plants Article Effect of Prohydrojasmon on the Growth of Eggplant and Komatsuna Haidar Rafid Azis 1, Shinya Takahashi 1,2,* , Masami Koshiyama 3, Hiroshi Fujisawa 4 and Hiroko Isoda 1,2 1 Alliance for Research on the Mediterranean and North Africa (ARENA), University of Tsukuba, Tsukuba 305-8572, Japan; haidar.rafi[email protected] (H.R.A.); [email protected] (H.I.) 2 Faculty of Life and Environmental Sciences, University of Tsukuba, Tsukuba 305-8572, Japan 3 Specialty Chemical Division, Zeon Corporation, Chiyoda-ku, Tokyo 100-8246, Japan; [email protected] 4 Headquarter, Zeon Corporation, Chiyoda-ku, Tokyo 100-8246, Japan; [email protected] * Correspondence: [email protected] Received: 8 September 2020; Accepted: 13 October 2020; Published: 15 October 2020 Abstract: Prohydrojasmon (PDJ) can improve the polyphenol and anthocyanin content and antioxidant activity of some crop plants, but it also shows a suppressive effect on the plant growth. This study aimed to investigate the inhibitory effect of PDJ on the growth of two crop plants: komatsuna (Brassica rapa var. periviridis) and eggplant (Solanum melongena). We applied various concentrations of PDJ drip-wise or by spraying to eggplant and komatsuna seedlings and made detailed observations of growth. In general, no significant suppressive effect of PDJ was observed in the aerial parts in both plants. However, a significant inhibitory effect was found in roots treated with PDJ at concentrations of 600 and 1000 ppm. Interestingly, komatsuna treated with PDJ at a concentration of 200 ppm in both approaches resulted in a significant increase in root weight up to 37%. At a concentration range of 200–400 ppm, PDJ showed no inhibitory effects, and in some cases slightly promoted root growth. Therefore, this could be the recommended concentration range. We conclude that application of PDJ can still be beneficial to the vegetable crops without causing serious inhibition or suppression effects on the growth, as long as it is kept at rather low concentrations. Keywords: prohydrojasmon; inhibitory effect; plant hormone; komatsuna (Brassica rapa var. periviridis); eggplant (Solanum melongena) 1. Introduction Jasmonates are among the most important plant hormones which play roles in many aspects of plant growth. As a class of oxylipins, jasmonates can induce a wide range of responses in higher plants. Since the first isolation of its methyl ester (jasmonic acid methyl ester), numerous jasmonates have been detected in various plant phyla. Jasmonates are found in algae, mosses, fungi, gymnosperms, and angiosperms [1–3]. Prohydrojasmon (PDJ) is a synthetic analog of jasmonic acid (JA) developed as a plant growth regulator. Plant growth regulators are defined as naturally or chemically synthesized substances that play roles in developmental or metabolic processes in plants [4]. The activity of PDJ may be similar to that of endogenous JA. It has been shown to affect several physiological processes, including senescence, leaf abscission, fruit ripening, coloration, and pigment accumulation [5,6]. Other studies have shown that PDJ functions as an elicitor in plants to stimulate the accumulation of secondary metabolites including phenolics, anthocyanins, terpenoids, and glucosinolates [7,8]. Numerous studies have reported that application of PDJ, like methyl jasmonate (MJ), can improve several aspects of crop quality. Application of PDJ has been reported to improve the hand-picking efficiency of satsuma mandarin [9]. PDJ applied to Gala and Braeburn apples a couple of days before Plants 2020, 9, 1368; doi:10.3390/plants9101368 www.mdpi.com/journal/plants Plants 2020, 9, 1368 2 of 15 harvest resulted in a significant increase in red color index [10]. Similarly, the red-blushed color in some cultivars of mangoes significantly improved after being treated with PDJ at low concentration a few weeks prior to harvest [11]. This kind of color development was also reported to have occurred in red pear after being sprayed with PDJ during the pre-color-change period [12]. The color improvements in those fruits are strongly associated with the accumulation of anthocyanins which is enhanced by the application of PDJ. To date, we found that studies using PDJ majorly are still dominated by its application to fruit crops. In our previous study, however, we reported that application of PDJ to vegetable crops significantly increased the phenolic compounds, anthocyanin content, and antioxidant activity in komatsuna and lettuce [13]. Despite improvements of those secondary metabolites, PDJ is also known to have some kind of suppressive or inhibition effects on plant growth, particularly in initial plant growth [14]. In fact, growth inhibition was among the first characterized physiological effects of