Title Impact of Saline Irrigation Water on Citrus Rootstocks in the Lower Rio Grande Valley Prime Agreement Number: 06HQGR0130

The Texas citrus industry in the Rio Grande Valley experiences periodic droughts. During such times, water restrictions from the Amistad and Falcon Reservoirs can reduce water available for agriculture. Irrigation in this region is primarily surface waters from the Rio Grande where citrus orchards are flood irrigated. Irrigation practices that use less water are being explored and evaluated. Water used to irrigate crops usually contains between 800 and 900 mg L of salt, the equivalent of adding between 2100 and 2400 lbs salt/acre foot. Limiting irrigation in agricultural areas may lead to salt accumulation in the crop rooting depth, especially where low water use systems like drip irrigation is utilized. Currently, Citrus trees are grafted onto hardy rootstocks in order to ensure tree survival and production. These rootstocks are used to reduce pathogen impacts and enhance their tolerance to thermal, saline and other environmental stressors. It is vital to find saline tolerant citrus rootstocks for soil and environmental conditions in the Lower Rio Grande Valley (LRGV). This study’s objectives are to assess the salinity tolerance of citrus rootstocks using typical soils found in the Rio Grande Valley. We will evaluate irrigation water salinity tolerance levels for these rootstocks during greenhouse trials. Problem and Research Objectives Drought and water restrictions are an ongoing problem for farmers in the Rio Grande Valley. Finding citrus rootstocks that are able to tolerate increasing salinity while using less water is vital for the agricultural community and water conservation in South Texas. Citrus (Citrus spp.) is an important economic crop in the LRGV, bringing in more than $50 million for growers annually (Sauls 2008). Citrus trees are traditionally grafted onto sour orange rootstock because of its ability to tolerate the calcareous, high pH soils and heavy soil conditions in south Texas (Sauls 2008; Louzada et al. 2008). However, the sour orange rootstock is susceptible to a variety of diseases and pathogens that were previously not a problem. Increasing concerns over Citrus tristeza virus transmitted by the new arrival, the Brown Citrus Aphid, and other diseases have initialized more research into finding alternative rootstocks. These pest and pathogen resistant rootstocks must be evaluated for the varying soil and water conditions found in the Rio Grande Valley. The decreasing availability of irrigation waters from surface waters due to drought and restrictions have put limitations on irrigation practices in the LRGV. On average, typical irrigation practices consist of flooding fields with 0.5 acre‐foot /acre between 4 and 6 times during the growing season. This could increase up to 9 times during the growing season in times of drought or water shortage. Given an average EC of 1.33 dS m (850 mg L), this means that in a growing season as much as 4624 to 10,404 lbs of salt/ acre are added annually to citrus orchards. While most salts will be leached away by excess water, some salts will continue to accumulate. This problem will be compounded if water restrictions limit the amount of water farmers will be able to apply to their land. The intrusion of salt water from the Gulf of Mexico causes high salinity levels in groundwater throughout the LRGV. Groundwater in this region has not typically been used for irrigation due to high spatial variability in water quality and quantity (Chowdhury and Mace). Surface water limitations may force farmers to resort to