A field‐experiment (2004/2005 and 2005/2006 seasons) was conducted in the coastal plain of south‐eastern Sicily (37°03′N, 15°18′E, 15 m a.s.l.), on a Calcixerollic Xerochrepts soil, aimed at quantifying the effect of shading on chlorophyll (Chl) content, Chl fluorescence, photosynthesis and growth of subterranean clover. Four levels of photosynthetically active radiation reduction (from 0 % to 90 %) were tested on Trifolium brachycalycinum cv. ‘Clare’ and Trifolium subterraneum ecotype ‘Ragalna’. In both species shading progressively increased Fv/Fm, internal CO2 concentration, diffusive leaf resistance and specific leaf area (up to 8 %, 34 %, 18 % and 68 %, respectively), and decreased Chl content, Tmax, photosynthetic rate and plant dry weight (up to 9 %, 24 %, 79 % and 39 %, respectively). As plants aged, characteristic bell‐shaped trends were evident for photosynthetic parameters, with Fv/Fm increasing up until the onset of flowering, and thereafter declining. This implies that Fv/Fm may be a useful indicator of earliness in subterranean clover genotypes. The aboveground dry biomass response to shading was both genotype‐ and season‐dependent, but was predictable from the measurement of relative leaf Chl content. Moreover, our results suggest that an improvement in the interaction between host‐rhizobium may represent a major potential breeding target for enhancing subterranean clover tolerance to shading.

No abstract

The influence of the holoparasite branched broomrape on the vegetative growth, leaf chlorophyll content, photosynthetic rate, and chlorophyll fluorescence of tomato was studied over two growing seasons on plants grown in a commercial greenhouse. The presence of the parasite strongly reduced the aerial biomass by acting as a competing sink for assimilate, but more importantly, by compromising the efficiency of carbon assimilation via a reduction in leaf chlorophyll content and photosynthetic rate. The chlorophyll fluorescence parameters F0, Fm, Fv, and Fv/Fm were all altered in parasitized plants, indicating that branched broomrape–infected plants are more susceptible to photoinhibition. The degree of damage to the host was not dependent on either the number or the biomass of parasitic plants per host plant. We suggest that the ability to maintain a high photosynthetic rate, leaf chlorophyll content, or both and the ability to minimize photoinhibition can be developed as indirect assays for improved tolerance to branched broomrape.

Tomato cultivation in the Mediterranean region is susceptible to infestation by the parasitic weed branched broomrape (Orobanche ramosa), and severe yield losses can result. The effectiveness of solarization, a soil disinfection technique that uses passive solar heating, to control the incidence of broomrape under greenhouse conditions was studied over two growing seasons. Solarization was accomplished by the application of clear polyethylene sheets to moist soil for 58 to 61 d during the hot season. The treatment increased maximum soil temperature by around 10 C, and at 5 cm below the soil surface, a temperature of more than 45 C was reached for 34 to 58 d, whereas this temperature was not reached at all in the first season and not for 20 d (second season) in unmulched soil. In solarized soil, no broomrape shoots emerged, and neither haustoria nor underground tubercles of the parasite were found on tomato roots. The treatment killed about 95% of buried viable seed, and induced secondary dormancy in the remaining 5%. In nonsolarized plots, broomrape shoots were present at a high density, decreasing plant growth and fruit production. Fruit yield was 133 to 258% higher in the solarized as compared with the nonsolarized treatment. Based on these results, we suggest that soil solarization, which precludes chemical contamination and is suitable for organic farming, is an appropriate technology where the risk of branched broomrape infestation is high.

-Soil solarization is a pre-planting treatment not based on chemicals, used in hot climates to control weeds and soil-borne pathogens. Its effectiveness has been widely demonstrated, for example, in the USA, Spain, Portugal, Egypt, Italy, Mexico, India and Iraq. However, an improvement in efficacy is needed before it can be widely adapted as a commercial practice. Supplementation of the soil with organic matter prior to solarization has been proposed as a management option, but its effectiveness has yet to be confirmed by any systematic study. Therefore, here we carried out a set of experiments in southern Italy over two seasons to study the effect of four levels of organic supplementation of 0, 0.35, 0.70 and 1.05 kg m −2 prior to solarization. Soil temperature and its chemical properties, as well as plant vegetation growth and fruit production were monitored for tomato plants grown under commercial greenhouse conditions. Organic supplementation increased the maximum soil temperature achieved through solarization by 3.9• C to 4.7 • C. At 5 cm below the soil surface, a temperature of over 52 • C prevailed for 22 to 23 days when 0.70 kg m −2 organic supplement was incorporated, and for 14 to 13 days in the presence of 0.35 kg m −2 supplement, but this temperature was attained only for one day in the absence of any supplement. Organic supplementation significantly increased the soil concentration of NO − 3 -N, exchangeable K 2 O, Ca 2+ and Mg 2+ and electrical conductivity. Increased available P 2 O 5 and total N at the end of the crop cycle were also associated with supplementation of solarized soil. Plant vegetative growth was improved by supplementation, with crop plant stem diameter enhanced by up to 18%, above-ground vegetative fresh and dry weight by up to, respectively, 53 and 44%, and the number of leaves per plant by up to 16%. As the supplementation rate was raised from 0 to 0.70 kg m −2 , fruit yield was increased by about 70% (from 4.9 to 8.3 kg plant −1 ). Organic matter supplementation may provide the basis for a more favorable sink/source balance for tomato cropping. We conclude that organic supplementation represents a beneficial management measure to increase the effectiveness of soil solarization, and that these results provide encouragement for the future commercial application of this environmentally-friendly technique.soil solarization / organic supplementation / soil properties / tomato / plant growth / fruit yield