FISIOLOGÍA

Leaf vein density and photosynthetic rate in Rosa: Is there a correlation?

 

Mario Pagano¹ and Paolo Storchi¹

¹ CREA – Council for Agricultural Research and Economics, Research Unit for Viticulture, Arezzo, Italy. Corresponding author: mario.pagano@crea.gov.it; tel.: +39-0575353021.

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Summary

A thorough knowledge of the leaf vascular network and its evolution is essential for understanding the fluid dynamics of water movement in leaves. Recent studies have shown how these networks can be involved in the performance of photosynthesis, which is linked to the density of the vascular network per unit of leaf area. The aim of this study is to highlight the correlation between leaf vein density (VD) and physiological parameters. The hypothesis proposed was tested on the leaves of the "Isabel Renaissance" (IR) rose cultivar. The main result concerned the VD, which was significantly correlated with the maximum leaf net photosynthetic rate (PN). This finding supports the hypothesis that the vascular system of IR leaves can be correlated with leaf photosynthesis performance.

Key words: Bundle sheath extensions; Leaves; Net photosynthetic rate; Vein density; Leaf temperature.

Resumen

Densidad de venas foliares y la tasa de fotosíntesis en Rosa: ¿Existe una correlación?

Un conocimiento profundo de la red vascular de la hoja y su evolución es esencial para la comprensión de la dinámica de fluidos del movimiento del agua en las hojas. Estudios recientes han demostrado cómo estas redes pueden estar implicadas en el rendimiento de la fotosíntesis, que está vinculada con la densidad de la red vascular por unidad de área de la hoja. El objetivo de este estudio es poner de relieve la correlación entre la densidad de venas de la hoja (VD) y parámetros fisiológicos. La hipótesis propuesta fue probada en las hojas del cultivar de rosa "Isabel Renaissance" (IR). El resultado principal referido a VD, se correlacionó significativamente con la tasa de fotosíntesis neta máxima de la hoja (PN). Este hallazgo apoya la hipótesis de que el sistema vascular de las hojas de IR se puede correlacionar con el rendimiento de la fotosíntesis de la hoja.

Palabras clave: Extensiones de la vaina del haz; Hojas; Tasa de fotosíntesis neta; Densidad de venas; Temperatura foliar.

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Introduction

A fundamental physiological aspect, which is shared among living organisms, is a high branching and structural complexity of the vascular architecture of leaves (Parsons-Wingerter, & Wickerman, 2011; Sack et al., 2012). The structure of the xylem, which is able to increase hydraulic efficiency and maintain water column continuity, is fundamental for the survival of plant species. However, the factors that have driven this improvement are still under study. The vascular architecture of a leaf has two main functions: to transport nutritive substances and to provide mechanical support. Water, solutes and hormones pass through the xylem, while carbohydrates (which are produced in the leaf) are conducted via the phloem (Roth-Nebelisck et al. 2001). The vein network plays a fundamental role in defining the maximum limit of photosynthetic efficiency (Brodribb et al., 2007; Sack & Sccofoni, 2013). Recent studies have highlighted how vein density (VD) per unit area and mesophyll thickness determine the efficiency of the hydraulic system (Brodribb et al., 2007). As observed by Brodribb et al. (2007), VD (defined as the total length of veins per unit area, Price et al., 2014) is closely associated with the rate of photosynthesis. VD can be correlated with the distance that the water must travel through the mesophyll. Furthermore, a novel study highlighted how the VD may be involved in the efficiency of leaf cooling (Pagano & Storchi, 2015). However, the factors that have led to these results are still under study. The aim of this study is to highlight the correlation between the leaf vein density (VD) and net photosynthetic rate (PN).

Materials and Methods

Abbreviations used: E – transpiration rate; gs – stomatal conductance; LA – leaf area; LED – light emitting diode; PN – net photosynthetic rate; ppm – parts per million; RH – relative humidity; VD – leaf vein density.
The research was carried out on one cultivar of Rosa L. (R. x anglais cv. Isabel Renaissance). Three plants were used for the analyses conducted in the laboratories of the CREA research centre located in Arezzo - Italy (N 43° 48’, E 11° 87’), which were exposed to the following environmental condition: 23.50 °C, 51.10 % of relative humidity. Ten leaves from each plant were randomly selected from the main shoots for the gas exchange measurements listed in Table 1. Leaf gas exchange measurements were performed with a CIRAS-I portable photosynthesis system (PP Systems, Hitchin), when the leaves were still attached to the plant. CIRAS determinations were made between 10:00 and 11:00 AM. The reference CO2 concentration and the relative humidity (RH) in the CIRAS device were set at 400 ppm and 70 %, respectively. Furthermore, the same leaves were temporarily placed in a refrigerated chamber at 4°C for 24 h before image acquisition in order to keep them fresh for the image analysis. The images were obtained using an EOS 700D camera (Canon) with an EF 100 mm f/2.8 Macro USM objective (Canon) and a Dinolite digital microscope (AM-313T5, AnMo Electronics Corporation, Hsinchu) placed over a LED illumination panel. The leaves were flattened in order to achieve maximum focus during image acquisition. After capturing the shape of the leaf on camera, the vein image was acquired with a Dinolite microscope at 230x magnification. DinoXcope 1.9.9 (AnMo Electronics Corporation) software was used to identify the VD. The microscope was set vertically to the sample and each image was acquired with the best focus obtainable with DinoXcope software. Image acquisition was performed by three rectangles of 1 mm2 areas into a virtual grid. The rectangles were drawn to include at least two 2° order of veins. We subsampled three rectangular boxes for each leaf (according to Sack et al., 2012). The boxes represented typical fragments and were located between the midvein and margin. The images were processed with Fiji 1.50i software (ImageJ, National Institutes of Health) which enables the user to measure the physical parameters that characterize the leaf network. VD data on the major veins were collected from the images manually. The image analysis was performed on the same leaves used for the gas-exchange measurement. The VD (measured from veins visible from their bundle sheath extensions) was measured using ImageJ segmented line tool on the images collected.

