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Effects of rootzone temperature and irradiance on the growth and photosynthetic characteristics of certain subtropical vegetable crops
Author
Choong, Tsui Wei
Supervisor
He, Jie
Abstract
Subtropical bell-pepper (Capsicum annuum Indra) Kai Lan or Chinese broccoli (Brassica alboglabra Bailey) and tomato (Lycopersicon esnrlentum L. Karst) plants were used in this study. They were aeroponically grown in a greenhouse m three different rootzone temperatures (RZTs) of 25oC, 30oC and fluctuating ambient, while their shoots were exposed to diurnal naturally fluctuating ambient temperatures under 100% and 70% prevailing solar radiation. The growth and photosynthetic characteristics of these plants were studied.
All the plants that had both shoots and roots exposed to hot ambient temperatures under 100% and 70% of prevailing solar radiation had photosynthetic rates that were much lower than those grown under cool RZTs with the same solar conditions. Further, these same plants had lower photosynthetic rates, under 100% prevailing solar radiation, indicating that high solar radiation may also contribute to the susceptibility of plants to hot ambient temperatures. Associated with these reduced photosynthetic rates were poor root system development, lower shoot production and smaller leaf areas. However, all the three crops flourished with faster growth (highest shoot and root fresh weights were obtained as well as highest total leaf area per plant) and higher photosynthetic rates at cool optimum RZTs of 30oC for Capsicum and tomato, and 25oC for Kai Lan, under 100% prevailing solar radiation.
Plants grown under different RZTs showed different patterns of photoassimilate partitioning which is expressed as a root/ shoot ratio. Capsicum exhibited higher ratios at their younger stages when they were grown at ambient-RZTs as compared to those grown at cool-RZTs. However, there was no significant difference (p > 0.01) among the root/ shoot ratios when the plants were in their mature stages. The changes in the root/shoot ratios of Kai Lan and tomato were similar. The ratios at cool-RZTs were very close in their early stages, increased three weeks after transplanting, peaked within a week and then declined. The rate of development of root systems in Kai Lan at ambient-RZTs was also much slower. All plants grown under cool-RZT had higher shoot water content. This could be due to reduced water uptake by the poorly-developed root systems when the plants were grown under hot-ambient temperature.
There were no changes in chlorophyll content and PS I1 efficiency (measured as F,/F, ratio)in Capsicum and tomato plants grown at all RZTs, indicating the absence of chronic photoinhibition. However, the decline in chlorophyll content and reduction in F,/F, ratios observed in Kai Lan at ambient-RZTs indicated chronic photoinhibition in their leaves.
In the second part of the study, Capsicum was grown at 30oC-RZT, ambient-RZT, 30oC-RZT but transferred to ambient-RZT and at ambient-RZT but transferred to 30oC-RZT at 2, 3, 4 and 5 weeks after transplanting. Similarly, Kai Lan was grown at 25oC-RZT, ambient-RZT, 25oC-RZT but transferred to ambient-RZT and at ambient-RZT but transferred to 25oC-RZT at 2, 3 and 4 weeks after transplanting. All plants were grown under 100% prevailing solar radiation in this study. The chlorophyll fluorescence ratio, chlorophyll and carotenoid content, relative water content and harvest yields were measured. All plants experienced no attenuation of light intensity during the middle of the day, resulting in dynamic photoinhibition which allowed the plants grown at cool-RZTs to recover rapidly while those grown at ambient-RZTs sustained more photoinhibition. Based on shoot fresh weight measurements, it was found that Kai Lan required root cooling throughout its whole growth period whereas Capsicum only required cooled RZTs for up to the first 3 weeks after transplanting, for the fruit fresh weights to be equivalent to those grown in 30oC-ET. Thus, this decrease in cooling time will allow commercial growers to reduce production costs.
All the plants that had both shoots and roots exposed to hot ambient temperatures under 100% and 70% of prevailing solar radiation had photosynthetic rates that were much lower than those grown under cool RZTs with the same solar conditions. Further, these same plants had lower photosynthetic rates, under 100% prevailing solar radiation, indicating that high solar radiation may also contribute to the susceptibility of plants to hot ambient temperatures. Associated with these reduced photosynthetic rates were poor root system development, lower shoot production and smaller leaf areas. However, all the three crops flourished with faster growth (highest shoot and root fresh weights were obtained as well as highest total leaf area per plant) and higher photosynthetic rates at cool optimum RZTs of 30oC for Capsicum and tomato, and 25oC for Kai Lan, under 100% prevailing solar radiation.
Plants grown under different RZTs showed different patterns of photoassimilate partitioning which is expressed as a root/ shoot ratio. Capsicum exhibited higher ratios at their younger stages when they were grown at ambient-RZTs as compared to those grown at cool-RZTs. However, there was no significant difference (p > 0.01) among the root/ shoot ratios when the plants were in their mature stages. The changes in the root/shoot ratios of Kai Lan and tomato were similar. The ratios at cool-RZTs were very close in their early stages, increased three weeks after transplanting, peaked within a week and then declined. The rate of development of root systems in Kai Lan at ambient-RZTs was also much slower. All plants grown under cool-RZT had higher shoot water content. This could be due to reduced water uptake by the poorly-developed root systems when the plants were grown under hot-ambient temperature.
There were no changes in chlorophyll content and PS I1 efficiency (measured as F,/F, ratio)in Capsicum and tomato plants grown at all RZTs, indicating the absence of chronic photoinhibition. However, the decline in chlorophyll content and reduction in F,/F, ratios observed in Kai Lan at ambient-RZTs indicated chronic photoinhibition in their leaves.
In the second part of the study, Capsicum was grown at 30oC-RZT, ambient-RZT, 30oC-RZT but transferred to ambient-RZT and at ambient-RZT but transferred to 30oC-RZT at 2, 3, 4 and 5 weeks after transplanting. Similarly, Kai Lan was grown at 25oC-RZT, ambient-RZT, 25oC-RZT but transferred to ambient-RZT and at ambient-RZT but transferred to 25oC-RZT at 2, 3 and 4 weeks after transplanting. All plants were grown under 100% prevailing solar radiation in this study. The chlorophyll fluorescence ratio, chlorophyll and carotenoid content, relative water content and harvest yields were measured. All plants experienced no attenuation of light intensity during the middle of the day, resulting in dynamic photoinhibition which allowed the plants grown at cool-RZTs to recover rapidly while those grown at ambient-RZTs sustained more photoinhibition. Based on shoot fresh weight measurements, it was found that Kai Lan required root cooling throughout its whole growth period whereas Capsicum only required cooled RZTs for up to the first 3 weeks after transplanting, for the fruit fresh weights to be equivalent to those grown in 30oC-ET. Thus, this decrease in cooling time will allow commercial growers to reduce production costs.
Date Issued
1998
Call Number
SB321 Cho
Date Submitted
1998