DEL SORGO DA BIOMASSA ALL’AUMENTO DELLA CONCENTRAZIONE DI CO
2Michele Rinaldi 1*, Carmen Maddaluno 1, Pasquale Garofalo 2, Laura D’Andrea 2
1 Consiglio per la ricerca in agricoltura e l’analisi dell’economia agraria (CREA) – Centro di Ricerca Cerealicoltura e Colture Industriali (CI) – S.S. 673, km
25,200 – 71122 Foggia, Italia
2 Consiglio per la ricerca in agricoltura e l’analisi dell’economia agraria (CREA) – Centro di Ricerca Agricoltura e Ambiente (AA), Via Celso Ulpiani, 5 –
70125 Bari, Italia * michele.rinaldi@crea.gov.it
Abstract
The aim of this study was to evaluate the effect of CO2 enrichment at leaf scale on the biomass sorghum cultivated in
Southern Italy. An increased concentration from 370 (ambient) to 760 µmol mol-1 of CO
2 was set. Leaf photosynthetic rate
(A), stomatal conductance (gs) and leaf transpiration rate (E) were measured, and water use efficiency (WUE) was calculated as ratio between A and E. The parameters were measured on fully expanded leaves during three vegetative phases (juvenile phase, full canopy expansion and beginning of senescence) using a Photosynthesis System LCpro+, Portable Infra- Red Gas Analyzer (IRGA). This research showed that as consequence of the increase in carbon dioxide concentration, the biomass sorghum led to an improvement in plant carbon assimilation and processing, by increasing the photosynthesis, reducing the water transpired by the plants and improving the water use efficiency.
Keywords: leaf photosynthetic rate, leaf transpiration rate, stomatal conductance, carbon dioxide, IRGA. Parole chiave: fotosintesi, traspirazione, conduttanza stomatica, anidride carbonica, IRGA.
Introduction
The atmospheric carbon dioxide concentration [CO2] has risen by more than 30% since pre-industrial times, from
equilibrium levels of about 280 ppm in 1880, to the currently observed levels of 365 ppm, and it will increase to 500-1000 ppm by 2100 (IPCC, 2007). The elevated concentration of CO2 tends to increase growth and yield of most agricultural
crops (Tubiello et al., 2007), with different paths for C3 and C4 plants, indeed, the final biomass accumulation is about 40-
44% in C3 plants and 22-33% for C4 plants (Poorter, 1993; Wand et al., 1999).
Among the C4 plants, there is the biomass sorghum (Sorghum bicolor L.). It is an energy crop, that could represent an
alternative renewable resource to fossil fuels, to produce bio-ethanol. Moreover, it is sustainable in Mediterranean environment, because of its low nitrogen and water requirement and its high efficiency in capturing and transforming energy into biomass.
The aim of this study was to evaluate the effect of the CO2 enrichment at leaf scale on the biomass sorghum cultivated in
Southern Italy.
Materials and Methods
The research was carried out in Foggia (lat. 41° 8’ 7’’ N; long. 15° 83’ 5’’ E, alt. 90 m a.s.l.) (Southern Italy) in 2010 at the experimental farm of CREA-SCA of Bari. The soil is a vertisol of alluvial origin, Typic Calcixeret, (Soil Taxonomy 10th
ed., USDA 2010), silty-clay with available soil water, 202 mm m-1. The local climate is “accentuated thermo- Mediterranean” as classified by FAO-UNESCO Bioclimatic Maps (1962), with daily temperatures below 0 °C in the winter and above 40 °C in the summer. Annual rainfall (average 550 mm) is mostly concentrated during the winter months. The sowing of biomass sorghum (Sorghum bicolor L., hybrid BIOMASS 133) was carried out in the first days of May and the plants were cut early in August at flowering. The crop was cultivated in well watered conditions, supplying the water consumed and was fertilized with 2 q ha-1 of diammonium phosphate in pre-sowing.
