Commercial cultivars Arugula Eruca sativa and lettuce Lactuca sativa (Canasta) were grown in aeroponics systems in tropical Singapore with root zone temperature (RZT) kept constantly chilled at 25°C. At 30 days after transplantation (DAT) leaf discs excised from the plants were subjected to heat stress from 26, 30, 34, 38, 42 and 46°C for 1 hour and the magnitude of heat shock protein synthesis was recorded. Based on the heat shock protein kinetic profile a sub-lethal heat shock regime was designed at 38°C. While control plants were kept at RZT 25°C, treatment plants were initially kept at 25°C RZT from transplantation till 10 DAT, and subsequently subjected to a daily 38°C RZT from 1000 hrs to 1600 hrs to simulate a noon-time heating without chiller effect from 11 DAT to 22 DAT. Root scan analysis was performed every 3 days to examine if the sub-lethal heat shock would affect root growth and morphology. From 21 to 30 DAT the plants were divided into four groups: (i) CC: plants were kept constantly at 25°C RZT from transplanting to harvest; (ii) CH: plants kept in 25°C RZT from 0 to 22 DAT and then subjected to a daily heat stress of 45°C of 6 hours (1000 hrs to 1600 hrs) from 23 to 32 DAT; (iii) HC: plants subjected to 38°C RZT sub-lethal heat shock from 11 to 22 DAT and kept in 25°C RZT from 23 to 32 DAT and (iv) HH: plants subjected to 38°C RZT sub-lethal heat shock 11 to 22 DAT and then subjected to a daily heat stress of 45°C of 6 hours (1000 hrs ~ 1600 hrs) from 23 to 32 DAT. Plants were harvested 33 DAT and examined for their productivity, maximal photosynthetic oxygen evolution (Pmax), chlorophyll fluorescence Fv/Fm ratio and other chlorophyll fluorescence parameters, total reduced nitrogen (TRN) and total sugar content. CH plants initially shows a significantly lower Fv/Fm ratio compared to the other treatment groups (26 DAT), but by 32 DAT there was no significant difference in the four treatment groups. CH treatment group for both vegetables were also significantly lowest for shoot fresh weight, Pmax and insoluble sugar. Conversely, Arugula HC and CH plants had the lowest soluble sugar and TRN respectively while there were no significant differences between the four treatment groups for Canasta for TRN and soluble sugar. Preliminary results of this study suggests that as long as root zone chilling is applied during the root formative period (0 DAT to 11 DAT) temperature is not a very important environmental stressor for later growth of these temperate vegetables as long as water in the form of nutrient solution is constantly available. A sub-lethal heat shock would also improve crop productivity to approach that of plants constantly kept at chilled RZT despite application of high heat stress (46°C for 6 hours daily).
Take Away Notes:
• Dosage response of the vegetables to heat stress and optimal thermal window is elucidated.
• Findings of this research could be directly applied in aeroponics practices especially in the tropical regions.
• Energy consumption in aeroponics systems could be reduced by 30~50% with this finding.
• Lethal thermal limit of studied vegetables established.