Physiological limitations to the growth response of bean plants (Phaseolus vulgaris L.) to carbon dioxide enrichment
Creators: name: Liu, Hung-Tsu (Paul)
Publisher: name: University of British Columbia
Year: 2011

description: Previous studies on dwarf bean plants have found a very limited growth response to CO₂ enrichment (Jolliffe and Ehret, 1985; Ehret and Jolliffe, 1985b). There was no increase in leaf area, and leaf injury was observed after about three weeks of CO₂ enrichment (Ehret and Jolliffe, 1985a). Although dry weight was increased, the increase may be limited due to restricted carbon utilization (e.g. no increases in leaf area). In this study, non-photosynthetic limitations, such as the partitioning of dry matter among plant parts, the partitioning of carbon among photosynthetic end products, and the interactive effects of nutrient and carbon supply on growth, that may contribute to the observed growth responses were investigated. Bean plants responded to CO₂ enrichment by increasing their total dry weights. This weight increase was caused by higher growth rate, at least at early growth stages, and higher unit leaf rate. The dry weight increase was mainly in the leaves, and was not evenly distributed among all plant parts. Leaf expansion and branching were not enhanced by CO₂ enrichment. The differential effects of CO₂ enrichment on growth of different parts caused significant increases in specific leaf weight and shoot root ratio, and a decrease in leaf area ratio. These results indicated that the bean plants used in this study have a limited ability to utilize the extra carbon that was fixed under CO₂ enrichment. There were small increases in glucose, fructose, and sucrose concentrations early in the CO₂ treatments. These increases became much larger after three weeks of CO₂ enrichment. The timing of the higher increases in leaf soluble sugars coincided with the timing of increases in stem and roots dry weight. There was also a large increase in starch concentration shortly after plants were transfered to CO₂ enriched condition. The higher starch concentration accounted for the majority of the weight increase in CO₂ enriched leaves, and this starch level was maintained for several days after plants were switched back to ambient CO₂ levels. A ¹⁴C study on the partitioning of carbon between leaf pools showed that carbon transfer out of the storage pool under CO₂ enrichment was limited. CO₂ enrichment had no effects on leaf protein and amino acid concentrations. No difference, or slight increases, were found in inorganic nutrient concentrations per unit leaf area. Plants grown under CO₂ enrichment, however, show a higher loss of nutrients (especially N and K) from older shoot parts (primary leaves and older trifoliates) to younger parts. High NO₃ ̄ supply increased plant dry weight and leaf area under both CO₂ enriched and ambient conditions. The dry weight increases of the stem and roots caused by CO₂ enrichment, however, were much higher and earlier for high NO₃ ̄treated plants. Furthermore, lower leaf starch concentration was also observed for those CO₂ enriched high NO₃ ̄ treated plants. High NO₃ ̄ supply also increased the leaf nutrient concentrations (N, K, Mg, Ca). Increased uptake of nutrients for high NO₃ ̄ treated plants may be partly contributed by the enhanced root growth. In addition to the growth responses, foliar abnormalities developed gradually in beans under CO₂ enrichment. Chlorosis, assessed by a loss in total chlorophyll concentration, was observed in the primary leaves after about three weeks of CO₂ enrichment. The disorder eventually appeared in the oldest trifoliate leaves after more prolonged CO₂ enrichment. The onset of leaf injury was correlated with the timing of the increases in leaf soluble sugars and the redistribution of nutrient elements from the older shoot parts to the younger parts. High NO₃ ̄ supply delayed the development of leaf injury induced by high CO₂. Results in the present studies indicate that growth responses of dwarf bean plants to CO₂ enrichment were affected by the limited carbon partitioning, and the restriction of starch degradation was indicated to be the probable cause. A higher carbon input under CO₂ enrichment may create a higher demand for inorganic elements. Effects of nutrient supply (NO₃ ̄) on growth responses and leaf injury of CO₂ enriched plants suggested that an imbalance between carbon and nutrient input could be partly related to the limited growth responses of dwarf bean plants to CO₂ enrichment.