Plant indicators of available soil water in the perennial herbaceous crop Miscanthus × giganteus Greef et Deu Salvatore Foti, Salvatore Cosentino, Cristina Patan`e, Venera Copani, Emanuele Sanzone

To cite this version: Salvatore Foti, Salvatore Cosentino, Cristina Patan`e, Venera Copani, Emanuele Sanzone. Plant indicators of available soil water in the perennial herbaceous crop Miscanthus × giganteus Greef et Deu. Agronomie, EDP Sciences, 2003, 23 (1), pp.29-36. .

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Agronomie 23 (2003) 29–36 © INRA, EDP Sciences, 2003 DOI: 10.1051/agro:2002066

Original article

Plant indicators of available soil water in the perennial herbaceous crop Miscanthus × giganteus Greef et Deu Salvatore FOTIa, Salvatore L. COSENTINOb*, Cristina PATANÈc, Venera COPANIa, Emanuele SANZONEa a

Dipartimento di Scienze Agronomiche, Agrochimiche e delle Produzioni Animali (DACPA) – Sezione Scienze Agronomiche, Università di Catania, via Valdisavoia 5, 95123 Catania, Italy b Dipartimento di Produzione vegetale – Università della Basilicata, via Nazario Sauro, 84, Potenza, Italy c Centro di Studio Sulle Colture Erbacee Strategiche del Mediterraneo, CNR, via Valdisavoia 5, 95123 Catania, Italy (Received 14 August 2001; revised 8 February 2002; accepted 26 Februray 2002)

Abstract – Miscanthus × giganteus is a potential crop for biomass. In order to evaluate the tolerance of the species to low soil water availability, which in the Mediterranean environment occurs during the warm and dry summertime, a 2-year research experiment was carried out in the field on the eastern coast of Sicily (Southern Italy), comparing the physiological responses of the crop to three levels of ETm restoration (25%, 50% and 100%). All the studied plant indicators (pre-dawn and midday leaf water potential, leaf transpiration and stomatal conductance) were affected by soil water deficit according to a relationship described by an asymptotic curve. However, the pre-dawn leaf water potential was more strictly related to soil water content, among those tested, due to its relative independence from weather conditions and thus may represent a valid indicator of plant water status for irrigation scheduling. Indeed, the variations in leaf water potential, transpiration and stomatal conductance, measured during the warmest hours of the day, were related to the air vapour pressure deficit in the well-irrigated treatment (100% of ETm restoration). Miscanthus × giganteus / mediterranean environment / plant water status / stomatal conductance / vapour pressure deficit Résumé – Indicateurs physiologiques de l’eau disponible du sol dans une culture herbacée vivace Miscanthus × giganteus Greef et Deu. Miscanthus × giganteus est une espèce à haut potentiel de production de biomasse. Dans le but d’évaluer la tolérance de l’espèce à la disponibilité d’eau réduite dans le sol qui se produit au cours de l’été sec et chaud en climat méditerranéen, une expérimentation de deux ans a été conduite dans un site de la côte orientale de la Sicile (Sud de l’Italie) où la réponse physiologique de la culture à trois niveaux de restitution de l’ETM (25 %, 50 % et 100 %) a été étudiée. Tous les indicateurs physiologiques étudiés (le potentiel hydrique foliaire mesuré au lever du jour et à midi, la transpiration, la conductance stomatique) ont été influencés par le déficit hydrique du sol selon une relation décrite par une fonction asymptotique. Toutefois, le potentiel hydrique mesuré au lever du jour montre une relation étroite avec la teneur en eau du sol, parmi les indicateurs testés, à cause de sa relative indépendance par rapport aux conditions climatiques et il pourrait donc être un indicateur valable de l’état hydrique des plants et un outil pour la programmation de l’irrigation. Par contre, la variabilité du potentiel hydrique foliaire, de la transpiration et de la conductance stomatique, mesurées dans les heures les plus chaudes de la journée, était liée au déficit de pression de vapeur de l’air dans le traitement bien irrigué (restitution du 100 % de l’ETM). Miscanthus × giganteus / environnement méditerranéen / état hydrique de la plante / conductance stomatique / déficit de pression de vapeur

1. INTRODUCTION The genus Miscanthus originates from tropical and subtropical regions of the southern coasts of Asia [7]. The climate of these areas is characterised by high temperatures and abundant and well-distributed rainfall, very different from the Mediterranean climate. However, the genus progressively spread over different altitudes up to 3 000 m [14].