saline or brackish groundwater in order to meet crop water demands. This has led to the need for further development and evaluation of saline tolerant rootstocks. The objectives of this study were to evaluate and assess the salinity tolerance for several citrus rootstocks. There is also a need to evaluate irrigation water deficits to determine an optimal salinity tolerance level that will also meet the crop’s water needs. To determine these factors we will set up greenhouse trials using various citrus rootstocks. We used rootstock varieties adapted to soil conditions in the Rio Grande Valley and water with varying electrical conductivities and apply them at different increments in order to evaluate the optimal salinity tolerance in a water deficit situation. This study’s purpose is to obtain preliminary data in order to further research that may be conducted during field trials of the same rootstocks. This can potentially be valuable information for growers in times of drought or water restriction when they may have few options. Water quality and availability is a problem that is escalating as increased population growth in the LRGV as well as drought and water restrictions occur. Materials/Methodology Initially, rootstock seeds were evaluated for the potential to be used in this study by germinating seeds in a nutrient agar supplemented with salt solutions at different concentrations. This initial evaluation evaluated germination by observing seeds collected from rootstock parent varieties grown on Texas A&M University‐Citrus Center property in Weslaco, TX. Four rootstock cultivars were evaluated based on their disease resistance, tolerance to calcareous clays, fruit quality and potential yield. These rootstock cultivars were Sour Orange, C‐146, C‐57 and C‐22. One scion variety was also tested to evaluate salinity on the scion cultivar. This in vitro study was conducted to minimize contamination and reduce any additive effects of repeated saline water additions. This part of the study evaluated the in vitro germination and growth of citrus seeds in a nutrient agar supplemented with sea salt solution (Instant Ocean©, Spectrum Brands, Inc., Madison, WI) to have salt concentrations that correspond to approximately 0, 1, 3, 5, and 10 dS/m (+/‐ 1 dS/m) electrical conductivities. Each cultivar had 10 seeds per box and 2 boxes per treatment for a total of 100 seeds per cultivar and 20 seeds per treatment. The seeds were sanitized in a solution containing 10% bleach and 0.1% Tween 20 and stirring continuously for 2 hours, then rinsed with deionized water four times. In sterile conditions, the seed testa and cotyledons were cut (without damage to the micropylar end) in order to promote optimal germination and rule out seed coat factors in germination hindrance. Seeds were placed in a Magenta‐7 vessel (Sigma‐Aldrich) and containing Murashige and Skoog basal medium (Murashige and Skoog, 1962) supplemented with Gambourg’s vitamins (Sigma‐Aldrich, St. Louis, MO), and 0.4% Phytagel (Sigma‐Aldrich, St. Louis, MO) along with the sea salt solution (Instant Ocean©). The seeds were kept in the dark at approximately 27°C for 2 weeks and then gradually introduced into natural light conditions. The germination was recorded daily until the 14 day point. The germinated seeds were measured for the following after 70 days total germination and growth. • Germination rate and percentage • Number of seedlings germinated per seed (polyembryony) • Root length and width • Shoot length and width • Fresh weight and dry weight (average moisture content) After the initial seed evaluation study, three rootstock varieties were