Abstract
The increase in events associated with drought constraints plant growth and crop performance. Cacao (Theobroma cacao L.) is sensitive to water deficit stress (DS), which limits productivity. The aim of this research was to characterise the response of seven (CCN51, FEAR5, ICS1, ICS60, ICS95, EET8, and TSH565) commercially important cacao clones to severe and temporal water deficit stress. Ten-month-old cacao trees were submitted to two treatments: well-watered and water-stressed until the leaf water potential (Ψleaf) reached values between −3.0 and −3.5 MPa. The effects of hydric stress on water relations, gas exchange, photochemical activity, membrane integrity and oxidative stress-related gene expression were evaluated. All clones showed decreases in Ψleaf, but TSH565 had a higher capacity to maintain water homeostasis in leaves. An initial response phase consisted of stomatal closure, a general mechanism to limit water loss: as a consequence, the photosynthetic rate dropped by approximately 98% on average. In some clones, the photosynthetic rate reached negative values at the maximum stress level, evidencing photorespiration and was confirmed by increased intracellular CO2. A second and photosynthetically limited phase was characterized by a drop in PSII quantum efficiency, which affected all clones. On average, all clones were able to recover after 4 days of rewatering. Water deficit triggered oxidative stress at the early phase, as evidenced by the upregulation of oxidative stress markers and genes encoding ROS scavenging enzymes. The effects of water deficit stress on energy metabolism were deduced given the upregulation of fermentative enzyme-coding genes. Altogether, our results suggest that the EET8 clone was the highest performing under water deficit while the ICS-60 clone was more susceptible to water stress. Importantly, the activation of the antioxidant system and PSII repair mechanism seem to play key roles in the observed differences in tolerance to water deficit stress among clones.
Highlights
Cacao (Theobroma cacao L.) is a fruited tree native to the Amazon basin of South America (Motamayor et al, 2008; Cornejo et al, 2018), and its cultivation represents the main economic livelihood for smallholder farmers and landowners in several producing countries in Africa, Central America, and South America (Phillips-Mora et al, 2007; Baligar et al, 2008)
In the WW clones, the leaf value remained between −0.25 and −0.16 MPa, while in clones under deficit stress (DS), a significant reduction was observed to an approximate value of −3.0 MPa at day 26 after treatment (D26) (Table 3) when the volumetric water content (VWC) reached 6% (Figure 1A)
The results of the current study confirmed that the irrigation suspension period applied in the seven cacao clones led to severe drought stress
Summary
Cacao (Theobroma cacao L.) is a fruited tree native to the Amazon basin of South America (Motamayor et al, 2008; Cornejo et al, 2018), and its cultivation represents the main economic livelihood for smallholder farmers and landowners in several producing countries in Africa, Central America, and South America (Phillips-Mora et al, 2007; Baligar et al, 2008). Cacao bean production reached 4.6 million tons in 2017/2018 (ICCO, 2020), which benefited approximately 40 to 50 million people (Carr and Lockwood, 2011; Voora et al, 2019) and the global market is growing predicted to increase 7.3% from 2019 to 2025 (Voora et al, 2019). Some important threats related to global warming may reduce this growth prediction during incoming decades, such as the higher incidence of water deficit and drought periods in current cacao cultivation areas (Läderach et al, 2013; WCF (World Cocoa Foundation), 2014; Schroth et al, 2016; Medina and Laliberte, 2017; Farrell et al, 2018; Gateau-Rey et al, 2018; Lahive et al, 2019; Hebbar et al, 2020). The transpiration rate exceeds water absorption by the roots, thereby reducing the water content in tissues and affecting both nutrient uptake and photosynthesis (Ahuja et al, 2010; Osakabe et al, 2014; Demidchik, 2015)
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