Communicated by Serge Rambal (Montpellier, France) * Correspondence and reprints [email protected]

The species Miscanthus × giganteus is a herbaceous, rhizomatous, perennial plant, with a C4 photosynthetic pathway and a productive period beyond ten years, with a high yield potential in biomass, good quality of the lignocellulosic material, low susceptibility to pathogens, high water use efficiency and low requirements in nutrient inputs [2, 4, 6]. This species has been indicated for many reasons as one of the most interesting biomass crops for different uses (energy, pulp of cellulose, compost, etc.) [16].

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This has led to the study of the possible adaptability of the Miscanthus × giganteus species to environmental conditions different from those of the original area. This aspect is particularly important in the regions of southern Europe, such as Sicily, which are suitable from a thermal point of view but are lacking in rains during summertime, when the vegetative and reproductive processes of the crop are active. In these regions, therefore, careful irrigation is needed. Useful parameters for crop irrigation scheduling are provided by measurements of the soil or plant water status. Nevertheless, the former is not always reliable, since different physiological behaviours can correspond to the same soil water content. Furthermore, because of variation in the soil water content in the area surrounding the roots, it is often difficult to know where to measure soil water content [18]. On the contrary, the physiological parameters based on plant water status are expressions of the influence exerted upon it, by both the soil and the climate [8, 12, 17]. Among these parameters, the most widely adopted for identifying water stress conditions are leaf water potential, stomatal conductance and canopy temperature [3]. Sometimes leaf water potential cannot represent a fully reliable index of the physiological or metabolic activity of the plant; some plants, in fact, reduce their assimilation activity despite keeping the same levels of leaf water potential, simply by closing the stomata [19]. Indirect indices of the plant water status are stomatal conductance and leaf temperature. The decrease in stomatal conductance determines a reduction in leaf transpiration and a subsequent increase in leaf temperature. These parameters, therefore, can be usefully adopted as indicators of plant water status [3]. To this end a three-year research experiment was carried out with the aim of defining the relationships between soil water content and some plant physiological parameters, in order to verify the possibility of using these as indicators of the most suitable moment for irrigation in Miscanthus × giganteus crop. 2. MATERIALS AND METHODS The study was carried out between 1994 and 1996 in Catania (Sicily, 10 m a.s.l., 37°25’ N Lat, 15°30’ E Long), on a vertic xerochrepts soil (Soil taxonomy) whose characteristics are reported in Table 1. Micropropagated plantlets of Miscanthus × giganteus provided by Piccoplant (Oldenburg, Germany) were transplanted in the field on the 10th of June 1993. After transplanting and until the autumnal vegetative stasis, the soil water content was kept at a good level in order to allow good plant establishment. Beginning in 1994, in order to determine different soil water availability, three different treatments, namely 100% (I2), 50% (I1) and 25% (I0) of ETm restoration were studied in a randomised block experimental design with plots measuring 80 m2 ( 8 × 10 m ) replicated three times. The water was distributed by means of a drip irrigation system. The irrigation was determined on the basis of the maximum available water content in the first 0.6 m of soil, where most of the root is expected to grow, calculated by means of the following formula [5]:

Table 1. Soil characteristics of the field site in the top 0–50 cm. Soil characteristic

Value

Sand (%) Loam (%) Clay (%)

49.27 22.43 28.30

}

(Gattorta method)

pH (in water solution)

8.6

Total calcareous (%) (gas-volumetric method)

