Notas de investigación

 

Water relations, gas exchange, and yield of processing tomato under reduced irrigation*

 

Relaciones hídricas, intercambio gaseoso y rendimiento de tomate para proceso bajo riego reducido

 

Jorge A. Zegbe^1§ and M. Hossein Behboudian²

 

¹Campo Experimental Zacatecas. Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias (INIFAP). Calera de V. R., Zacatecas, México. A. P. 18. C. P. 98500. ^§Corresponding author: jzegbe@zacatecas.inifap.gob.mx.

²Hort Science Group. INR 433. Massey University. Palmerston North, New Zealand.

 

* Recibido: enero de 2010
Aceptado: julio de 2010

 

Abstract

Partial root zone drying is a reduced irrigation technology to save water. The objective of this research was on measurement of hitherto unreported values of root water potential along with of processing tomato (Solanum lycopersicum L.) under reduced irrigation. Four irrigation treatments were applied: treatment 1, daily full irrigation as control, where both sides of the root system were well watered; treatment 2, daily irrigation on one side of the root system with half the volume of water given to daily full irrigation; treatment 3, full irrigation every other day of both sides of the root system (deficit irrigation); and treatment 4, irrigation on one side of the root system every other day with half the volume of water given to daily full irrigation. Root water potential was the same for both sides of root system in the treatments 2 and 4. Leaf water potential of treatment 2 was the same as that of treatment 1. Partial stomatal closure and lower transpiration and photosynthetic rates were observed in treatment 2 plants compared of treatment 1. Dry mass of fruit was higher in treatments 1 and 2, than in 3 and 4 treatments. It is concluded that any alteration in the physiological parameters of treatment 2 could have resulted from root signals because leaf water potential was the same in the treatments 1 and 2.

Key words: CO₂ assimilation, root and leaf water potential, stomatal conductance, transpiration.

 

Resumen

El riego parcial de la raíz (RPR) es una tecnología de riego reducido para el ahorro de agua. El objetivo de esta investigación; además de otros parámetros relacionados, fue medir valores del potencial hídrico de la raíz (?raíz) que hasta ahora no han sido publicados en tomate para proceso (Solanum lycopersicum L.), bajo el régimen de riego reducido. Se aplicaron cuatro tratamientos de riego: tratamiento 1, riego completo diario como testigo, donde ambos lados del sistema radical fueron irrigados adecuadamente; tratamiento 2, riego diario en un solo lado del sistema radical con la mitad del volumen aplicado al tratamiento 1; tratamiento 3, riego completo cada tercer día en ambos lados del sistema radical (riego deficitario); y tratamiento 4, riego cada tercer día en un lado del sistema radical con la mitad del volumen aplicado al tratamiento 1. El potencial hídrico de la raíz fue similar en ambos lados del sistema radical en los tratamientos 2 y 4. El potencial hídrico de la hoja del tratamiento 2 fue igual al observado en el tratamiento 1. Se observó un cierre estomático parcial y tasas bajas de transpiración y fotosíntesis en el tratamiento 2 comparado con el tratamiento 1. La materia seca de la fruta fue más alta en los tratamientos 1 y 2 que en los tratamientos 3 y 4. Se concluyó que cualquier alteración en los parámetros fisiológicos en el tratamiento 2, pudo haber resultado por información enviada desde la raíz, porque el potencial hídrico de la hoja fue igual en los tratamientos 1 y 2.

Palabras clave: asimilación de CO₂, conductancia estomática, potencial hídrico de raíz y hoja, transpiración.

 

In research on plant water relations, the assessment of root water potential (ψ_root) is often neglected because roots are less accessible than shoots (Steudle, 2000) and because of the difficulties involved in measurement of ψ_root (Gee et al., 1974). Nevertheless, ψ_root has been measured using a pressure chamber in plants undergoing water deficit (Gee et al., 1974). But this has not been done for plants either in split-root system (SRS) experiments or in partial root zone drying (PRD) research.

The root development of some deciduous trees under SRS was reported to be similar to fully irrigated trees (Poni et al., 1992; Dry et al., 2000). These authors suggested that water moves from roots in wetted soil to roots in drying soil and this was confirmed in grapevines by using deuterium-labelled water (Stoll et al., 2000; Smart et al., 2005).

Water redistribution within root system (RS) may occur at night when transpiration rate is negligible (Green et al., 2006). We were interested in learning how ψ_root in wetted and drying sides of PRD treatments compare with each other while transpiration is happening during the day. Our other objective was to compare water relations, gas exchange parameters, and yield of processing tomato under PRD and deficit irrigation (DI).

Plants of 'petopride' processing tomato (Solanum lycopersicum L.) were used in an experiment conducted in a naturally-lit glasshouse, with ventilation/heating set points of 25/15 ºC, at the Plant Growth Unit, Massey University, Palmerston North (40º 2' S, 175º 4' E), New Zealand. Details of the experimental set up could be found in Zegbe et al. (2006). The experiment was conducted from July to December 2001. Seeds were sown on 31 July 2001 and 40-day-old individual tomato plants were transplanted as described in Zegbe et al. (2007).

