Baby Juby, Minimol J S, Santhoshkumar A V, Joseph Jiji, Abd-ElGawad Ahmed M, Ullah Fazal
Department of Plant Breeding and Genetics, College of Agriculture, KAU, Vellanikkara, Thrissur, 680656, India.
Cocoa Research Centre, Kerala Agricultural University, Vellanikkara, Thrissur, 680656, India.
BMC Plant Biol. 2025 Apr 21;25(1):501. doi: 10.1186/s12870-025-06448-3.
Water stress affects the most important determinants of yield-canopy architecture, photosynthesis, and partitioning of assimilates. Being a perennial crop, the water requirement in cocoa is fairly high. Therefore, identifying drought-tolerant plant varieties within cocoa species is critically important, especially in the context of ongoing climate variability attributed to climate change. This research aimed to develop cultivars that efficiently utilize available/minimal water resources and maintain optimal yields despite environmental stressors. Hence, efforts were taken to identify drought-tolerant genotypes, taking physiological characteristics as the main parameters. Drought-tolerant parents (M 13.12, G I 5.9, G II 19.5, and G VI 55) identified in previous studies were crossed in all possible combinations to attain drought-tolerant hybrids. All the crosses were obtained except for GV1 55 x M 13.12, which may be due to an incompatibility reaction between these genotypes, which is a common mechanism in cocoa. The hybrids were then subjected to drought stress (under 40% field capacity) and were analyzed for various physiological parameters such as chlorophyll stability index, membrane stability, relative water content, photosynthetic rate, transpiration rate, leaf temperature and chlorophyll content.
The highly tolerant and tolerant hybrids in almost all the crosses studied had relatively high chlorophyll stability index (highest being 86.73% in highly tolerant hybrid of cross M 13.12 x G I 5.9), membrane stability (highest value of 86.36% observed in tolerant hybrids of cross M 13.12 x G I 5.9), relative water content (highest value being 79.43% observed in the highly tolerant hybrid of cross G II 19.5 x G VI 55 while the lowest value of 23.51% being shown by the susceptible hybrids of cross G VI 55 x G II 19.5), photosynthetic rates (highest being 1.627 µmol CO ms observed in cross M 13.12 x G I 5.9) and chlorophyll content (highest being 41.27 SPAD units observed in the highly tolerant hybrids of G II 19.5 x G VI 55) as compared to susceptible hybrids. Tolerant hybrids had lower transpiration rates (lowest being 0.306 mmol HO m s in cross G VI 55 x G II 19.5) than susceptible hybrids (highest being observed in the susceptible hybrids of cross M 13.12 x G I 5.9 having the value of 2.067 mmol HO m s) indicating their efficiency in handling water stress. However, all the tolerant, susceptible and fully irrigated hybrids showed comparable values ranging between 30 and 33 C for the leaf temperature indicating the efficiency of cocoa plants in regulating the water stress even during drought. Correlation and path analysis revealed that relative water content and photosynthetic rate were positively associated with the dependent variable, the number of leaves retained. However, the transpiration rate showed a negative correlation with several parameters such as cell membrane stability (-0.550), relative water content (-0.528) and chlorophyll stability index (-0.319). Binary regression analysis indicated that relative water content and photosynthetic rate will show 51.87 and 66.29% improvement over the base population, respectively, if used as selection criteria for a new population in future drought breeding programmes.
This experiment indicated how the plants were able to regulate various physiological mechanisms under drought stress and how these parameters can be utilized in distinguishing between drought tolerant and susceptible plants. Ranking based on the values of physiological parameters revealed that crosses M 13.12 x G I 5.9, G II 19.5 x G VI 55, and G II 19.5 x G I 5.9 produced hybrids with favourable responses to drought tolerance. It indicated that these crosses could be further utilized in developing drought-tolerant genotypes. Further, the binary regression studies indicated that relative water content and photosynthetic rate can be used as selection parameters to identify drought-tolerant hybrids more efficiently in the future cocoa stress breeding programmes.
水分胁迫会影响产量的最重要决定因素——冠层结构、光合作用和同化物分配。作为一种多年生作物，可可的需水量相当高。因此，在可可品种中鉴定耐旱植物品种至关重要，尤其是在气候变化导致气候持续变化的背景下。本研究旨在培育出能够有效利用可用/最少水资源并在环境胁迫下保持最佳产量的品种。因此，以生理特征为主要参数，努力鉴定耐旱基因型。将先前研究中鉴定出的耐旱亲本（M 13.12、GI 5.9、G II 19.5和G VI 55）进行所有可能的杂交组合，以获得耐旱杂种。除了GV1 55×M 13.12之外，所有杂交组合均已成功获得，这可能是由于这些基因型之间的不亲和反应，这在可可中是一种常见机制。然后让杂种经受干旱胁迫（田间持水量为40%），并分析各种生理参数，如叶绿素稳定性指数、膜稳定性、相对含水量、光合速率、蒸腾速率、叶片温度和叶绿素含量。
在所研究的几乎所有杂交组合中，高度耐旱和耐旱的杂种具有相对较高的叶绿素稳定性指数（在杂交组合M 13.12×GI 5.9的高度耐旱杂种中最高，为86.73%）、膜稳定性（在杂交组合M 13.12×GI 5.9的耐旱杂种中观察到的最高值为86.36%）、相对含水量（在杂交组合G II 19.5×G VI 55的高度耐旱杂种中观察到的最高值为79.43%，而在杂交组合G VI 55×G II 19.5的敏感杂种中最低值为23.51%）、光合速率（在杂交组合M 13.12×GI 5.9中最高，为1.627 μmol CO₂ m⁻² s⁻¹）和叶绿素含量（在G II 19.5×G VI 55的高度耐旱杂种中最高，为41.27 SPAD单位），与敏感杂种相比。耐旱杂种的蒸腾速率（在杂交组合G VI 55×G II 19.5中最低，为0.306 mmol H₂O m⁻² s⁻¹）低于敏感杂种（在杂交组合M 13.12×GI 5.9的敏感杂种中观察到的最高值为2.067 mmol H₂O m⁻² s⁻¹），表明它们在应对水分胁迫方面的效率。然而，所有耐旱、敏感和充分灌溉的杂种叶片温度在30至33℃之间显示出可比的值，表明可可植物即使在干旱期间调节水分胁迫的效率。相关性和通径分析表明，相对含水量和光合速率与因变量保留的叶片数呈正相关。然而，蒸腾速率与几个参数呈负相关，如细胞膜稳定性（-0.550）、相对含水量（-0.528）和叶绿素稳定性指数（-0.319）。二元回归分析表明，如果在未来的干旱育种计划中用作新群体的选择标准，相对含水量和光合速率分别比基础群体提高51.87%和66.29%。
本实验表明了植物在干旱胁迫下如何调节各种生理机制，以及这些参数如何用于区分耐旱和敏感植物。根据生理参数值进行排名显示，杂交组合M 13.12×GI 5.9、G II 19.5×G VI 55和G II 19.5×GI 5.9产生的杂种对耐旱性有良好反应。这表明这些杂交组合可进一步用于培育耐旱基因型。此外，二元回归研究表明，相对含水量和光合速率可作为选择参数，以便在未来的可可胁迫育种计划中更有效地鉴定耐旱杂种。