15.24

Organic matter (%) (Walkley and Black method)

1.40

Total N (‰) (Kjeldahl method)

1.00

P2O5 avail. (ppm) (Ferrari method) K2O avail. (ppm) (Dirks and Sheffer method)

5 244.8

Field capacity at –0.03 MPa (%)

27

Wilting point at –1.5 MPa (%)

11

V = 0.66 ( FC – WP ) × Φ × D

(1)

where: V = water amount in mm; 0.66 = fraction of readily available soil water permitting unrestricted evapotranspiration; FC = soil water at field capacity, equal to 27% of dry soil weight; WP = soil water at wilting point, equal to 11% of dry soil weight; Φ = apparent volumetric mass (kg·m–3); D = rooting depth, equal to 0.6 m. The ETm was estimated by means of the pan-evaporation method, which assured a prompt and immediate timing of the date of irrigation and avoided crop stress [5]. The irrigation was done when the sum of daily ETm, calculated as follows, corresponded to V: ETm = E0 × Kp× Kc

(2)

where: E0 = class “A” pan-evaporation in semi-arid environment; Kp = pan-coefficient, equal to 0.80 (average relative humidity 40–70%, low wind speed, fetch 1 m) [5]; Kc = crop coefficient, ranging between 0.4 and 0.7 from plant emergence to beginning of jointing, between 0.7 and 1.1 from beginning to end of jointing, equal to 1.1 from end of jointing to bloom, and between 1.1 and 0.7 from bloom until October. The crop coefficients adopted refer to those of C4 plants (e.g. sorghum and corn) for the Mediterranean environments [5]. In 1995 a prolonged lack of irrigation water negatively affected the crop growth and development in all treatments. In 1994, during the growing season, 215.5 mm, 440.9 mm and 881.7 mm of total water were distributed to I0, I1 and I2, respectively. In 1995, 76.4 mm, 146.9 mm, and 287.9 mm were supplied to the above mentioned water treatments.

Plant indicators of soil water in Miscanthus

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Figure 1. Changes in max and min air temperature, evaporation, vapour pressure deficit (VPD) (mean ten-day values) and rainfall (total tenday values) during the field trials over the two years.

For the fertilisation a total of 120 kg·ha–1 of nitrogen, half as ammonium sulphate, at the spring crop regrowth and half as ammonium nitrate, at the beginning of jointing, was distributed. Throughout the growing season, using a meteorological station located close to the experimental field (100 m distant), the following meteorological variables were recorded: air temperature, air humidity, rainfall, global solar radiation and class “A” pan-evaporation. From beginning of plant jointing until bloom, various plant physiological variables were measured. Leaf water potential was measured by means of a pressure chamber (soilmoisture equipment corp., Santa Barbara, Ca, USA) before sunrise (05:00 h solar time, “pre-dawn” ψb) and at the highest solar intensity (14:00 h solar time "midday"ψm). Measurements were made in the field immediately after cutting the distal portion of the leaf blade (15–20 cm) of the youngest and fully expanded leaves in at least three samples in each treatment. Leaf transpiration and stomatal conductance were measured by means of a null balance "steady state" porometer (Model LI-1600, Li-Cor, Inc., Lincoln, Nebraska, USA) [11]. The measurements were carried out between 13:00 h and 14:00 h (solar time), on the lower surface of the last fully expanded leaf on three samples per plot. All measurements were carried out at periodic intervals during the growing season. In order to estimate the soil water deficit of each date of plant physiological measurements, the soil water content (θ) was measured gravimetrically. In order to monitor the soil water content, soil samples were collected up to 0.8 m depth every 0.2 m in three replications for each treatment, before and after each irrigation treatment, throughout the whole growing season. Soil samples were dried in an oven at 105 °C and weighed when a constant weight was reached.