There were four treatments: treatment 1, daily full irrigation (FI) considered as control where both sides of RS were well irrigated; treatment 2, daily irrigation on one side of the RS with half the volume of water given to FI (PRD₁); treatment 3, full irrigation every other day of both sides of the RS (DI); and treatment 4, irrigation on one side of the RS every other day with half the volume of water given to FI (PRD₂). Irrigation design and management, and volumetric soil water content determination are detailed in Zegbe et al. (2006; 2007).

Water potential of leaf (ψ_leaf) and ψ_root were measured using a Scholander pressure chamber (Soil Moisture Equipment Corp., Santa Barbara, California, USA). Three replicates per treatment of mature plants were measured on 118 and 136 days after sowing (DAS).

The ψ_leaf was measured on two leaflets before disturbing the plants for ψ_root determinations. For measurement of ψ_root, two root branches from two opposite sides (wetted and drying sides of PRD treatments) were selected. They were excised and carefully removed from the soil minimising root damage and placed in the pressure chamber. All water potential measurements were taken between 09:00 to 11:30 h for each sampling date.

The roots were approximately 25 cm long and had an average diameter of 1.44 ±0.4 mm; when root damage was noticed, a new root was excised and measured. After ψ_leaf determinations, stomatal conductance (g_s), transpiration rate (E), net photosynthetic rate (A), and photosynthetic photon flux (PPF) were measured with a portable photosynthesis system (LI-6200, Li-Cor Inc., Nebraska, USA) on two mature and exposed leaflets. At harvest, fruits were cut into halves and oven-dried at 85 °C to constant weight to determine their total dry weight.

The data were analysed by a completely randomised model using the ANOVA procedure of Statistical Analysis System software (SAS, 2001-2003). Treatment means were separated by multiple t tests at p≤ 0.05 and when F test of treatments was significant at p≤ 0.05. Means separation between wetted and drying part of root system for each PRD treatment was confirmed by orthogonal contrasts. There were 48 plants in the experiment with 24 being used for measurements three replicated plants were sampled.

Volumetric soil water content (θ) was significantly higher in FI plants than in DI and PRD plants (Table 1). The difference in θ between the wet and drying sides of RS in PRD₁ plants was also significant. But θ was the same in the wet and drying side of the RS in PRD₂ plants being similar to that of DI plants (Table 1). After re-watering, θ was still lower in PRD and DI treatments that in FI. We used a potting mixture composed of bark: pumice: peat with a ratio of 6:2:1. Complete re-hydration of this mixture did not occur upon re-watering possibly due to hysteresis as defined by Brady and Weil (2000); this has also been observed in other PRD experiments in tomato (Kirda et al., 2004; Zegbe et al., 2006 and 2007) and in apple (Zegbe et al., 2008).

On 118 and 136 days after sowin (DAS), ψ_root tended to be lower in DI and in the drying side of PRD₁ plants than that of FI plants (Table 1). Both sides of the RS in PRD2 had a significantly lower ψ_root than that of FI and PRD1 treatments. ψ_root of PRD plants tended to be lower in drying side than in the wetted side of the RS, but the difference was not significant using the orthogonal contrast analysis (p ≤ 0.05). On both sampling dates, plants in PRD₂ (which was the most water stressed treatment) had the lowest ψ_leaf, g_s, E, and A of all the treatments (Table 1).

On 118 DAS; g_s, E, and A were the same in DI, PRD₁, and FI plants. This was due to low solar radiation at measurement time (Behboudian et al., 1994). Radiation was twice higher on 136 DAS than on 118 DAS and in the former day, compared to the FI plants, DI and PRD₂ plants had a lower ψ_leaf accompanied by a reduction in g_s, E, and A (Table 1). The PRD₁ plants had the same ψ_leaf as FI plants suggesting that ψ_leaf of PRD₁ plants must have equilibrated with the irrigated part of RS (Green et al., 2006; Skaggs et al., 2006). But reductions of gs by 30%, E by 15%, and A by 31% (opposite to Tan et al., 1981) could be due to signals produced in non irrigated RS (Dry et al., 2000).

This result is compatible with the assumption that PRD treatment maintains a higher ψ_leaf and dry mass of fruit (DMF) than DI treatments even if both are being irrigated with the same amount of water. The DMF values in this experiment (LSD= 89.7 g) were 452.3, 425.2, 291.7, and 217.2 for FI, PRD₁, DI, and PRD₂, respectively. PRD₁ will therefore be a better reduced irrigation strategy to adopt.

In conclusion, ψ_root for both sides of RS were the same for PRD treatment and ψ_leaf in less severe PRD treatment (PRD₁) equilibrated with wet side of the RS being the same as fully watered controls. Any aberrations in the physiological properties of PRD₁ could have resulted from root signals.

 

ACKNOWLEDGEMENTS

This research was partially supported by the Secretaría de Educación Pública (SEP)-PROMEP-México; Universidad Autónoma de Zacatecas (UAZ); Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias (INIFAP) en México. We thank Jorge Omar and Miriam Zegbe for their help and support.

 

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