Applications of a Commercial Extract of the Brown Seaweed Ascophyllum nodosum Increases Drought Tolerance in Container-grown ‘Hamlin’ Sweet Orange Nursery Trees
Hamlin’ sweet orange trees on ‘Carrizo’ citrange and ‘Swingle’ citrumelo rootstocks were treated weekly with a commercial extract of the brown seaweed Ascophyllum nodosum at 5 and 10 mL·L−1 as either a soil drench or foliar spray. Half of the trees in each treatment were subjected to drought stress [irrigated at 50% of evapotranspiration (ET)], whereas the other half remained fully irrigated (100% ET). Drought stress reduced shoot growth and leaf photosynthesis but increased root and total plant growth relative to the amount of water applied, thus increasing whole plant water use efficiency. Trees treated with seaweed extract and drought-stressed had significantly more total growth than untreated drought-stressed trees for both rootstocks. The maintenance of growth by the seaweed extract under drought stress conditions was unrelated to photosynthesis. However, the seaweed extract treatment did have a significant effect on plant water relations. Soil drench-treated trees had more growth and higher stem water potential than foliar-treated or control trees after 8 weeks of drought stress. These results indicate that seaweed extract may be a useful tool for improving drought stress tolerance of container-grown citrus trees
Numerous physiological functions are affected by drought stress in citrus trees (Gómez-Cadenas et al., 1996). Stomatal closure has been reported to occur within 2 h in response to severe drought stress in citrus (Tudela and Primo-Millo, 1992) and continuous stress may lead to reduced CO2 assimilation and inhibit growth (Brakke and Allen, 1995). However, Bradford and Hsiao (1982) concluded that restriction of canopy development was the most sensitive parameter to moderate drought stress. Thus, it is important to distinguish between moderate and severe drought stress when assessing physiological responses.
In Florida, citrus nursery trees are primarily produced in 2.65 L (10 cm × 10 cm × 34.3 cm) containers commonly referred to as Citra-pots (CPOT5H; Steuwe and Sons, Tangent, OR). As of Jan. 2007, all citrus propagation in Florida must occur in greenhouses that meet specific state requirements for pest and disease exclusion (Florida Department of State, 2010). Most nurseries currently use overhead irrigation and as trees grow, the expanding leaf canopy reduces irrigation uniformity by blocking and deflecting water away from the roots. Uneven irrigation results in a portion of young trees receiving less water than optimal, resulting in uneven tree growth (Spann, unpublished data). Unless trees are rearranged on a bench, which is not a common practice, some trees experience relatively constant moderate drought stress under current production practices. Therefore, it is desirable to find ways of improving drought tolerance to improve production uniformity.
Seaweed and seaweed extracts (SWE) have been used as soil amendments and fertilizers in agriculture for centuries, particularly in coastal temperate regions (Blunden and Gordon, 1986). The most commonly used species in agriculture belong to the brown seaweed family (Blunden and Gordon, 1986) and include the species Ascophyllum nodosum. SWEs are often classified as plant biostimulants (Khan et al., 2009) and are generally thought to contain trace amounts of macro- and micronutrient elements, amino acids, vitamins, cytokinins, auxins, abscisic acid-like compounds, and quaternary ammonium compounds (Crouch et al., 1992; Crouch and van Staden, 1993; Durand et al., 2003; Ördög et al., 2004; Reitz and Trumble, 1996; Stirk et al., 2003). However, some doubt has recently been shed on the actual levels of plant growth hormones in SWEs (Craigie, 2010).
Commercial extracts of A. nodosum have been reported to increase fruit yield and quality of citrus (Fornes et al., 1995, 2002; Koo, 1988; Koo and Mayo, 1994) and grapes (Norrie et al., 2002). Extracts from A. nodosum have also been reported to increase drought stress tolerance of grasses (Schmidt and Zhang, 1997; Zhang and Ervin, 2004) and vegetable and ornamental crops (Neily et al., 2010), but there are no published reports about their effects on drought stress tolerance in woody plants in general or citrus specifically.
The objective of this study was to determine if a commercially available alkaline extract of A. nodosum (Stimplex® Crop Biostimulant; Acadian Seaplants, Nova Scotia, Canada) improves drought stress tolerance and maintains shoot growth under drought conditions of sweet orange nursery trees grown on commonly used rootstocks.
Timothy M. Spann, Holly A. Little
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