The soil water deficit was expressed as a percentage of maximum available water according to the following formula [3]: water deficit = 1– ((WC – WP)/(FC – WP)) × 100

(3)

where: WC = soil water content as % of dry soil; WP = soil water content at wilting point as % of dry soil; FC = soil water content at field capacity as % of dry soil. The air vapour pressure deficit (VPD) was calculated, for each date of plant physiological measurements, from minimum air humidity and maximum air temperature values recorded between 13:00 h and 14:00 h (solar time) [1]. The non-linear regressions between the soil water deficit and physiological parameters were calculated by means of the SIGMAPLOT 5.0 software [15].

3. RESULTS 3.1. Weather The meteorological data were typical of the Mediterranean environment (Fig. 1). The temperatures, in the three years of the experiment, reached maximum values (30 °C–35 °C) during the months of July and August, when daily water panevaporation was 10–12 mm, corresponding to approximately 7–8 mm·day–1 potential evapotranspiration. The average temperature at the beginning of the month of May, the period of early crop growth, was around 17 °C in all three years. The rainfall, almost absent between May and October in 1994, was higher in the subsequent year.

S. Foti et al.

Pre-dawn leaf water potential (MPA)

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Figure 3. Changes in pre-dawn leaf water potential (MPa ± SE) in Miscanthus × giganteus in relation to the studied treatments.

Figure 2. Trend of percentage of available soil water content measured in the three irrigation regimes of the two experimental years. Values refer to the average of the measurements carried out at four soil depths (0.2, 0.4, 0.6, 0.8 m).

The vapour pressure deficit (VPD) appeared to be higher in 1994 with two peaks at the end of May and of August (almost 4.0 kPa). In the following year, in relation to higher rainfall, the values of the VPD were lower, with a maximum around 3.0 kPa. During the spring-summer-autumn period the temperatures encountered by the crop were not limiting for growth and development; the almost total absence of rainfall during summertime rendered the use of irrigation absolutely necessary. 3.2. Soil water content In the 2 years of measurement the amount of available water in the I2 treatment did not fall below 30%, whereas in the I1 treatment it dropped to 20% (Fig. 2). Substantially dryer was the I0 treatment, subjected to a more reduced water regime, with available water in the June-August period ranging between 60 and 20% in 1994 and between 40 and 10% in 1995. 3.3. Physiological variables Figures 3, 4 and 5 report the variations in pre-dawn and midday leaf water potential, leaf transpiration and stomatal conductance measured throughout the growing season. In the I2 treatment the pre-dawn leaf water potential ranged between –0.05 and –0.12 MPa in 1994 and between –0.05 and

Figure 4. Changes in midday leaf water potential (MPa ± SE) in Miscanthus × giganteus in relation to the studied treatments.

–0.13 MPa in 1995 (Fig. 3). In the I1 treatment the values were slightly lower and not significantly different from the I2 treatment in both years, while in the I0 treatment the values, constantly lower than in the previous treatments, were higher in 1994 (comprising between –0.10 and –0.17) than in 1995 (values ranging between –0.06 and –0.39 MPa). The midday leaf water potential measured in all years showed a clearer difference between the studied treatments (Fig. 4). In the I2 treatment, leaf water potential ranged from –1.65 to –2.07 MPa (average –1.93 MPa) in 1994 and from

Pre-dawn leaf water potential (MPA)

Plant indicators of soil water in Miscanthus

33

2

Figure 5. Changes in leaf transpiration (mg H2O · m–2 · s–1 ± SE) in Miscanthus × giganteus in relation to the studied treatments.

2

Figure 7. Relationship between available soil water deficit (100% of available soil water) and leaf water potential pre-dawn (A) and at mid-day (B) in Miscanthus × giganteus in relation to the studied treatments.

Figure 6. Changes in stomatal conductance (× 10–3 m · s–1 ± SE) in Miscanthus × giganteus in relation to the studied treatments.

