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Root-zone temperature effects on the physiology of certain temperate and subtropical crops
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Type
Thesis
Author
Tan, Lay Pheng
Supervisor
He, Jie
Lee, Sing Kong
Abstract
The aeroponic system allows temperate Lactuca sativa cv Panama (lettuce) and subtropical Capsicum annuum Indra F1-hybrid (capsicum) to grow all year round in the tropics by simply cooling their roots while their aerial portions are subjected to the hot fluctuating greenhouse temperature. Cool root-zone temperature (C-RZT) plants had higher total leaf area, shoot and root weight. By contrast, hot and ambient RZT (A-RZT) resulted in poor shoot and root development of lettuce and capsicum. Reciprocal RZT transfer experiments were conducted to investigate the mechanisms of RZT effects on lettuce and capsicum physiology. The studies on both stomatal (A, gs and RWC) and non-stomatal (Pmax, chlorophyll, soluble protein, Rubisco protein and Rubisco activation state) effects of photosynthesis revealed that stomatal limitation was the primary event for lettuce transferred from C=>A-RZT. For instance, the rapid decline of A in C=>A-RZT lettuce following RZT transfer was largely attributed to stomatal closure. The stomatal closure may be due to perturbed water relations. C=>A-RZT lettuce experienced moderate water deficits, exhibited by decreased RWC. This was also supported by the rapid recovery of the above parameters in A=>C-RZT lettuce plants. However, C=>A-RZT and A=>C-RZT capsicum displayed delayed responses in A, gs and midday RWC compared to lettuce. Generally, A=>C-RZT resulted in higher Pmax, quantum yields, total chlorophyll contents, leaf soluble proteins, Rubisco proteins and Rubisco activities for lettuce and capsicum as compared to A-RZT plants. C=>A-RZT had reverse effects on these parameters. The overall productivity of A-RZT capsicum may be associated with lower availability of Rubisco proteins and activities. This was found to be associated with N deficiency, an important element involved in the synthesis of these proteins for the light-independent pathway of photosynthesis. However, for lettuce, the lower productivity at A-RZT could be due to both stomatal and non-stomatal limitations.
High RZT created a greater metabolic drain on the lettuce. This was confirmed by the determination of root and shoot fresh and dry weights, root/shoot ratios and using 14C labeling techniques. The C=>A-RZT caused lettuce and capsicum to have lower leaf area, shoot and root fresh weight (FW) and dry weight (DW) and 14C partitioning to developing leaves than C-RZT plants. Conversely, A=>C-RZT lettuce and capsicum had higher leaf area, shoot and root FW and DW and more 14C photosynthate translocated to shoot apices than A-RZT. The higher root/shoot ratio and 14C partitioning to roots recorded in A-RZT lettuce suggested that more photoassimilates were translocated to the roots for respiration and root cell thickening than for root elongation. However, the trend of the capsicum root/shoot ratios and 14C partitioning were different from the lettuce because A-RZT capsicum had lower root/shoot ratio and 14C partitioning to roots.
In this study, the roots of C-RZT lettuce and capsicum were longer with a greater number of root tips and total root surface area; and smaller average root diameter than A-RZT plants. Using reciprocal RZT transfer experiments, the changes in root initiation (root tips) preceded those of contents of chlorophyll, soluble proteins and Rubisco proteins. High RZT-induced alteration in roots was likely to decrease root hydraulic conductivity as well as mineral absorption. Generally, it was found that shoot N and NO3- concentrations were more responsive to RZT than other minerals measured such as Ca, K, Mg, B, Cu, Fe, Mn, Mo and Zn. C-RZT lettuce and capsicum had higher shoot N and NO3- concentrations on per unit dry weight basis compared with A-RZT plants. C=>A-RZT caused a decline in shoot N and NO3- concentrations in both lettuce and capsicum. Meanwhile, lettuce and capsicum exhibited increased N and NO3- concentrations following A=>C-RZT. The decline in soluble protein and Rubisco of A-RZT plants may also be attributed to lower NRA. The general NRA results indicated that there was higher shoot NRA than root NRA. The role of N deficiency in mediating the decline in plant growth of A-RZT plants was studied. A comparison was made between C-RZT subjected to N deprivation (-N C-RZT) and A-RZT capsicum. From both leaf growth and shoot dry matter partitioning data, it may be postulated that poor shoot development observed in A-RZT capsicum was mediated to a large extent, by N deficiency. The decline in the N status of A-RZT capsicum may have accounted for stomatal closure, decreased A, lesser N available for manufacture of soluble proteins and Rubisco proteins and perhaps sensitivity to phytohormones such as CK. It was observed that although N deprivation also limited root growth, the degree of inhibition was greater in A-RZT capsicum than -N C-RZT plants. Therefore, the inhibition in capsicum development grown at A-RZT may not be solely due to N nutritional disturbance.
High RZT created a greater metabolic drain on the lettuce. This was confirmed by the determination of root and shoot fresh and dry weights, root/shoot ratios and using 14C labeling techniques. The C=>A-RZT caused lettuce and capsicum to have lower leaf area, shoot and root fresh weight (FW) and dry weight (DW) and 14C partitioning to developing leaves than C-RZT plants. Conversely, A=>C-RZT lettuce and capsicum had higher leaf area, shoot and root FW and DW and more 14C photosynthate translocated to shoot apices than A-RZT. The higher root/shoot ratio and 14C partitioning to roots recorded in A-RZT lettuce suggested that more photoassimilates were translocated to the roots for respiration and root cell thickening than for root elongation. However, the trend of the capsicum root/shoot ratios and 14C partitioning were different from the lettuce because A-RZT capsicum had lower root/shoot ratio and 14C partitioning to roots.
In this study, the roots of C-RZT lettuce and capsicum were longer with a greater number of root tips and total root surface area; and smaller average root diameter than A-RZT plants. Using reciprocal RZT transfer experiments, the changes in root initiation (root tips) preceded those of contents of chlorophyll, soluble proteins and Rubisco proteins. High RZT-induced alteration in roots was likely to decrease root hydraulic conductivity as well as mineral absorption. Generally, it was found that shoot N and NO3- concentrations were more responsive to RZT than other minerals measured such as Ca, K, Mg, B, Cu, Fe, Mn, Mo and Zn. C-RZT lettuce and capsicum had higher shoot N and NO3- concentrations on per unit dry weight basis compared with A-RZT plants. C=>A-RZT caused a decline in shoot N and NO3- concentrations in both lettuce and capsicum. Meanwhile, lettuce and capsicum exhibited increased N and NO3- concentrations following A=>C-RZT. The decline in soluble protein and Rubisco of A-RZT plants may also be attributed to lower NRA. The general NRA results indicated that there was higher shoot NRA than root NRA. The role of N deficiency in mediating the decline in plant growth of A-RZT plants was studied. A comparison was made between C-RZT subjected to N deprivation (-N C-RZT) and A-RZT capsicum. From both leaf growth and shoot dry matter partitioning data, it may be postulated that poor shoot development observed in A-RZT capsicum was mediated to a large extent, by N deficiency. The decline in the N status of A-RZT capsicum may have accounted for stomatal closure, decreased A, lesser N available for manufacture of soluble proteins and Rubisco proteins and perhaps sensitivity to phytohormones such as CK. It was observed that although N deprivation also limited root growth, the degree of inhibition was greater in A-RZT capsicum than -N C-RZT plants. Therefore, the inhibition in capsicum development grown at A-RZT may not be solely due to N nutritional disturbance.
Date Issued
2002
Call Number
QK755 Tan
Date Submitted
2002