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Photosynthetic water-use efficiency of leaves (WUE"1) and canopy (WUE"c) of sweet sorghum (Sorghum bicolor [L.] Moench) was investigated to gain insight into the crop ability to optimize water use and to provide explanations for the high values of biomass WUE reported in the literature. The Keller cultivar was grown for two consecutive years (1993 and 1994) in open field in southern Italy. Two consecutive water stress cycles were imposed on the crop in each year. In the investigation, emphasis was placed on the leaf scale during the first year, and on the canopy scale during the second year. Leaf area index (LAI), predawn leaf water potential (@j"b), leaf and canopy gas-exchange by a portable photosynthesis system and by a Bowen-ratio/energy-balance method were the major measurements carried out in the field. The variation in soil-water status, estimated by @j"b, ranged from -0.1 to -0.8 MPa in 1993 and from -0.2 to -1.1 MPa in 1994. The investigation at the leaf scale revealed that the ratio of leaf internal CO"2 partial pressure (c"i) to ambient CO"2 partial pressure (c"a) was maintained generally within a range 0.2-0.27, with the leaf conductance ranging from 0.08 to 0.35 mol m^-^2 s^-^1; and was about constant (average = 0.24) over a range of leaf-to-air vapor pressure difference (VPD"1) between 2.3 and 5.8 MPa. Due to this fairly stable c"i, WUE"1 was inversely proportional to VPD"1 and its value, normalized for VPD"1, was constant at 11.8 @mmol "C"O"""2mmol ^-^1"H"""2"O kPa. In terms of gas-exchange responses, a parallel behavior was observed at both the leaf and canopy scale. This was particularly evident when comparing diurnal trends of WUE"1 and WUE"c. The resulting value of WUE"c, normalized for the evaporative demand of the atmosphere (E"o), was 1.56mol "C"O"""2 m^-^2. While WUE"1 of sweet sorghum compared well with that of other C"4 crops, values at the canopy scale were higher than other C"4 crops, especially maize and grain sorghum, and were consistent with the high biomass WUE observed in the field by other authors. A possible explanation for a high WUE may be found in the differences in dark respiration of the whole canopy between sweet sorghum and other C"4 crops.

L'action d'une contrainte hydrique intervenant ? diff?rents moments du cycle v?g?tatif du poivron a ?t? ?tudi?e en serre sur des plantes cultiv?es dans des conditions climatiques stables. La m?thode retenue pour provoquer la contrainte hydrique est celle de Robelin, elle consiste ? appliquer un coefficient de s?cheresse identique (rapport des transpirations des plantes en cours de dess?chement et des transpirations maximales des m?mes plantes bien irrigu?es) pendant un certain temps et une certaine p?riode de v?g?tation. Cette m?thode a ?t? appliqu?e sur 4 stades ph?nologiques : v?g?tatif, d?but de floraison, d?but de nouaison et formation des fruits. L'analyse th?orique du coefficient de s?cheresse a montr? que les conditions climatiques (temp?rature, vitesse du vent) et la surface foliaire sont des ?l?ments intervenant dans l'intensit? de la contrainte hydrique provoqu?e lors de l'application de ce coefficient pour des courtes dur?es (heure, jour). Dans nos conditions exp?rimentales (conditions climatiques stables, coefficient de s?cheresse appliqu? ? l'?chelle d?cadaire), nous avons not? ? partir des mesures directes de l'?tat hydrique de la plante, que la contrainte hydrique est d'intensit? assez voisine au cours des stades ph?nologiques ayant une surface foliaire proche (stades floraison, nouaison et formation des fruits); en revanche, l'intensit? de la contrainte hydrique est nettement inf?rieure lorsque la surface foliaire est faible (stade v?g?tatif). Ces observations ont ?t? ? l'origine de quelques suggestions permettant de mieux adapter la m?thode de Robelin ? l'objectif recherch?. L'?volution de la sensibilit? des stades ph?nologiques soumis ? une contrainte hydrique identique a ?t? obtenue en comparant 4 composantes du rendement, analys?es ? l'?chelle de la plante (poids frais et sec, la taille, et le nombre des fruits) par rapport ? un t?moin bien irrigu?. La sensibilit? est plus accus?e au stade de nouaison qu'aux stades de floraison et de formation des fruits. Ces observations peuvent avoir des incidences sur la gestion de l'irrigation du poivron. Consequences of water stress applied at different growth stages in the production of pepper plants. The effects of water stress occuring at various periods during the growth cycle of peppers was studied in greenhouse-grown plants under stable climatic conditions (table II). The method used to provoke water stress was the Robelin method, which consists of applying an identical dryness coefficient (ratio of plant transpiration during drying-out phase to maximum transpiration of the same well-irrigated plants) for a certain time period and during a certain vegetative phase. This method was applied to 4 phenological stages: vegetative growth, early flowering, early fruit setting, and fruit formation (table I, fig 1). A theoretical analysis of the dryness coefficient shows that climatic conditions (temperature, wind speed) and leaf surface are elements affecting the intensity of the water stress provoked when this coefficient is applied for short time periods (h, d). Under the present experimental conditions (stable climatic conditions, dryness coefficient applied every 10 d) and using direct measurements of plant water status, the authors note that water stress is of comparable intensity during phenological stages with high leaf surfaces (flowering, early fruit setting, and fruit formation stages) and that, on the other hand, water stress intensity is markedly reduced when leaf surface is low (fig 2). These observations are the basis for certain suggestions aiming at a better adaptation of the Robelin method to research objectives. An evaluation of the sensitivity of phenological stages submitted to identical water stress was obtained by comparing (fig 3, table IV) 4 yield components analyzed on a plant scale (bulk and dry weight, fruit size and number) to those of a well-irrigated plant (table III). The sensitivity is greater at the early fruit setting stage than at the flowering or fruit formation stages. These observations could have an effect on methods for care and irrigation of peppers.