The parameters were measured on two new fully expanded and sunlight leaves of three plants randomly chosen in the field, using a Photosynthesis System LCpro+, Portable Infra-Red Gas Analyzer (IRGA) (ADC, BioScientific Ltd., Hoddesdon, Herts, UK). The [CO2] in the leaf chamber (cref) was increased step by step, starting with 370 (ambient), followed by: 450,
580, 680 and 760 µmol mol-1.
The measurements were carried out in the three phases: 60 days (juvenile phase), 70 days (full canopy expansion) and 90 days (beginning of senescence) after the sowing.
The following parameters were measured: leaf photosynthetic rate (A; µmol m-2 s-1), stomatal conductance (gs; mol m-2 s-1) and leaf transpiration rate (E; mmol m-2 s-1). Water use efficiency (WUE; µmol CO
2 mmol-1 H2O) was calculated as ratio
Results and Discussion
The stomatal conductance (gs; mol m-2 s-1) and the leaf transpiration rate (E; mmol m-2 s-1) remain constant with increasing CO2, while decreasing during the crop cycle.
The photosynthesis increases with the increase of CO2, continuously in the phase of senescence beginning, while in the
other two phases (juvenile phase and full canopy expansion) the increase is only after the first two increments of CO2 (450
and 580 µmol mol-1) compared to the environmental one (370 µmol mol-1), then there is the flattening after the other two following increments of CO2 (680 and 760 µmol mol-1).
The WUE (ratio of photosynthesis and transpiration) increases with the increase in CO2 concentration and higher values
have been found especially during the full expansion phase. The trend is similar to that of photosynthesis.
Fig. 1: Photosynthetic components observed on biomass sorghum at 60 days (juvenile phase; blue diamonds), 70 days (full canopy expansion; red squares) and 90 days (beginning of senescence; green triangles) after the sowing.
Fig. 1: Componenti della fotosintesi osservate su sorgo da biomassa a 60 giorni (stadio giovanile; rombi blu), 70 giorni (piena espansione; quadrati rossi) e 90 giorni (inizio di senescenza; triangoli verdi) dopo la semina.
Conclusions
The increase of [CO2], as predicted by the
future climate scenarios, will be accompanied by raise of temperature and rainfall reduction. In these conditions, the C4 plants through the abatement of the stomatal conductance and leaf transpiration, combined with a faster intercellular CO2 processing, could fit better to the changing environmental conditions than C3
species, enhancing leaf A and growth via increases in Ci improvements of shoot water relations and increases in leaf temperature.
However, the increases in carbon dioxide concentration led to an improvement in plant carbon assimilation and processing, by: (i) increasing the photosynthesis, (ii) reducing the water transpired by the plants and (iii) improving the water use efficiency. These effects indicate that the biomass sorghum (C4 plant) has a better flexibility, in the environmental short term CO2 enrichment.
Acknowledgement
This work has been supported by the Italian Ministry of Finance and Economy, the Ministry of Education, University and Research, the Ministry of Environment and Territory, the Ministry of Agricultural, Food and Forestry Policies, through the contract no. 285 – 20/02/2006 (CLIMESCO Project, Coordinator: dr. D. Ventrella).
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Wand S.J.E., Midgley G.F., Jones M.H. and Curtis P.S., 1999. Responses of wild C4 and C3 grass (Poaceae) species to elevated atmospheric CO2 concentration: a meta-analytic test of current theories and perceptions. Global Change Biol.. 5: 723-741. Stomatal conductance 0.0 0.5 1.0 1.5 2.0 0 300 600 900 cref (ppm) gs ( m ol m -2 s -1)
Leaf transpiration rate
0 2 4 6 8 10 0 300 600 900 cref (ppm) E ( m ol m -2 s -1)
Leaf photosynthetic rate
0 10 20 30 40 50 60 70 0 300 600 900 cref (ppm) A ( µ m ol m -2 s -1) WUE = A / E 0 4 8 12 16 0 300 600 900 cref (ppm) WU E ( m m ol m -2)