Determination of Crop Water Stress Index by Infrared Thermometry in Grapefruit Trees Irrigated with Saline Reclaimed Water Combined with Deficit Irrigation

Cristina Romero-Trigueros,José María Bayona Gambín,Pedro Antonio Nortes Tortosa,Emilio Nicolás Nicolás,Juan José Alarcón Cabañero

doi:10.3390/rs11070757

Abstract

Water is not always accessible for agriculture due to its scarcity. In order to successfully develop irrigation strategies that optimize water productivity characterization of the plant, the water status is necessary. We assessed the suitability of thermal indicators by infrared thermometry (IRT) to determine the water status of grapefruit in a commercial orchard with long term irrigation using saline reclaimed water (RW) and regulated deficit irrigation (RDI) in Southeastern Spain. The results showed that Tc-Ta differences were positive in a wide range of vapor pressure deficits (VPD), and the major Tc-Ta were found at 10.00 GMT, before and after the highest daily values of VPD and solar radiation, respectively, were reached. In addition, we evaluated the relationships between Tc-Ta and VPD to establish the Non-Water Stressed Baselines (NWSBs), which are necessary to accurately calculate the crop water stress index (CWSI). Two important findings were found, which include i) the best significant correlations (p < 0.005) found at 10.00 GMT and their slopes were positive, and ii) NWSBs showed a marked hourly and seasonal variation. The hourly shift was mainly explained by the variation in solar radiation since both the NWSB-slope and the NWSB-intercept were significantly correlated with a zenith solar angle (θZ) (p < 0.005). The intercept was greater when θZ was close to 0 (at midday) and the slope displayed a marked hysteresis throughout the day, increasing in the morning and decreasing in the afternoon. The NWSBs determination, according to the season improved most of their correlation coefficients. In addition, the relationship significance of Tc-Ta versus VPD was higher in the period where the intercept and Tc-Ta were low. CWSI was the thermal indicator that showed the highest level of agreement with the stem water potential of the different treatments even though Tc and Tc-Ta were also significantly correlated. We highlight the suitability of thermal indicators measured by IRT to determine the water status of grapefruits under saline (RW) and water stress (RDI) conditions.

Highlights

Citrus is one of the most important commercial fruit crops in the world, including Southeastern Spain, where the climate is semi-arid Mediterranean and water is not always accessible due to its scarcity
The results showed that the canopy (Tc)-Ta differences were positive in a wide range of vapor pressure deficits (VPD), and the major Tc-Ta were found at 10.00 GMT, before and after the highest daily values of VPD and solar radiation, respectively, were reached
The canopy temperature measured by infrared thermometry was higher than air temperature, which results in positive differences between Tc-Ta for a wide range of VPD during the middle hours of the day

Summary

Introduction

Citrus is one of the most important commercial fruit crops in the world, including Southeastern Spain, where the climate is semi-arid Mediterranean and water is not always accessible due to its scarcity. In order to overcome this problematic scarcity of water resources for agriculture, the use of non-conventional water sources such as reclaimed water (RW) could become an alternative for farmers [4] since these waters are beneficial to crops when used as fertilizers due to their high macronutrient concentration [5] even though an excess of them could promote their loss through leaching [6]. In Murcia, which is a semi-arid region located in Southern Spain, there are 93 operational wastewater treatment plants (WWTP) that deliver almost 109 hm per year [8]. The negative effects of salinity and water stress on leaf gas exchange and chlorophyll content have been reported in citrus [11]

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