Abstract
BackgroundPhotosynthesis in the green leafless blade tissues or organs of plants has been studied in some plants, but the photosynthetic characteristics of stems and petioles are poorly understood. Cucurbitaceous plants are climbing plants that have substantial stem and petiole biomass. Understanding the photosynthetic contribution of cucumber stems and petioles to their growth and the underlying molecular mechanisms are important for the regulating of growth in cucumber production.ResultsIn this study, the photosynthetic capacity of cucumber stems and petioles were determined by 14CO2 uptake. The total carbon fixed by the stems and petioles was approximately 4% of that fixed by one leaf blade in the cucumber seedling stage, while the proportion of the carbon accumulated in the stems and petioles that redistributed to sink organs (roots and shoot apexes) obviously increased under leafless conditions. The photosynthetic properties of cucumber stems and petioles were studied using a combination of electron microscopy and isotope tracers to compare these properties of stems and petioles with those of leaf blade using two genotypes of cucumber (dark green and light green). Compared with those of the leaf blades, the chlorophyll contents of the cucumber stems and petioles were lower, and the stems and petioles had lower chloroplast numbers and lower stoma numbers but higher thylakoid grana lamella numbers and larger stoma sizes. The Chl a/b ratios were also decreased in the petioles and stems compared with those in the leaf blades. The total photosynthetic rates of the stems and petioles were equivalent to 6 ~ 8% of that of one leaf blade, but the respiration rates were similar in all the three organs, with an almost net 0 photosynthetic rate in the stems and petioles. Transcriptome analysis showed that compared with the leaf blades, the stems and petioles has significantly different gene expression levels in photosynthesis, porphyrin and chlorophyll metabolism; photosynthetic antenna proteins; and carbon fixation. PEPC enzyme activities were higher in the stems and petioles than in the leaf blades, suggesting that the photosynthetic and respiratory mechanisms in stems and petioles are different from those in leaf blade, and these results are consistent with the gene expression data.ConclusionsIn this study, we confirmed the photosynthetic contribution to the growth of cucumber stems and petioles, and showed their similar photosynthetic patterns in the terms of anatomy, molecular biology and physiology, which were different from those of cucumber leaf blades.
Highlights
Photosynthesis in the green leafless blade tissues or organs of plants has been studied in some plants, but the photosynthetic characteristics of stems and petioles are poorly understood
In this study, we confirmed the photosynthetic contribution to the growth of cucumber stems and petioles, and showed their similar photosynthetic patterns in the terms of anatomy, molecular biology and physiology, which were different from those of cucumber leaf blades
The main questions that we address in this paper are as follows: do cucumber stems and petioles have similar photosynthetic structures and functions compares to those of leaf blades? What is the photosynthetic contribution rate of cucumber stems and petioles compared to that of leaf blades, and how many of these structures contribute to sink organs? What is the difference in photosynthetic gene expression among cucumber stems, petioles and leaf blades? We hypothesized that the photosynthesis is similar in stems and petioles but different in leaf blades, and the carbon distribution shifts when plants are leafless
Summary
Photosynthesis in the green leafless blade tissues or organs of plants has been studied in some plants, but the photosynthetic characteristics of stems and petioles are poorly understood. Some plants, such as Brachychiton (Malvaceae), during the dry part of the year to adapt to water stress, and the trunk photosynthesis may support the carbon needs of this plant when it does not have leaf blades [13, 14] This scenario suggests that photosynthetic stems confer some physiological advantages, such as extra carbon fixation capacity, improved water-use efficiency (WUE) during periods when many plants are leafless, and balanced respiratory costs due to reassimilation of C O2 [1, 15,16,17,18,19,20,21,22]. It is necessary to pay attention to the photosynthetic capacity of the leafless, green organs (stems and petioles) of cucumbers both under normal conditions and conditions when there are no leaf blades, and this capacity is an important supplement to the production capacity of leaf photosynthetic [1]
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