| Summary: | 碩士 === 國立臺灣大學 === 園藝學研究所 === 94 === The leaf responses of Chinese mustard (Brassica rapa L. var. chinensis (Rupr.) Olsson) seedlings to water stress were investigated. When the third leaf emerged, seedlings were transplanted to modified Snow and Tingey system for four levels of water availability: control (CK, 4 cm below root screen), mild water stress (A, 22 cm below root screen), medium water stress (B, 30 cm below root screen), and severe water stresss (C, 47 cm below root screen). The measured parameters consists of the following: leaf numbers, the leaf area, the net photosynthetic rate (AN), the stomatal conductance (gs), the transpiration rate (E), the intercellular CO2 concentration (Ci) and some chlorophyll fluorescence parameters (Fv/Fm, qP, qN, NPQ and ΦPSII). It appears that the leaf responses of seedlings of mild water stress were identical with those of control. In this study the non-photochemical quenching (qN or NPQ) of plants which suffered medium and severe water stress performed significantly higher than those of control plants in about 4th sampled day, while the quantum yield of PSII electron transport (ΦPSII) was much lower in 6th day. These two chlorophyll fluorescence patterns are more sensitive to water stress than AN, gs and E. And fluorescence patterns are closely related to photosynthetic patterns. This leads to the possibility that qN, NPQ and ΦPSII are all well indicators to reflect the drought damage of photosynthetic patterns. Despite the fact that the leaf area of plants of medium and severe drought was significantly decline in 6th sampled day. If it is impossible to test the theory in any meaningful way, such claims have little credibility. There are flaws in attributing such importance to the cell enlargement alone. It must be noted that the decline of leaf numbers is more early than changes of qN, NPQ and ΦPSII. Stated another way, water deficit may effect the cell division very early. Thus these fluorescence patterns are less likely to indicate the damage of the leaf area. To conclude, results of this study showed that in Chinese cabbage qN, NPQ and ΦPSII are more likely to indicate the stress effect of photosynthetic patterns, but are less likely to work in leaf numbers and leaf area.
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