A. Ganis's research while affiliated with University of Padova and other places

Two field trials were carried out in two years in heavy soils of NE Italy, with the aim of studying the effects of water and nitrogen management on fibrous root distribution and dynamics in sugar beet (cv. Dorotea). In conditions of moderate water deficit (year 2002, Conselice, Ravenna, clay soil), two water regimes (irrigation to 100% of potential evapotranspiration, and rainfed) were factorially combined with three rates of nitrogen application (180, 90, 0 kg ha−1). Irrigation increased volumetric root length density (RLDv) without N application and at the medium N rate – a common amount in beet cultivation – but reduced it at the maximum N dose. The medium N rate increased RLDv and shifted root distribution towards shallow layers, regardless of water regime.In the conditions of marked drought of 2003 (Legnaro, Padova, silty-loam soil), at a single rate of N supply (90 kg ha−1) irrigation increased total production (length) of fibrous roots throughout the soil profile (1.8 m), except in the 0.5–1 m interval, and improved the length of standing living roots during the season. Although the maximum root depth at the end of the season was similar in the two water regimes (about 1.9 m), irrigated roots reached the saturated soil layers 10 days earlier than in rainfed plants. The main result was reduced root turnover in deep soil layers (>1 m) and an increase at the surface in the rainfed treatments in conditions of drought, a probable mechanism of adaptation to a more marked gradient of soil moisture compared with irrigation.

Geostatistics was applied to a set of RLD (Root Length Density) data of maize obtained from auger sampling. A reciprocal linear-logarithmic function was found to fit the spatial trend of RLD, and was subtracted from the original data for correct application of geostatistics. Subsequent use of variography enabled description of the relations of root density with space and provided information on the size of root clusters. Spatial relations also served as a basis for subsequent estimation by kriging of RLD in unsampled locations, thus yielding bi-dimensional RLD maps with higher accuracy than those obtained by traditional methods of interpolation. Comparing two maize hybrids of different yields, the maps showed substantial differences in the proportion of root length in peripheral positions of the cross-row vertical profile and in the incidence of zones with critical density of colonisation. The trend of variograms also indicated larger root clusters in the higher-yielding hybrid, together with more uniform colonisation of soil – a profitable feature in conditions of limited soil resources. The trial highlighted some drawbacks in the application of geostatistics to feasible schemes of soil-coring in wide-spaced crops.

Manual line-intersect methods for estimating root length are being progressively replaced by faster and more accurate image analysis procedures. These methods even allow the estimation of some more root parameters (e.g., diameter), but still require preliminary labour-intensive operations. Through a task-specific macro function written in a general-purpose image analysis programme (KS 300 – Zeiss), the processing time of root images was greatly reduced with respect to skeletonisation methods by using a high-precision algorithm (Fibrelength). This has been previously proposed by other authors, and estimates length as a function of perimeter and area of the digital image of roots. One-bit binary images were acquired, aiming at large savings in computer memory, and automatic discrimination of roots against extraneous objects based on their elongation index (perimeter2/area), was performed successfully. Of four tested spatial resolutions (2.9, 5.9, 8.8, 11.8 pixel mm–1), in clean samples good accuracy in root length estimation was achieved at 11.8 pixel mm–1, up to a root density of 5 cm cm–2 on the scanner bed. This resolution is theoretically suitable for representing roots at least 85 m wide. When dealing with uncleaned samples, a thick layer of water was useful in speeding up spreading of roots on the scanner bed and avoiding underestimation of their length due to overlaps with organic debris. A set of fibrous root samples of sugar beet (Beta vulgaris var. saccharifera L.) collected at harvest over two years at Legnaro (NE Italy) was analysed by applying the above procedure. Fertilisation with 100 kg ha–1 of nitrogen led to higher RLD (root length density in soil) in shallow layers with respect to unfertilised controls, whereas thicker roots were found deeper than 80 cm of soil without nitrogen.

The root system of plants is subject to fast cycles of renewal and decay within the growing season. In water and/or nutrient stress conditions, this turnover may become strategic for plant survival and productivity, but knowledge about its mechanisms is still insufficient. In order to investigate the effects of nitrogen fertilization on growth and turnover of sugar beet roots, an experiment was carried out over two growing seasons in northern Italy with two levels of N supply (0, 100 kg ha–1). Biomass production and partitioning were followed during growth, and fibrous root dynamics were inspected by means of computer-aided procedures applied to minirhizotron images.In conditions of N shortage, lower yields (storage roots) were associated with greater allocation of biomass to tap roots (final tap-root/shoot ratio = 5.6 vs. 4.1) and shallower distribution of fibrous root length density. The maximum depth of roots was not affected by N, but unfertilized plants reached it more slowly.The ratio of cumulative root dead length to produced length at the end of the growing period (TDL max/TPL max) was used as the most suitable approach for assessing overall root turnover. This ratio was greater in controls (0.73 vs. 0.50), which showed lower root longevity (–34% life-span on average), indicating that a greater proportion of root growth was renewed by unfertilized plants over the season.

Root responses of maize (Zea mays L.) to limited nutrients and water availability were evaluated in two highly productive full-season hybrids, DK585 and Santos (Dekalb – Monsanto), in laboratory, pot and field tests. In the laboratory, under optimal nutrient and water supply, seedlings of DK585 had higher growth (leaves and roots). Under nitrate or sulphate deprivation, DK585 showed better ability in adapting its root/shoot ratio to stress conditions, whereas Santos showed less plastic behaviour. This morphological trait of DK585 was associated with higher sulphate and constitutive nitrate influxes. In pot trials (plants with four to five leaves), DK585 maintained a high transpiration level to very low values (around 0.2) of FTSW (fraction of transpirable soil water), whereas Santos showed a higher response to soil drying. The latter reduced the rate of transpiration starting from a FTSW of about 0.6. In the open field (trial in 2000, Legnaro, NE Italy), in conditions of fluctuating combined water and nitrogen stress, DK585 at flowering reached greater root length density (RLD) than Santos in deep layers (50–100-cm interval depth) of positions further from the plant. However, in these conditions, the yield of DK585 was found to be only slightly higher than that of Santos (8.88 vs. 8.49 t ha–1 d.w.). An overall evaluation of the two hybrids indicates the more conservative strategy towards limited water and nutrient resources in Santos, and a greater tendency towards stress avoidance in DK585.

Minirhizotrons speed up research on root demography, but image quality often hampers standardization of the image processing method. A simple procedure working on the blue band of colour images was tested on fibrous roots of sugarbeet (Beta vulgaris var. saccharifera). With respect to green and red, the blue band allows better detection of roots when their luminance is very similar to that of the background. The method makes use of an exponential algorithm of contrast stretching, which takes luminance frequency distribution into account. Based on a single threshold level, the procedure includes skeletonization. A minimum segment length was adopted to discriminate roots from extraneous objects. Although a specific minimum root length (MRL) value was calculated for each soil type, results show that a single value can be applied, indicating that this method can be profitably used for processing large samples of images.