THE EFFECT OF ROOTS CONFINEMENT ON THE RELATIVE GROWTH OF ROOTS AND CANOPY OF OPUNTIA FICUS-INDICA

Summary The influence of soil volume on roots and canopy growth performance of cactus pear (Opuntia ficus-indica) was studied at Palermo University. In November 2014, 1-year-old Opuntia ficusindica cladodes were planted in five different volumes of soil 50, 33, 18, 9 and 5 Liters. Three replicates (plants) per pot size were dug out at 6 and 12, 18 and 24 month intervals, thus in total, there were 5 x 3 x 4= 60 experimental plots. The resulting experimental design was a completely randomized design with combinations of two factors, soil volume and month of the sampling, with three replications. Roots of each plant were washed and visually divided into three groups depending on their diameters: Fine roots less ≤ 2 mm; medium roots (2-5 mm); large roots >5 mm, the roots of each group was manually separated and measured. Roots surface area was measured using image processing VegMeasure software®. Root volumes were calculated from surface area and root length by assuming that roots are cylindrical. Root measurements were taken prior to root dry mass estimation. Cladode surface area, thickness, number of new cladodes, cladodes fresh and dry mass were measured and recorded for each plant. Roots: shoot mass, root density, root length density and specific root length were calculated. Mother cladode and roots starch content estimation was performed using the perchloric acid method while the natural signature of δ13C and the roots turnover was determined depending on the portion of C in soil that was derived from the cactus pear root. Results indicated a significant effect of soil volume and sampling dates and their interaction (P<0.01) on: total roots length, roots surface area, dry mass, volume, specific roots length, the large roots surface area, medium roots surface area, dry mass of the large, medium roots, number second generation cladodes, canopy dry mass, total canopy surface area, carbon isotopic signature δ 13C and carbon derived by roots per soil unite. Whereas, root density, roots length density, fine roots dry mass, total number of cladodes, mother cladodes and roots starch content and roots turnover were significantly affected by soil volume and sampling dates only. Increasing the soil volume enhanced the total roots length, surface area, roots dry mass, and total roots volume. However, the smallest soil volume showed stable roots growth over time in terms of the total roots length, total surface area and the total roots volume, as well as the total dry mass. In contrast, soil volume restriction enhanced root density as well as the root length density and the specific root length. On the other hand, the large, medium and fine roots dry mass and surface area and length tended to increase with the soil volume. The number of the first generation cladodes was affected by the soil volume restriction. The lower number of the second generation cladodes produced in the lower soil volume, plants in the smallest soil volume stopped producing new second generation cladodes after the first sampling date. Moreover, the total number of the new cladodes increased with soil volume over time and ranged between (3-15 cladodes per plant). Linear canopy dry mass increases were observed with respect to the soil volume increase. The roots: canopy dry mass and the roots volume: canopy dry mass ratios increased with the soil volume increase, this is because of positive effect of the soil volume increase on both roots and canopy. Results showed an increase in starch accumulation in both roots and the mother cladodes along with soil volume decrease. Furthermore, there was an increasing negative δ13C signature values over time as result of the contribution of cactus pear root (CAM-C) to the soil organic matter (C3-soil). The CAM-C contribution increased from 27 C (g of soil kg-1) in the biggest soil volume to 57 C (g of soil kg-1) in the smallest soil volume. This can be explained by the higher roots mortality in the small soil volume which increased the turnover percentage with time ranging between (10-15.4%). These results suggest that the limitation of soil availability has resulted in root and canopy growth limitation and greater root turnover.