Response of potato varieties to an episode of high temperature

Hassan Sardar

Hunger is on the rise because the global population is increasing more rapidly than food production (Chapter 1). Potato is one of the four most important food crops in the world, along with rice, wheat and maize. Importantly, potato is vulnerable to high temperature. The exposure of potato crops to high temperature has been increasing because of increasing global temperature and its increasingly wide spread cultivation (See Section 2.1.3-2.1.5). In the field situation, potato crops face episodes of high temperature rather than a continuous high temperature (See Section 2.2.1). However, most of the potato and high temperature studies have used continuous high temperature (See Section 2.1.4). While studies have shown that potato varieties respond differently to high temperature, there is very little information about why genotypes differ in their response to an episode of high temperature (See Section 1.2, 2.3.1, 2.3.2). In this study I investigated the difference in the response of four Western Australian commercial potato varieties to an episode of high temperature. All experiments were carried out in naturally lit glasshouses of Murdoch University. A temperature of 20 ± 2°C was used as the control because the optimum temperature range for potato is 14-22°C (Chapter 2). A high temperature episode of 30 ± 1°C was imposed for a pre-determined period (9 days in Experiments 1-3 and 14 days in Experiments 4 and 5). All plants were initially grown in a single compartment at 20 ± 2°C. Photoperiod ranged from 10h to 14h throughout the study (Fig. 3.1) and average light intensity ranged from 153 J s-1 m-2 to 391 J s-1 m-2 in the different experiments as they were performed in different seasons of the year (Table 3.1). Relative humidity ranged from 37% to 87% during the experimental period in different experiments (Table 5.2). To impose the episode of high temperature, half of the plants in each experiment were transferred to a different compartment of the same glasshouse and brought back to the base temperature at the end of the episode. After screening seven different potato varieties (Laura, Nadine, Carisma, Atlantic, Maranca, Royal Blue, Rodeo), four varieties (Atlantic, Maranca, Nadine & Laura) were selected for further study based on their contrasting response to the high temperature treatment. In Experiment 3, a 9-day episode of high temperature imposed after tuber initiation (43-52 DAP) resulted in the total dry matter accumulation and partitioning to tubers being reduced to a similar extent in all varieties when measured at the end of episode (52 DAP). However, the varieties recovered differently (Chapter 5). Atlantic recovered well, accumulating 18% greater DM than the control after the episode (53-81 DAP). Nadine and Laura accumulated respectively, 52% and 69% less DM than control in the same period (53-81 DAP). Maranca also accumulated 36% less DM than control but showed a noticeable recovery in terms of the number of large size tubers. The difference in the dry matter accumulation of the varieties between 52 and 81 DAP was related to the relative impact of the high temperature on the green leaf area of the varieties at the end of episode (52 DAP) (Chapter 6). No impact on the number of tubers was detected at the end of episode (52 DAP) but differential effects of high temperature on the number of tubers of varieties were observed at the end of Experiment 3 (81 DAP) (Chapter 5). The number of tubers remained unaffected in Atlantic and Nadine, and were reduced by high temperature in Laura. Interestingly, in Maranca, the number of tubers was increased by high temperature. Atlantic also recovered in terms of the mean tuber dry weight having only 20% reduction relative to control at the end of experiment while which was significantly smaller than the reduction caused in Maranca (-60%), Laura (-47%) and Nadine (-42%). In the final two experiments, the varieties were studied in pairs to allow more detailed measurements: Atlantic and Maranca were studied in Experiment 4, and Laura and Nadine were studied in Experiment 5. In both experiments, high temperature was imposed before the start of tuberization, i.e., at stolon hooking (HTH). In each experiment, the episode was continued until at least one group of plants started tuberization. In each case, the duration of the episode was two weeks, i.e., 24-38 DAP in Experiment 4 and 21-35 DAP in Experiment 5. In Experiment 4, there was an additional high temperature treatment which involved an episode imposed on a separate set of plants at the start of stolon tip swelling (HTI). This allowed a direct comparison to see whether the relative thermotolerance level of varieties differed between the stages of development. In Experiment 4 total plant dry matter of Atlantic and Maranca was not affected by the end of episode (38 DAP) but was reduced after the end of the episode (Chapter 5) in a manner consistent with the green leaf area (Chapter 8). The total plant dry matter accumulation, partitioning and stolon growth and development of the varieties were affected similarly by HTH but not their tuber size (Chapter 5, 7 & 8). Tuberization was significantly delayed by HTH in both varieties which was associated with the reduced expression of StSP6A in HTH plants, however, Atlantic gradually improved its expression of StSP6A under HTH conditions, thus showed a tendency to tuberize earlier than Maranca towards the end of episode (Chapter 7). Ultimately, the tuber size of Atlantic was affected less than that of Maranca. In contrast to HTH, the HTI treatment did not affect the number of tubers nor the total plant DM or leaf area. It did however reduce the partitioning of DM to tubers. Control plants had 54% harvest index while HTI plants had only 29% harvest index at the end of experiment. Importantly, HTI and HTH caused similar reductions in the tuber DM in Maranca but Atlantic was affected less by HTI than by HTH. Experiment 5 ended with the following key results; (i) HTH reduced the number of swollen tips of stolons at the end of episode (35 DAP) more in Laura than in Nadine (Chapter 7), (ii) number of tubers reduced similarly in both varieties at the end of experiment (Chapter 7) (50 DAP) but number of large size tubers reduced only in Nadine while Laura remained unaffected (Chapter 7), (iii) HTH affected the total plant dry matter and partitioning of dry matter to stolons and tubers at the end of episode (35 DAP) as well as at the end of experiment (50 DAP) (Chapter 5) which coincided well with the green leaf area (Chapter 8), (iv) however, total plant dry matter and partitioning of dry matter to tubers was affected more in Nadine than in Laura at the end of experiment (50 DAP) (Chapter 5) which may have been due to the shift of a large proportion of leaf area to branches in Nadine (Chapter 8), (iv) tuber size of Laura was affected less than Nadine which may have been because of an adjustment of tuberization initially according to the total plant dry matter (Chapter 7). Throughout the study, the major differences between the response of varieties to high temperature were observed after the end of high temperature episode; reflecting a variety’s capacity to recover from the impact of an episode of high temperature. The large number of studies in which the high temperature regime is continued through to the end of the experiment cannot account for this aspect. Given that crops in the field are likely to experience episodes of high temperature, this may have significant practical implications when considering the performance of a variety under field conditions. Across the four cultivars, the capacity to recover from a high temperature episode was inversely related to their loss of leaf area during the episode. This needs to be tested across a wider set of germplasm. Importantly, the relative response of the varieties to a high temperature episode differed depending on the growth stage when the episode was imposed. This points to the value of thinking about ‘thermotolerance’. There may be greater value in considering the situation-specific thermotolerance in screening and breeding programs rather than looking for general or overall thermotolerance. Key words: High temperature, episode, potato, varieties, dry matter, tubers

Response of potato varieties to an episode of high temperature

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