jasmonates, particularly root growth inhibition [15,16]. Previous studies have found that exogenous application of JA can inhibit various aspects of seedling growth including primary root growth, leaf expansion, and hypocotyl elongation [17–19]. One study emphasized that root growth is more sensitive to MJ [20]. Although this suppressive effect of PDJ is not as strong as that of MJ [21], it could still interrupt the plant growth, particularlyPlants if it 2020 is, applied9, x FOR PEER at REVIEW higher concentrations. To answer how far the suppressive2 of e16ff ect of PDJ could interrupt the growth of plants, here we conducted a study aimed at investigating the effect of picking efficiency of satsuma mandarin [9]. PDJ applied to Gala and Braeburn apples a couple of days PDJ at variousbefore concentrations harvest resulted onin a thesignificant growth increase and developmentin red color index of [10]. komatsuna Similarly, the and red-blushed eggplant seedlings. color in some cultivars of mangoes significantly improved after being treated with PDJ at low 2. Results concentration a few weeks prior to harvest [11]. This kind of color development was also reported to have occurred in red pear after being sprayed with PDJ during the pre-color-change period [12]. The 2.1. Effect ofcolor PDJ improvements Applied Drip-Wise in those onfruits Growth are strongly of Komatsuna associated with the accumulation of anthocyanins which is enhanced by the application of PDJ. The applicationTo date, ofwe PDJfound drip-wise that studies onusing komatsuna PDJ majorly atare thestill lowestdominated and by its highest application PDJ toconcentrations fruit (200 and 1000crops. ppm, In our respectively)previous study, showedhowever, we a tendencyreported that towards application a of slight PDJ to increase vegetable incrops shoot length significantly increased the phenolic compounds, anthocyanin content, and antioxidant activity in and its intermediatekomatsuna and concentrations lettuce [13]. (400 and 600 ppm) showed an opposite tendency towards a slight decreaseDespite in shoot improvements length. All of those results secondary regarding metabolites, this PDJ parameter is also known were to have statistically some kind insignificantof (Figure1a).suppressive The highest or inhibition concentration effects on of plant PDJ growth, drip-wise particularly (1000 inppm) initial plant showed growth a tendency[14]. In fact, towards a slight increasegrowth in inhibition leaf width, was whileamong the the first concentrations characterized physiological below that effects (200, of jasmonates, 400, and particularly 600 ppm) showed a root growth inhibition [15,16]. Previous studies have found that exogenous application of JA can tendency towardsinhibit various a slight aspects decrease of seedling in leaf growth width. including Similarly, primary all root results growth, regarding leaf expansion, this parameter and were statisticallyhypocotyl insignificant elongation (Figure [17–19].1b). One Interestingly, study emphasized PDJ drip-wise that root growth at concentrations is more sensitive of to 600 MJ and[20]. 1000 ppm significantlyAlthough reduced this root suppressive length byeffect 38% of PDJ and is 43%,not as respectively;strong as that of the MJ remaining[21], it could still concentrations interrupt the (200 ppm plant growth, particularly if it is applied at higher concentrations. To answer how far the suppressive and 400 ppm) showed statistically insignificant change in the root length (Figure1c). PDJ applied by effect of PDJ could interrupt the growth of plants, here we conducted a study aimed at investigating drip-wise approachthe effect seeminglyof PDJ at various didnot concentrations show a distinctive on the growth change and todevelopment the dimensional of komatsuna growth and of the aerial part of komatsuna,eggplant seedlings. however, at higher concentrations, the root length was significantly suppressed. (a) (b) (c) Figure 1. ShootFigure length 1. Shoot ( alength), leaf (a), width leaf width (b), (b and), and root root lengthlength (c () cof) komatsuna of komatsuna seedlings seedlings treated with treated PDJ with PDJ applied drip-wise at various concentrations. Komatsuna seeds were germinated and grown in soil, applied drip-wiseand PDJ was at variousapplied to concentrations.seedlings at 1 week before Komatsuna harvest. Shoot seeds length were and germinated leaf width values and are grownthe in soil, and PDJ wasaverage applied difference to seedlings between characteristics at 1 week before measured harvest. before and Shoot after lengthtreatment and (n = leaf4). Root width length values is are the average diffaverageerence value between (n = 4) characteristicsmeasured on the final measured day of pot before culture and(day after21).