The effect of short-term vs. long-term soil moisture stress on the physiological response of three cocoa (Theobroma cacao L.) cultivars

Abstract

Understanding water stress signaling mechanisms and screening for tolerant cocoa cultivars are major challenges when facing prolonged dry and rainy seasons in cocoa-producing areas. While abscisic acid (ABA) and proline are supposed to enhance drought tolerance in cocoa, the role of polyamines remains unclear. The aim of this study was to investigate the biochemical response and phenological adaptation of cocoa (Theobroma cacao) on different soil moisture conditions, with a focus on short-term (20 days) and long-term (89 days) stress conditions, and to compare the performance of three cocoa cultivars. In a split plot design with four blocks, cocoa seedlings of an international high-yielding cultivar (TSH-565) and two locally selected cultivars (IIa-22 and III-06) from the drought-exposed Alto Beni region, Bolivia, were arranged in pots under a roof shelter (cultivar: three levels). The seedlings were exposed to strong (VERY DRY) and moderate (DRY) soil moisture deficits, water logging (WET) and regular irrigation (MOIST) that served as a control (moisture: four levels). We examined the growth performance and the levels of ABA, proline, and polyamines in the leaves. Growth was reduced already at a moderate drought, while severe drought enhanced seedling mortality. Severe drought increased the levels of ABA by 453% and of proline by 935%, inducing a short-term stress response; both compounds were degraded over the long-term period. The polyamine concentration was unrelated to soil moisture. The cocoa cultivars did not differ in their biochemical response to soil moisture stress (proline: p-value = 0.5, ABA: p-value = 0.3), but the local cultivar III-06 showed a stronger height growth increment than the international cultivar TSH-565 (237%, p-value = 0.002) under drought conditions.

Proline content in the leaves of poplar and willow under water deficit

“…Probably, stressed plants adapted their proline rates up to the control level in less than 30 days. Similar studies were conducted by Niether et al (2020) on leaves of Theobroma cacao L. when during prolonged drought stress (more than 21 days) the amount of proline decreased to control values. Diaz et al (2005) in their studies on the leaves of Lotus japonicus, observed that the rate of proline accumulation was always higher during acute drought than during chronic drought.…”

Roles of brassinosteroids in plant growth and abiotic stress response

“…Drought stress is one of the main adverse factors affecting plant growth and metabolism (Todaka et al 2015;Liu et al 2012;Song and Wang 2015;Niether et al 2020). It can destroy the enzyme system, lead to stomatal closure, reduce water content, decrease seed germination potential, and significantly diminish the length of roots and stems (Guo et al 2018b;You et al 2019).…”

Effect of flooding stress on cocoa (Theobroma cacao L.)

“…Furthermore, the content of starch and soluble sugars, in general, was reduced in the roots of plants subjected to waterlogging 13 , pointing out the high susceptibility of this organ to excessive water. In addition, proline might be an indicator of stress due to excessive water, but only for short term response, although it depends on cultivar studied 8 .…”

“…Therefore, there are two phenomena associated to climate change: drought and flooding. Although, the damage caused by water deficit is more intense than water excessive 8 , however, the waterlogging can disrupt the good performance of photosynthesis 9 and it decreases the yield of crops 10 . For that reason, it can be considered a threat to the food security 11 .…”