–1.68 to –2.39 MPa (average –2.16 MPa) in 1995. In the I1 treatment the values ranged, in 1994, between –1.88 and –2.30 MPa (average –2.16 MPa), and in 1995, between –1.84 and –2.62 MPa (average –2.39 MPa). The higher water stress caused by the I0 treatment determined values of midday leaf water potential ranging between –2.22 and –3.00 MPa (average –2.58 MPa) and between –2.00 and –3.02 MPa (average – 2.68 MPa) in the two years, respectively. Leaf transpiration in each of the two years ranged between 56 and 191 mg ·m–2 ·s–1 in the I2 treatment, and between 43

and 166 mg ·m–2 ·s–1 in the I1 treatment, whereas it did not exceed 69 mg ·m–2 ·s–1 in the I0 treatment. The minimal values in the 3 treatments were around 20 mg ·m–2 ·s–1. The same results were observed for the stomatal conductance, where the differences between the three treatments were similarly evident. In the I2 treatment, the values were maintained between 3.1 × 10 –3 and –2 1.02 × 10 m·s–1, while in the I1 treatment the values were comprised between 1.1 × 10 –3 and 5.5 × 10 –3 m·s–1. In the I0 treatment the values ranged between 0.9 and –3 2.8 × 10 m·s–1 (Fig. 6). 3.4. Relation between soil water content and physiological variables Figure 7 shows the relationship between soil water deficit and pre-dawn leaf water potential and midday leaf water potential. In optimal soil water conditions (I2 treatment), where the soil water deficit never went below 60% of the maximum available water, the pre-dawn leaf water potential ranged between –0.03 and –0.12 MPa. The plants subjected to the I1 treatment, where the soil water deficit reached almost 70%, showed values between –0.04 and –0.16 MPa. In

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2

-3

2

-2 -1

2

Figure 8. Relationship between available soil water deficit (100% of available soil water) and leaf transpiration (A) and stomatal conductance (B) in Miscanthus × giganteus in relation to the studied treatments.

the I0 treatment where the soil water deficit attained almost 90%, the pre-dawn leaf water potential attained its lowest values (–0.39 MPa). A non-linear equation significantly fits the relation between the two variables (R2 = 0.78). The curve is parallel to the x-line until the soil water deficit reaches 60–65% of the maximum available water and then drops suddenly beyond 70% and reaches very low values around 90%. The relation between the soil water deficit and the midday leaf water potential is, in general, comparable to the previous one (R2 = 0.75). However, in the range between 0 and 60% of soil water deficit, the leaf water potential showed a higher variability ranging between –1.65 MPa and –2.17 MPa. A high variability was also observed in the interval between 0 and 65% of soil water deficit for the other two physiological indices: the leaf transpiration which ranges, within this interval, between 56.3 and 176.3 mg·m–2 ·s–1 (R2 = 0.58), and the stomatal conductance, ranging between 1.6 × 10 –3 and 9.9 × 10 –3 m·s–1 (R2 = 0.55) (Fig. 8). In conditions of high soil water deficit (> 80%) the values of leaf transpiration and stomatal conductance, which specifically refer to the I0 treatment, were low, the former being between 65.9 and 15.1 mg·m–2 ·s–1 and the latter between 3.2 × 10 –3 and 0.7 × 10 –3 m·s–1.

Figure 9. Relationship between air vapour pressure deficit (VPD) and midday leaf water potential (A), leaf transpiration (B) and stomatal conductance (C) in Miscanthus × giganteus in no water stress condition (I2 treatment).

3.5. Relation between the vapour pressure deficit (VPD) and the studied physiological variables The physiological variables measured during the day (midday leaf water potential, leaf transpiration and stomatal conductance) in good soil water conditions (soil water deficit < 65% of the maximum plant available water) were correlated to the vapour pressure deficit, and computed taking into account the air temperature and air humidity at the moment of measurement. The derived relations are significantly and effectively fitted by a function which highlights how the increase in the vapour pressure deficit determines a corresponding increase in the stomatal conductance and leaf transpiration. With an increase in the VPD from 1.67 to 3.41 kPa, there was a corresponding increase in leaf transpiration from 56 to 171 mg·m–2 ·s–1 and in stomatal conductance from 3.0 × 10 –3 to 1.02 × 10 –2 m·s–1 (Fig. 9). Inversely, the increase in VPD determined a decrease in the midday leaf water potential from –1.65 to –2.39 MPa.

