). 0378-4290/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII:S0378-4290(00)00122-2