HORTSCIENCE 50(8):1192–1195. 2015. the ornamental value. Thus, it is of commer- cial importance to understand the physiolog- ical causes of this constraint. Here, we The Differential Response of Two describe the contrasting behavior of the shad- ing intolerant cultivar Puma Sunny and the Chrysanthemum Cultivars to Shading: more tolerant cultivar Gongzi. Our aim was to contrast their responses to suboptimal illumi- Photosynthesis, Chloroplast, and Sieve nation at the level of plant morphogenesis, photosynthetic activity, and chloroplast and Element-companion Cell phloem companion cell ultrastructure. Ultrastructure Materials and Methods Plant growth and shading treatment. Shuang Han Samples of the chrysanthemum cultivars College of Horticulture, Nanjing Agricultural University, Nanjing 210095, Puma Sunny and Gongzi were obtained from China; and College of Life Science, Shangqiu Normal University, Shangqiu, the Chrysanthemum Germplasm Resource Henan 476000, China Preserving Center, Nanjing Agricultural Uni- versity. Rooted cuttings at the seven-leaf Jiafu Jiang, Huiyun Li, Aiping Song, Sumei Chen, and Fadi Chen1 stage were planted in 4-L pots containing College of Life Science, Shangqiu Normal University, Shangqiu, Henan a 2:1:1 mixture of garden soil, vermiculite, 476000, China and perlite (Han et al., 2013). The two cultivars are obviously different in morphol- Additional index words. Chrysanthemum morifolium, low light, electron transport, phloem, ogy, growth, florescence, especially flower companion cell quality: ‘Puma sunny’ with anemone petal owns the higher ornamental value, while Abstract . The response to reduced light intensity of two contrasting cultivars Puma Sunny ‘Gongzi’ exhibits short duration tubular (shade intolerant) and Gongzi (shade tolerant) was characterized in terms of plant height, flower. Shading was conducted in net-house the root/shoot ratio, photosynthetic capacity, and the morphology and ultrastructure of (2.5-m high, 4-m long and 4-m wide) covered their chloroplasts and phloem companion cells. The initial response to shading of cultivar with various thicknesses of commercial black Puma Sunny plants was to extend their stems, and while the equivalent response of cultivar shading nets, one restricted the incident light Gongzi was less marked. Shading depressed the maximum relative electron transport rate to 55% of full sunlight (referred to as ‘‘CK,’’ (rETR) in both cultivars, and while the efficiency of light capture in cultivar Puma Sunny the preliminary experimental result showed was compromised by shading, this was not the case for cultivar Gongzi. Low levels of the maximum PAR of 55% full sunlight was incident light inhibited the formation of starch grains in the chloroplast and increased the close to the light saturation point) and the volume of interspace between the grana lamellae. In cultivar Puma Sunny, but less so in other to 15% of full sunlight (referred to as cultivar Gongzi, the chloroplasts became more slender and the stroma lamellae more ‘‘Shade’’). Diurnal variations of photosyn- swollen. Adjusting chloroplast morphology by developing extra layers of grana lamellae thetic photon flux density (wavelength 400– and maintaining the integrity of the phloem companion cells are both adaptations which 700 nm) under two light conditions were help make ‘Gongzi’ a more shade-tolerant cultivar. frequently monitored with external quantum sensor connected to photosynthesis system (LI-6400XT, LI-COR, Lincoln, NE), and Light intensity significantly affects plant’s enzyme activation, the plant’s circadian displayed in Fig. 1. For this experiment, the morphology, so reduced levels of illumination rhythm, and activity in the lutein epoxide rooted cuttings were subjected to two irradi- are likely to have an impact on both the plant’s cycle (Ralph et al., 2002). ance levels for 20 d. Average canopy temper- morphology, anatomy (Craven et al., 2010; Although the effect of suboptimal illumi- atures under two contrasting light treatments Dai et al., 2009; Deng et al., 2012), physiology, nation on photosynthetic activity, chlorophyll were always within <1.0 °C of each other, cellular biochemistry, phenology, and eco- content, and chlorophyll fluorescence is well and in no case were consistent differences in nomic yield (Craven et al., 2010; Favaretto established (Craven et al., 2010; Deng et al., temperature detected between treatments. et al., 2011; Raveh et al., 1998). The relation- 2012), the relationship—if any exists— The average minimum/maximum tempera- ship between the level of photosynthetically between photosynthesis and chloroplast ultra- ture, photoperiod (day/night), and air humidity active radiation (PAR) and the plant’s rela- structure has not been widely characterized. were 30/21 °C, 14/10 