Statistical analysis

Statistical analyses were performed with Prism v. 5.0 (GraphPad software San Diego, California). In particular, we tested for differences in correlation between net photosynthesis and VD (Fig. 1) and net photosynthesis vs leaf temperature (Fig. 2) in order to determine the best fit. The best agreement was observed using a nonlinear regression (second order polynomial; GraphPad Prism 5.0). The principal component analysis (PCA) was carried out with PAST software (http:// folk.uio.no/ohammer/past).

Results

The plants were characterized by the following density/area of veins per unit: 3.08 ± 0.63 mm mm-2. Furthermore, maximum PN correlated with the VD (mm mm-2), thus confirming its positive link with the density (Fig. 1). Each point in the correlation graph contains an average of three crops (30 measurements) and three CIRAS measurements. The IR showed the following statistical mean values ± SD: PN [5.69 ± 1.04 μmol m-2 s-1], stomatal conductance (gs) [156.75 ± 36.70 mmol (H2O) m-2 s-1] and transpiration (E) [2.18 ± 0.21 mmol (H2O) m-2 s-1], photosynthetic active radiation (PAR) [393.17 ± 13.76 μmol (photon) m-2 s-1] and leaf area (LA) of [13.37 ± 3.89 cm2] (see Table 1). Finally, the PCA graph shows a correlation between leaf temperature, VD and the PN maximum (Fig. 3) in mature leaves of the IR cv.

Fig. 1. Relationship between VD and PN. Each point into the graph contains an average of three crops and three CIRAS measurement (± Standard Deviation, r2 = 0.50, p<0.05).

 

Fig. 2. Relationship between leaf temperature and PN. Each point into the graph contains an average of three crops and three CIRAS measurement (± Standard Deviation, r2 = 0.58, p<0.05).

Fig. 3. PCA analysis of PN, VD, leaf temperature (°C) and LA. Each point into the graph contains an average of three crops and one CIRAS measurement (PC1 91.54%; PC2 6.24%).

Table 1. Physiological parameters measured in "Isabel Renaissance" cultivar of Rosa

Discussion

The aim of the study is to highlight the possible correlations between leaf anatomic architecture with physiological parameters. The vein network of the IR cultivar analysed was characterized by a lower mean density/area of vein per unit when compared to other species such as Vitis vinifera L.: Sangiovese cv (4.98 ± 0.51 mm mm-2, P<0.05) and Montepulciano cv (5.70 ± 0.40 mm mm-2, P<0.05) (Pagano et al., 2016). A higher density of the vascular system plays an important role in water conduction (Sack & Holbrook, 2006; Zwieniecki et al., 2007) and in biomechanical support of the leaf. A significantly positive correlation was observed between VD and maximum PN. The relationship shown in figure 1 is not linear: after one point of the VD the curve of the PN no longer increases. In fact, the nonlinear relationship probably means that the saturation point has reached its maximum rate beyond which the PN decreases. This result supports the hypothesis that the vascular system is directly linked to PN performance. We are aware that the photosynthesis can be influenced by leaf age and various environmental factors such as temperature, CO2, light, drought stress, cultivar genotype (Crafts-Brandner & Salvucci, 2000; Palliotti et al., 2009), but we also believe that VD may play a fundamental role in PN performance. As regards to the effect of temperature, Lieth observed that temperatures (Cara Mia cv) above 30 °C caused a sharp decline in net photosynthesis and that the photosynthetic process was totally interrupted in very high temperatures (Lieth & Pasian, 1990). Similarly, the decline in net photosynthesis became very pronounced above 21.5 °C (Fig. 2). The PCA graph (Fig. 3) shows that the leaf area was not correlated with the other parameters. In fact, the PCA supports the hypothesis that only PN, VD and leaf temperature (°C) are associated. This finding supports the hypothesis that the vascular system in IR cv leaves can be correlated with leaf photosynthesis performance. Other studies should be carried out to determine which correlation exists between stomata density per unit and vein density and how they are linked to transpiration (Brodribb, 2009). The image analysis could also be used for the characterization of the VD in order to determine the variety. In conclusion, the main goal of this study was to determine that the PN maximum correlated with the VD in IR cv.

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Recibido el 10 de mayo de 2016,
aceptado el 2 de Agosto de 2016.