Abstract
Keeping in view the yield losses instigated by heat stress in several crops, we carried out an experiment to explore the curative effect of exogenous applications of proline on the morpho-physiological, biochemical, and water-related attributes of okra genotypes under high-temperature stress (controlled conditions). Four contrasting genotypes C1, C2, C3, and C4 heat tolerant and heat sensitive genotypes were selected from a diverse panel of okra genotypes (n = 100) to examine plant responses to high-temperature stress and exogenous application of proline. Four-week-old seedlings were subjected to heat stress by gradually increasing the temperature of a growth chamber from 28/22 °C to 45/35 °C (day/night) and sprayed with an optimized proline concentration 2.5 mM. The experiment consisted of a factorial arrangement of treatments in a completely randomized design. The results showed that there were maximum increases in shoot length (32.7%), root length (58.9%), and shoot fresh (85.7%). The quantities of leaves per plant were increased by 52.9%, 123.6%, 82.5%, and 62.2% in C1, C2, C3, and C4 after proline application. On the other hand, only root fresh weight decreased in all genotypes after proline application by 23.1%, 20%, 266.7%, and 280.8% (C1, C2, C3, C4). A lower leaf temperature of 27.72 °C, minimum transpiration of 3.29 mmol m−2 s−1, maximum photosynthesis of 3.91 μmol m−2 s−1, and a maximum water use efficiency of 1.20 μmol CO2 mmol H2O were recorded in the genotypes C2, C1, C3, and C4, respectively. The highest enzymatic activity of superoxide dismutase, peroxidase and catalase were 14.88, 0.31, and 0.15 U mg-protein in C2, C1, and C3, respectively. Maximum leaf proline, glycinebetaine, total free amino acids, and chlorophyll content 3.46 mg g−1, 4.02 mg g−1, 3.46 mg g−1, and 46.89 (in C2), respectively, due to foliar applications of proline. Another important finding was that heat tolerance in okra was highly linked highly linked to genotypes’ genetic potential, having more water use efficiency, enzymatic activities, and physio-biochemical attributes under the foliar applications of proline.
Highlights
Climate change is a great threat to the food security of the globe
The results indicated that proline application significantly enhanced the water potential of okra genotypes (Figure 3B)
A similar case was reported in a study conducted on wheat under copper stress [39,40] The number of leaves per plant was higher in heat-tolerant genotypes, as compared to heat-sensitive genotypes, because heat stress-tolerant okra genotypes retained their leaves and accumulated high proline in the leaf, while most of the leaves in heat-sensitive okra genotypes dropped their leaves, as discussed in the Results section (Figure 4)
Summary
Climate change is a great threat to the food security of the globe. It has been observed that there were increases of 0.85 ◦ C and 0.6 ± 2 ◦ C in the average temperature of the surface of the globe over the past two centuries [1,2]. High-temperature stress leads to drought stress, and a combination of both these stresses give birth to a third stress, namely salt stress or soil salinization All of this stress causes crop production losses, imposing food shortage for future generations [6,7]. Each degree rise in seasonal temperature cause a crop loss of 17% [2,9]. When plants are exposed to high temperature, at the anatomical level, there is a reduction in cell size, stomatal closure, excessive water loss, and increases in trachomatous and stomatal densities [12]. There is reduced water potential of the leaf [12], increased transpiration rate, water loss, and many physiological processes halt [15]. Heat stress causes damage to every step of plant life, starting from seed sowing to crop production and yield
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