60 Ahmad Nazarudin et al: Leaf Attribute Changes from Paclobutrazol and Potassium Nitrate 2011; Ahmad Nazarudin et al. 2012). In addition, a darker green color in leaves was exhibited as a response to PBZ and KNO3 treatments (Figure 2). This was due to enhanced relative chlorophyll con- tent associated with smaller leaf area, which further intensified the foliage color. Increased relative chlo- rophyll content as a response to PBZ was consistently reported in various plant species including Hibiscus rosa-sinensis (Ahmad Nazarudin 2012), Lagerstro- emia indica (Mohammed et al. 2016), and Paeonia lactiflora (Xia et al. 2018). Furthermore, the combination of PBZ and KNO3 produced relatively thicker leaves as compared to those treated with either PBZ or KNO3 alone. This study corroborates findings by Gopi et al. (2008) and Gao et al. (2011) which indicated that PBZ increased the leaf thickness of Amorphophallus campanulatus and Triticum aestivum, respectively. Increased leaf thickness also resulted from inhibition of leaf size in combination with further thickening of the palisade mesophyll cells. An enhanced palisade layer in other woody species such as S. campanulatum (Ahmad Nazarudin et al. 2007) and T. ciliata (Rodrigues et al. 2016) following PBZ application has also been docu- mented. A previous study has also shown that PBZ resulted in thicker leaves in Chrysanthemum due to an additional layer of palisade, although individual palisade cells were shorter (Burrows et al. 1992). In the present study, observation through a scanning electron microscope proved that the X. chrysanthus leaf retained a single layer of palisade parenchyma. Therefore, the enhancement of leaf thickness in this species was actually influenced by the increased pali- sade parenchyma thickness rather than the additional layer of the palisade. Other than PBZ, potassium was also proven to increase chlorophyll content and leaf thickness in Gossypium hirsutum (Akhtar et al. 2009). Enhanced leaf thickness due to PBZ and KNO3 as observed in this study might be beneficial for X. chry- santhus to prevent biotic and abiotic stresses. Leaf thickness plays an essential role in plant functioning and relates to a species’ strategy for resource acquisi- tion and use (Vile et al. 2005). Increased tissue thick- ness in the leaf enabling enhanced water-holding capacity with the presence of potassium helped improve metabolic activities and production of pho- tosynthates like carbohydrates and proteins and their translocation to respective sinks. For example, Aus- tralian desert plants have relatively thick, long-lived ©2021 International Society of Arboriculture leaves (Wright and Westoby 2002) and grow in soils that are uncommonly low in nutrients (Morton et al. 2011). Under such conditions, prolonging the leaf life span could be achieved through the production of leaves that are not only structurally tough and herbi- vore resistant, but also resistant to thermal damage (Leigh et al. 2012). In addition, physical modification of leaves as a response to PBZ, for example thicker leaves, smaller stomatal pores, and an expanded epi- cuticular wax layer on the leaf surface, may provide additional protections against various fungal, bacte- rial, and insect infestations (Chaney 2005). In the present study, a significant reduction in photo- synthetic rates, transpiration rates, and stomatal con- ductance was demonstrated in the presence of the PBZ compound. Similar results of reduced photosynthetic rate as a response to PBZ treatment were also reported in L. indica (Mohammed et al. 2016) and S. myrtifo- lium (Ahmad Nazarudin et al. 2012). This could be the indirect effect of modified cell arrangement in the leaves caused by PBZ, which eventually restricted gas exchange. It was proven in our previous experi- ment that the parenchyma cells of the PBZ-treated leaves were tightly packed because the decreased leaf size forced these tissues into such an arrangement (Ahmad Nazarudin et al. 2015). Fortunately, reduc- tion in the transpiration rate and stomatal conduc- tance could protect the plant against abiotic stresses related to water limitations or drought incidents. It could possibly decrease the amount of water lost through stomata. Fletcher et al. (2000) stated that tri- azole-treated plants had lower transpiration, required less water, and were able to adapt better to drought than untreated plants. In addition, KNO3 , which sup- plies potassium, may also be beneficial in other aspects of plant biochemical processes, such as acti- vating enzymes, regulating osmosis, and transporting photosynthates in the plant. As a consequence, the treated trees have greater tolerance to environmental stresses. Thus, PBZ and KNO3 may help protect this species when it is planted in harsh urban areas that expose it to the above-mentioned limitations. CONCLUSION In conclusion, the presence of PBZ produced a more compressed arrangement of cells in the leaves. Darker green leaves were observed due to enhanced relative chlorophyll content following PBZ and KNO3 ment. The combination of PBZ and KNO3 treat- also
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