Plant indicators of soil water in Miscanthus

4. DISCUSSION The changes of plant physiological indicators varied according to the changes in soil water availability. The estimate of soil water deficit by means of measurements (leaf transpiration, stomatal conductance and midday leaf water potential) carried out during the day raised many problems related to the reliability of the measurements. In fact, the relations between the soil water deficit and leaf gas exchange rate showed a high variability around the asymptotic curve whereas a lower, but still marked variability around the curve was also observed for the relation regarding midday leaf water potential and soil water deficit. This means that factors other than soil water content affect the physiological indicators measured during the day. According to the observed relations (Fig. 9), during the day the air VPD appeared to be, in good soil water conditions, the main factor controlling the studied physiological indicators, as the stomatal opening is affected by the VPD [18]. The soil water deficit was therefore more reliably estimated by pre-dawn leaf water potential. The use of a mathematical function which takes into account the VPD of the atmosphere could allow a reliable estimate within the limits of the conducted experiment of the soil water deficit, also using the plant physiological measurements carried out during the day. In fact, the relation between the VPD and daily measured plant physiological indicators in well-watered conditions (Fig. 9) could represent a water-stress “base-line” under which it is possible to assume that the plant is water-stressed and the soil water content is below the readily available content for the plant [9, 13]. It is necessary to say something about porometer results. In good soil water conditions and in high VPD values a certain shifting from linearity was observed. As a matter of fact, stomatal conductance at a high level of VPD may be affected by the closure of stomata, even if at good soil water content [9]. The transpiration rate (E) is calculated from stomatal conductance by the following formula [11]: ∆e E = g m ∆ w = g m -----(4) P where gm is the conductance in mole units (mol·m–2 ·s–1), and ∆w is the leaf-to-air water vapour mole gradient derived from the leaf-to-air vapour pressure difference (∆e) and barometric pressure (P). The transpiration rate, which then relates to gm directly and positively, moves from linearity as well. It should be noted, however, that errors of measurements may arise in high VPD conditions (low air humidity) when the incoming air inlet flow has lower values of relative humidity. In our case, since few points move from linearity, further experiments are needed to investigate thoroughly this phenomenon. Moreover, the transpiration rate measured in a porometer can never estimate transpiration rate ‘in situ’. The field transpiration rate can be computed from in situ values for air and leaf temperature, relative humidity, and boundary layer conductance according to the above mentioned formula. Alternatively, transpiration or conductance can be computed from energy balance [9].

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5. CONCLUSIONS The two-year study carried out in a Mediterranean environment to study the relationship between soil water content and some plant physiological indicators on Miscanthus × giganteus Greef et Deu, a crop for biomass and energy, in order to determine the right moment of water supply, allowed us to draw the following conclusions: – all the measured physiological indicators showed a more or less significant relationship with soil water deficit, described by an asymptotic non-linear function; – stomatal conductance, leaf transpiration and midday leaf water potential, strongly affected by the vapour pressure deficit (VPD), were not suitable for estimating soil water deficit; – the pre-dawn leaf water potential showed, nevertheless, within the interval going from 0 to 70% of the deficit of the available water, a lower variability compared with the other three variables. In the framework of the obtained results, the soil water deficit, and thus the best moment to supply water to the studied crop, may be efficiently estimated by measuring the pre-dawn leaf water potential. This is especially so in a Mediterranean environment, where the variability of the meteorological parameters during the summer period (air temperature and humidity) determines a high variability in the gaseous exchanges between plant and atmosphere. Acknowledgements: This work was carried out with financial support from the Commission of the European Community, AIR3 CT92-0294 “Miscanthus productivity network” project.

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