chosen to determine the salinity tolerance of grafted and non grafted citrus trees. In this study the Sour Orange, C‐22 and C‐146 cultivars were evaluated. Grafted rootstocks had the scion variety ‘Olinda’ a Valencia sweet orange variety grafted onto the previously mentioned rootstocks, while the non‐grafted varieties had no such treatment. The trees were watered bi‐weekly with a sea salt solution 0, 1, 3, 5 and 10 dS/m (+/‐ 1 dS/m). Each treatment contained three rootstock cultivars and 5 replications. The experimental setup as shown below was set up in a random complete block design. Grafted Rootstocks Grafted 0 dS/m 1 dS/m 3 dS/m 5 dS/m 10 dS/m Rep 1 C22‐R1G‐ 0dS SO ‐R1G‐ 1dS C22‐R1G‐ 3dS C146‐ R1G‐ 5dS SO ‐R1G‐ 10dS C146‐ R 1 G‐ 0dS C22‐R1G‐ 1dS SO ‐R1G‐ 3dS C22‐R1G‐ 5dS C146‐ R1G‐ 10dS SO ‐R1G‐ 0dS C146‐R 1G‐ 1dS C146‐ R1G‐ 3dS SO ‐R1G‐ 5dS C22‐R1G‐ 10dS Rep 2 C146‐ R 2G ‐0dS SO ‐R2G‐ 1dS SO ‐R2G‐ 3dS C146‐ R2G‐ 5dS C22‐R2G‐ 10dS SO ‐R2G‐ 0dS C22‐R2G‐ 1dS C146‐ R2G‐ 3dS C22‐R2G‐ 5dS SO ‐R2G‐ 10dS C22‐R2G‐ 0dS C146‐ R2G‐ 1dS C22‐R2G‐ 3dS SO ‐R2G‐ 5dS C146‐ R2G‐ 10dS Rep 3 C22‐R3G‐ 0dS C146‐ R3G‐ 1dS C146‐R 3G‐ 3dS C22‐R3G‐ 5dS SO ‐R3G‐ 10dS SO ‐R3G‐ 0dS SO ‐R3G‐ 1dS C22‐R3G‐ 3dS SO ‐R3G‐ 5dS C146‐ R3G‐ 10dS C146‐ R3G‐0dS C22‐R3G‐ 1dS SO ‐R3G‐ 3dS C146‐ R3G‐ 5dS C22‐R3G‐ 10dS Rep4 SO ‐R4G‐ 0dS SO ‐R4G‐ 1dS C146‐ R4G‐ 3dS C22‐R4G‐ 5dS C22‐R4G‐ 10dS C22‐R4G‐ 0dS C146‐ R4G‐ 1dS C22‐R4G‐ 3dS SO ‐R4G‐ 5dS C146‐ R4G‐ 10dS C146‐R 4G‐0dS C22‐R4G‐ 1dS SO ‐R4G‐ 3dS C146‐ R4G‐ 5dS SO ‐R4G‐ 10dS Rep 5 C22‐R5G‐ 0dS C146‐R5G‐1dS C22‐R5G‐3dS SO‐R5G‐5dS SO‐ R5G‐ 10dS C146‐ R5G‐ 0dS C22‐R5G‐1dS SO‐R5G‐3dS C22‐R5G‐ 5dS C146 ‐R 5G‐ 10dS SO‐R 5G‐ 0dS SO‐R 5G‐ 1dS C146‐R5G‐3dS C146‐ R5G‐ 5dS C22‐R5G‐ 10dS SO=Sour Orange, R=Replication, G= Grafted Non‐Grafted Rootstocks Non Grafted 0 dS/m 1 dS/m 3 dS/m 5 dS/m 10 dS/m Rep 1 C146‐ R1NG‐ 0dS SO ‐R1NG‐ 1dS SO ‐R1NG‐ 3dS C146‐ R1NG‐ 5dS C22‐R 1NG‐ 10dS SO ‐R1NG‐ 0dS C22‐R1NG‐ 1dS C146‐ R1NG‐ 3dS C22‐R1NG‐ 5dS SO ‐R1NG‐ 10dS C22‐R1NG‐ 0dS C146‐R 1NG‐ 1dS C22‐R1NG‐ 3dS SO ‐R1NG‐ 5dS C146‐R 1NG‐ 10dS Rep 2 SO ‐R2NG‐ 0dS SO ‐R2NG‐ 1dS C146‐ R2NG‐ 3dS C22‐R2NG‐ 5dS C22‐R 2NG‐ 10dS C22‐R2NG‐ 0dS C146‐R 2NG‐ 1dS C22‐R2NG‐ 3dS SO ‐R2NG‐ 5dS C146‐R 2NG‐ 10dS C146‐ R2NG‐ 0dS C22‐R2NG‐ 1dS SO ‐R2NG‐ 3dS C146‐ R2NG‐ 5dS SO ‐R2NG‐ 10dS Rep 3 C22‐R3NG‐ 0dS SO ‐R3NG‐ 1dS C22‐R3NG‐ 3dS C146‐ R3NG‐ 5dS SO ‐R3NG‐ 10dS C146‐R 3NG‐ 0dS C22‐R3NG‐ 1dS SO ‐R3NG‐ 3dS C22‐R3NG‐ 5dS C146‐ R 3NG‐ 10dS SO ‐R3NG‐ 0dS C146‐R 3NG‐ 1dS C146‐ R3NG‐ 3dS SO ‐R3NG‐ 5dS C22‐R 3NG‐ 10dS Rep 4 C22‐R4NG‐ 0dS C146‐ R4NG‐ 1dS C146‐ R4NG‐ 3dS C22‐R4NG‐ 5dS SO ‐R4NG‐ 10dS SO ‐R4NG‐ 0dS SO ‐R4NG‐ 1dS C22‐R4NG‐ 3dS SO ‐R4NG‐ 5dS C146‐ R 4NG‐ 10dS C146‐ R4NG‐ 0dS C22‐R4NG‐ 1dS SO ‐R4NG‐ 3dS C146‐ R4NG‐ 5dS C22‐R 4NG‐ 10dS Rep 5 C22‐R5NG‐0dS C146‐R5NG‐1dS SO‐ R5NG‐3dS C22‐R5NG‐ 5dS SO ‐R5NG‐ 10dS C146‐R5NG‐0dS C22‐R5NG‐1dS C22‐R5NG‐3dS C146‐ R5NG‐ 5dS C146‐ R 5NG‐ 10dS SO‐R5NG‐0dS SO‐R5NG‐1dS C146‐R5NG‐3dS SO ‐R5NG‐ 5dS C22‐R 5NG‐ 10dS SO=Sour Orange, R= Replication, NG=Non‐grafted The trees are part of a continuing 6 month study, and the preliminary data will be presented in this report. Salt water solutions are applied bi‐weekly at a volume determined by transpiration rate and soil moisture. The soil electrical conductivity (EC) is measured monthly and the soil will be periodically flushed with reverse osmosis water when soil EC is above the treatment levels. Physiological effects are assessed on an incremental basis. The data presented in this report is incomplete, but salinity effects have been noted and will be discussed later. The evaluations presented in this report are for the following measurements.