h and 70%, respectively. tive rETR is frequently assessed by means of Phloem sieve elements are nonnucleated cells, Rapid light curves were measured on the day the ‘‘rapid light curve’’ (RLC) technique, largely controlled by their nucleolated com- 20. Each treatment involved 60 pots, at each produced by plotting ETR against actinic panion cells, and their function is concerned conducted time point, three or above plants of irradiance (Belshe et al., 2007; Ralph and with the loading and transport of assimilate each treatment was selected randomly and Gademann, 2005; Serodio^ et al., 2006), and from the leaf into the phloem (Wang and determined. thereby providing a measure of photosynthetic Huang, 2003). In maize, companion cells in Plant growth measurement. The height of themainveinhavebeenshowntobe each plant was measured at 4-d intervals responsible for the loading and transport between days 4 and 20 following the estab- of sucrose synthase into the phloem (Nolte lishment of the cuttings, at which time both Received for publication 10 Apr. 2015. Accepted and Koch, 1993). The rice gene OsSUT1 is cultivars expressed symptoms of shade for publication 11 June 2015. expressed in companion cells and is known avoidance in the ‘‘Shade’’ treatment, with This study was supported by the National Natural to be regulated both by light and the filmy and bigger leaf, elongated petiole and Science Foundation of China (Grant No. 31071825, presence of sugar (Matsukura et al., 2000). stem. At day 20, the material was harvested 31272196), the Chinese Ministry of Education ‘‘Pro- Whether there is any influence of incident and divided into leaf, stem, and root, dried gram for New Century Excellent Talents in Univer- light on the phloem’s ultrastructure has yet ° sity’’ (Grant No. NCET-10-0492), Sci-Tech Support at 80 C for 48 h and weighed, 10 plants per Plan of Jiangsu Province (BE2011325, BE2012350), to be determined. treatment. and the Fund for Independent Innovation of Agricul- Recently, chrysanthemum has been widely RLC analysis. A portable pulse-amplitude tural Sciences in Jiangsu Province (CX(12) 2020). cultivated, especially in shaded greenhouse in modulation fluorescence system (MINI- 1Corresponding author. E-mail: [email protected]. China. The low light growth conditions affect PAM, Walz, Germany) was used to measure 1192 HORTSCIENCE VOL. 50(8) AUGUST 2015 Fig. 1. Curves of diurnal variation of photosynthetic photon flux density (PPFD) under full sunlight and Fig. 2. Effect of low-level illumination on plant 55% irradiance (wavelength 400–700 nm) during one day in July 2012 in Nanjing, China. Bars height in cultivars Puma Sunny and Gongzi. represent mean ± SE (n =3). *, ** significant difference based on Duncan’s multiple range test, in comparisons between fluorescence characteristics associated with RLC analysis. RLC describe the re- the two cultivars both under shade, at P <0.05 photosynthesis. Rapid light curves were sponse of plants to a range of light level. and P < 0.01, respectively. Bars represent obtained using a preinstalled software rou- Figure 4 shows clearly that average rETR is mean ± SE (n = 3). tine, which required the actinic illumination significantly influenced by light irradiance. to be increased in eight incremental steps Under the ‘‘Shade’’ regime, the Pm and Ek starting from effective darkness. The RLC values of both cultivars were substantially was based on the relationship P = Pm · [1 – lower than under the ‘‘CK’’ regime; how- exp (–a·PAR/Pm)] · exp (–b·PAR/Pm) ever, although the a value of cultivar Puma (Platt et al., 1981), where Pm (equivalent to Sunny was substantially affected by the rETRmax) expresses the photosynthetic ca- light intensity, that of cultivar Gongzi was pacity at saturation, a the initial slope not (Table 1). (representing the maximum light-use effi- Chloroplast ultrastructure. Both chloro- ciency) and b is a photoinhibition parameter. plast size and shape were materially influ- Parameter values were derived using a non- enced by the level of incident light (Fig. 5). linear regression method. There are three The abundance of chloroplasts and the num- replicates per treatment. ber of starch grains within each chloroplast Chloroplast and phloem companion cell were both inhibited by low light conditions. ultrastructure. Chloroplast ultrastructure was In cultivar Puma Sunny exposed to the investigated in the fourth leaf from the apex ‘‘Shade’’ treatment, the chloroplasts became of each plant after fixation in 2.5% (v/v) elongated (Fig. 5E), their stroma appeared Fig. 3. Effect of low-level illumination on the root/ glutaraldehyde in 0.1 M phosphate buffer irregular and swollen and their grana lamella shoot ratio in cultivars Puma Sunny and Gongzi. (pH 7.2) for at least 48 h. Phloem companion