Understanding Functional Relationships Affecting Growth and Quality of Field Grown Leaf Lettuce in the Greenbelt of Buenos Aires, Argentina

Earlier studies in the horticultural production area around Buenos Aires (Argentina) indicated that field grown leaf lettuce fertilised with 150 kg N ha accumulated NO3-N in the leaves up to concentrations well above the reference limits adopted by European countries. Previous studies also showed that the planning of sowing dates and a sound management may help overcome nutritional quality problems, reduce risks of environmental pollution and increase the efficiency of the system. As an aid to crop planning, a management decision-tool is being developed following a modelling approach to predict growth and quality (i.e. NO3 content) of field-grown lettuce in the area. The model was conceptually based on that of Seginer et al. (1997) for greenhouse lettuce, in which a negative correlation between C assimilates and NO3 concentrations in vacuole is used. The first step in the development of the model was to asses the simplest (and satisfactorily accurate) approach to model growth of a leaf lettuce crop under field conditions relying on few, easily available parameters, and then to identify source-limited conditions along the year. The model was calibrated with data from several experiments in the area. The model predicted crop (R = 0.97) and plant (R = 0.98) growth quite accurately when compared with independent data. A first analysis revealed that since leaf lettuce does not need to form a “head” to be commercially mature, growth can be fairly described by a simple exponential model. NO3 contents varied widely (from 990 to 7590 ppm) under different growing conditions, suggesting that there is room to model the interaction of factors creating source-limited conditions that eventually lead to NO3 accumulation in leaves. INTRODUCTION Lettuce (Lactuca sativa L.) is a major crop in the horticultural ‘belts’ round the main urban centres of Argentina. Due to a sustained demand for fresh food items and to relatively mild winter conditions, fast-growing, leaf lettuce cultivars are grown all-yearround under open field conditions. To achieve marketable plants in a short period (ca. 30 days in spring-summer), lettuce crops are initiated by transplanting, well irrigated and heavily fertilised with different N sources. The efficiency and the consequences of such a system have been strongly questioned, as concerns on food quality, environmental health and agricultural sustainability arose. Earlier studies in the area indicated that winter-grown leaf lettuce (cv. Grand Rapids) fertilised with 150 kg N ha accumulated NO3-N in the leaves up to concentrations well above the reference limits adopted by European countries (Chiesa, 2002). However, environmental and management conditions such as temperature, radiation and plant density affected the relationships between crop growth and NO3-N concentration in leaves, when different levels of N fertilisation (from 0 to 150 kg ha) where tested. Planning of sowing dates and sound adjustments of plant density and N Proc. 3 IS on Model IT Eds.: M.L.A.T.M. Hertog and B.M. Nicolaï Acta Hort. 674, ISHS 2005 368 fertilisation may help overcome quality problems, reduce risks of environmental pollution and increase the efficiency of the system, aiming also at avoiding the frequent low prices due to intermittent oversupply of lettuce in the market. A decision-tool is being developed following a modelling approach to predict growth and quality (i.e. NO3 content) of field-grown lettuce in the area, as an aid for crop management planning. The model was conceptually based on that of Seginer et al. (1997) for greenhouse lettuce, in which a negative correlation between C assimilates and NO3 concentrations in vacuole is used. Thus, NO3 accumulation occurs under source-limited conditions (i.e. winter), when C demands for growth and maintenance are not covered by C assimilation. This article reports the first steps in the development of a simple model for leaf lettuce, for which functional relationships affecting growth rates and NO3 concentration are studied, as affected by environmental and management factors. MATERIALS AND METHODS The greenbelt of Buenos Aires (GBA) is representative of other vegetable production areas surrounding densely populated urbanisations, with climatic conditions of temperate to subtropical environments (Fig. 1). Crop growth is likely limited by radiation rather than by temperature during the mild, rainy winters; during summer, temperatures are high, shortening the growing period and affecting lettuce quality. Dry weight and leaf area data from experiments in the region was used to calibrate a simple model to mathematically describe growth and predict growing periods up to harvest maturity. Experimental factors included: sowing date, plant density, shading and N fertilisation (Table 1). The model approaches the functional source-sink relationships at crop level, by first analysing growth regulation by environmental (E) and management (M) factors, and statistically relating that to measured NO3 levels and to the N balance at crop level. A growth analysis (Hunt, 1982) was performed using the experimental data. Variables and parameters from a single experiment where subjected to analysis of variance, and regression and correlation analysis were performed with GenStat 6. Modelling Approach Two approaches were followed to describe crop growth: (i) considering a variable growth rate (GR, in g m d) that depends on incident radiation and the fraction of it absorbed by the crop, for which measurements of leaf area index (LAI) and extinction coefficient (K) are needed, using a maximum growth rate or a radiation use efficiency (RUE, in g MJ) index; and (ii) assuming a constant relative growth rate (RGR, in d) and describing growth exponentially as TDW = TDWinitial * e RGR * , where TDW is the total dry weight of the aboveground biomass of the crop (in g m), which also implies that GR = TDW * RGR. By choosing for the second approach, crop growth can be predicted from a single parameter, RGR, and the effect of E and M factors on this may be easily studied to calibrate the model for a wider range of conditions. RESULTS AND DISCUSSION Plant growth could be well described by exponential models under widely different, contrasting growth conditions (Fig. 2). RGR (for individual plants) decreased with crop age, though the decrease was stronger in spring and RGR remained almost constant during winter (Table 2); the main factor affecting RGR was sowing date, followed by shading and N fertilisation. The relationship between TDW and LAI was approximately linear (R = 0.75) for non-shaded plants, for which it holds that: LAI = SLA * LWR * TDW, where SLA is the specific leaf area (m g) and LWR is the leaf weight ratio. Both SLA and LWR decreased with the development stage (leaf thickening, stem growth) and were affected by the different E and M factors. Since RGR can also be expressed as the product: RGR = RLER * SLA * LWR, where RLER (d) is the relative expansion rate of the leaf area, knowing the functional variation of these parameters when changing the growing conditions is of main importance to predict crop growth for onfarm situations.