Effects of Exogenous Application of Kaolin on Physiology, Growth and Yield of Potato Plants under High Temperature

Mahjuba Akter

Potato (Solanum tuberosum L.) is a key crop for global food security. As global temperatures rise due to climate change, and production expands into warmer regions, potato is expected to face more frequent and prolonged high-temperature episodes. This is a major concern because potato has a low optimum temperature for tuber development. Temperatures above 25 ˚C have been shown to impair potato tuber growth and yield. Applying kaolin to leaves has been shown to mitigate the impact of high temperature in some crops by reducing leaf and fruit temperature, but there are no studies on the benefits of using kaolin to reduce leaf temperature in potato. Therefore, four experiments were undertaken in a temperature controlled, partitioned greenhouse in Perth, Western Australia to explore whether kaolin applied at 35 days after planting (DAP) (after the confirmation of tuber initiation) could ameliorate the negative impact of high temperature (30 ˚C) stress in potato. Potato cultivars Royal Blue (RB) and Nadine (N) were chosen for the study because they are putatively different in heat tolerance; within the industry in Western Australia, RB is considered sensitive to high temperature and N is considered to be somewhat tolerant. Experiments 1 (RB) and 2 (N) were undertaken concurrently with the aims of determining the optimal kaolin application rate for potatoes and assessing its impact on leaf temperature and plant performance. Plants were sprayed with four rates of kaolin (0, 4, 6, and 8%; hereafter 0K, 4K, 6K, and 8K) before being transferred to 30 ˚C for 10 days. Leaf temperature (Tleaf) and stomatal conductance (gs) of the 4th and 7th leaves were measured on Days 2, 4, 6, 8, and 10 during the 10-day episode of high temperature. Plant height and dry mass were measured at the end of the episode. In both cultivars, 6K was the most effective in mitigating the negative impact of high temperature on plant and tuber growth, reducing Tleaf in two leaves of RB by 1.6 ˚C and N by 1.3 to 1.4 ˚C, and maintaining higher gs in RB (36 to 44%) and N (16 to 31%). The 6K treatment resulted in increased leaf and tuber dry mass. The greater dry mass may have been due to an increase in leaf area per plant or an increase in net photosynthesis (A) per unit leaf area. Therefore, in the following experiment (Experiment 3), the impact of kaolin on A and the area of newly emerged and expanded leaves were explored. Six percent kaolin was chosen for use in all subsequent experiments. Experiment 3 was designed to determine whether the higher dry mass production in the kaolin treated plants was due to a change in A or leaf area of the two contrasting potato cultivars, RB and N, under a 30 ˚C episode of 15 days. This experiment also revealed how high temperature reduce dry mass and showed that kaolin partially protected against this loss, significantly aiding in its recovery. The losses due to high temperature were greater in RB than N but the impact of kaolin was greater in RB. Kaolin application did not completely negate the adverse effects of high temperature (30 ˚C) when compared to the control group maintained at 21 ˚C. However, it is evident that kaolin provided significant benefits to the sensitive cultivar when exposed to 30 ˚C, making it better able to cope with the high-temperature stress than it would have without the kaolin application. For example, at 30 ˚C, kaolin maintained higher A (RB by 42%; N by 24%), new leaf area (RB by 37%; N by 11%), and tuber dry mass (above 60% in both cultivars). The higher amount of tuber dry mass in kaolin-treated plants was attributed to higher levels of A. The A in kaolin-treated plants might be facilitated by the increased growth of new leaves during a high-temperature episode. Experiment 4 was conducted to determine the extent to which kaolin affects the photosynthetic performance of new leaves (e.g., the 13th leaf) that emerged and grew during a high-temperature episode. Additionally, the experiment examined the differences in A among leaves of three different ages (e.g., the 4th, 7th, and 13th leaves) in response to kaolin application during a period of high-temperature stress. It was hypothesized that the lower leaf temperature due to the application of kaolin would alleviate the loss of chlorophyll from the leaves, which, in turn, would be related to higher A. So, this experiment explored whether kaolin application has a differential effect on the chlorophyll contents in leaves of three different ages. Furthermore, this experiment aimed to evaluate how kaolin application during a high-temperature episode continues to influence plant growth, yield, and leaf senescence 30 days after exposure to high temperature. There were four treatments in this experiment: (1) 0K-21: no kaolin, plants were continuously at 21 ˚C; (2) 0K-30: no kaolin, plants were at 21 ˚C until 34 DAP, then shifted to 30 ˚C on 35 DAP for 15 days, and finally returned to 21 ˚C for an additional 30 days (until 80 DAP); (3) 1K-30: similar to 0K-30 except with a kaolin application on 35 DAP; and (4) 2K-30: similar to 1K-30 except with a second application of kaolin on 45 DAP. The final destructive sampling was done on 80 DAP due to onset of leaf senescence in high-temperature-treated plants. The results of this experiment indicated that the 13th leaf was better suited to high-temperature condition. However, applying kaolin had a significant positive impact on its A and chlorophyll levels, similar to the impact observed on leaves that had formed before the high-temperature episode started (i.e., the 4th and 7th leaves). This experiment also revealed that kaolin application during a high-temperature episode led to increased whole plant green leaf area, reduced leaf senescence, and higher tuber dry mass, with these benefits persisting for 30 days after the stress ended. For example, at the end of the high-temperature episode, the plants treated with kaolin maintained a 21-24% greater whole plant green leaf area, exhibited 53-64% less leaf senescence, and had 90-138% more tuber dry mass than the untreated plants. Thirty days after the episode ended, the treated plants still showed an 8-9% greater whole plant leaf area, 37-66% less leaf senescence, and 52-60% more tuber dry mass. These findings underscore the efficacy of kaolin as a protective strategy against both the immediate and lasting negative impacts of high-temperature stress on the potato plant performance. This is the first study on applying kaolin to potato shoots under high temperature to explore how it influences Tleaf and A, and how these changes affect tuber yield. Although at 30 ˚C, the kaolin-treated plants had lower Tleaf and higher levels of A and tuber dry mass compared to plants without kaolin, the kaolin application did not fully offset the negative impact of 30 ˚C compared to the control (21 ˚C). However, at 30 ˚C, the application of kaolin led to an increase in leaf area, potentially facilitating A. The lower Tleaf in kaolin-treated plants was also associated with higher A, which in turn was related to higher leaf chlorophyll content during a high-temperature episode. Kaolin application during a high-temperature episode had both concurrent and ongoing positive effects on plant growth and yield. These results suggest that kaolin application could be an effective approach to mitigate stress in potato plants during high-temperature episodes. This is particularly relevant due to the global warming challenges that agriculture is facing. Further study is necessary to determine whether there are commercial benefits to be gained by applying kaolin to potato crops in the field when high temperatures are forecast.

Effects of Exogenous Application of Kaolin on Physiology, Growth and Yield of Potato Plants under High Temperature

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