Reactions of Drought-Affected Oil Palm Variety Seedlings during the Nursery Phase: Bud and Root Responses

Drought has a major impact on oil palm on vegetative growth and productivity. Hence, the cultivation of drought-tolerant and efficient water use oil palm planting material becomes imperative to address this issue. FTSW method can show the soil and plant moisture condition based on the transpired water by crops. This study was structured using a Randomized Block Design, incorporating two factors - three levels of FTSW (Fraction of Transpirable Soil Water) and three different oil palm varieties. Its primary objective was to assess the plant’s reaction to drought-induced stress. The results showed that drought affected plant performance in terms of the number of leafs, root volume, and wet and dry weights of buds and roots. DxP Simalungun variety showed better performance than DxP PPKS 540 and DxP Langkat under drought stress conditions.


Introduction
Due to the exploitation of the oil palm plantation's ideal planting area today, marginal and sub-optimal areas are being developed [1].Oil palm is a drought sensitive plant.Drought has a significant influence on the vegetative growth and productivity of oil palm.Drought also affects oil palm in various phases of its life, both nursery, immature plants to mature plants.Oil palms are significantly impacted by drought stress, and these impacts are closely related to morphological, physiological, and biochemical processes.
Drought stress bring on decrease in the rate of cell division, the rate of CO2 absorption, nutrient uptake, [2] [3], stomatal conductance, increase of proline content [4] [5] and decreased chlorophyll content [6] [3].Drought also causes decrease in plant water content, decrease in the rate of photosynthesis, decrease in the production of dry matter (dry matter) and stunted growth both the seedling and mature plants [7] [8].
There is continuous discussion regarding how to assess plant phenotypes under drought or other stresses [9].Usually, the testing procedure uses young plants cultivated in tiny pots.One of the anticipated outputs of the study, in addition to choosing an acceptable testing method to evaluate plant phenotypes, is the measurement of biomass accumulation during stress [10].Drought tests are frequently performed in controlled conditions like greenhouses due to the restricted opportunities for conducting plant testing directly in the field.To enable the initial selection of droughttolerant plants, it is crucial that these greenhouse studies adequately reflect the climatic conditions of drought encountered in the real field condition.
Based on how much water is transpired by plants, the Fraction of Transpirable Soil Water (FTSW) technique can show the soil moisture conditions.The fraction of transpirable soil water, or FTSW, is measured using a scale from 0 to 1.When the FTSW score is 1, the plant is operating at field capacity and has sufficient access to soil water.A FTSW result of 0 on the other hand indicates that the plant has wilted permanently and that there is extreme water shortage in the soil, which will cause irreparable damage to the plant.To see how plants react to drought stress, water In this research, a Randomized Block Design (RBD) was employed with two factors: plant variety and FTSW treatment.The first factor included three oil palm varieties, namely DxP Simalungun, DxP PPKS 540, and DxP Langkat.The second factor involved three FTSW levels: FTSW 1.0 (100% field capacity); FTSW 0.4 (40% field capacity); and FTSW 0.15 (15% field capacity).To determine the FTSW value, we measured the initial weight of the polybag when it was at 100% field capacity and referred to it as the "initial polybag weight."Then, we monitored the decrease in polybag weight on a daily basis until the targeted plant transpiration reached 10% of the field capacity plant weight, which we termed the "final polybag weight."This daily reduction in polybag weight was used to calculate the soil water availability as a percentage of soil water transpiration, also known as FTSW.The FTSW grade was counted using a specific formula: Drought stress was applied to the seedlings once they reached 6 months of age.The stress treatment involved withholding water until the seedlings reached the desired FTSW target weight.Meanwhile, routine watering was kept up for the FTSW 1 treatment.Weighing each seedling until it achieved the desired target weight allowed us to carefully track the daily decline in polybag weight.The seedlings were watered and weighed every day for 40 days after the target weight was reached to maintain it.Each seedling's completely opened leafs were counted in order to estimate the number of plant leafs.A ruler was used to measure the length of the roots from the base to the tip.Root volume was measured using the gravimetric method.First, attach the soil from the roots.100 mL of water was poured into the measuring cup, then the roots were put into the measuring cup.Root volume is calculated by count the final volume in the measuring cup minus the initial volume.Wet weight of plants was calculated by weighing the buds and roots of the plants before the water content in the plants was significantly reduced or immediately after the plants were harvested.Dry weight was counted per plant organ (buds and roots).Dry weight was calculated by weighing the plants after being ovened at 80 o C for 48 hours.The parameters observed were number of plant leafs, root length, root volume, wet and dry weight of bud and root.The F test was used to analyze observational data, and the Tukey test (P 0.05) was used to compare the treatments mean values when there was a significant treatment impact on variance.Statistical software called Minitab [11] was used to carry out the statistical procedures.

Leaf Number
The variance analysis results of the plant leaf number showed that both the drought treatment and oil palm varieties had a significant impact on the leaf number, but there was no interaction between both treatments (Table 1).Significant variations were seen from 42 DAT (Days After Treatment) through the conclusion of the observation period, highlighting the impact of the drought treatment.The FTSW 1 treatment exhibited the best performance in terms of plant leaf count, while the lowest plant leaf number was seen with the FTSW 0.15 treatment (indicating extreme stress).Drought-stressed plants experienced a 19% inhibition in leaf number growth compared to seedlings under field capacity treatment.Moreover, the variation in plant varieties also bring a significant role in the number of seedling leafs.In particular, the DxP Simalungun variety outperformed the DxP PPKS 540 and DxP Langkat in terms of leaf number performance.

Root Length and Root Volume
The variance analysis of root length parameters revealed that the drought stress treatment and plant types had no substantially influence on the variable root length of oil palm seedlings, and there was no interaction between the two treatments.The variance analysis results of root volume parameters indicated drought stress and plant varieties had significant impact on the variable root volume of oil palm seedlings, but no interaction was found between the two treatments (Table 2).FTSW 1 treatment showed the best seedling root length compared to FTSW 0.4 and FTSW 0.15.Compared to plants that were not under drought stress, the root volume development of drought-stressed plants was inhibited by up to 36%.Comparing DxP Simalungun to DxP PPKS 540 and DxP Langkat, it demonstrated the largest root volume.

Bud Weight
The weight of the plant bud provides insight into the state of the plant's canopy during observation.It reflects the presence of fresh photosynthetic assimilates and indicates the plant's high water content and the bud's capacity for growth.Essentially, the weight of the plant bud serves as an important indicator of the plant's overall health and vitality, revealing its ability to sustain photosynthesis and support new growth.The bud wet and dry weight of the oil palm were affected by drought stress and plant varieties, according to the variance analysis findings, however there was no interaction between the two treatments (Table 3).Treatment of FTSW 1 (100% field capacity) showed better bud wet weight and dry weight than FTSW 0.4 and FTSW 0.15 treatment.Drought-stressed plants experienced a reduction in bud wet weight of up to 50% and a decrease in bud dry weight of up to 59% compared to seedlings/ in field capacity.DxP Simalungun showed the highest bud wet weight and dry weight compared to DxP PPKS 540 and DxP Langkat varieties.

Root Weight
The weight of plant roots provides valuable insights into the state of the roots when observed, particularly during drought stress.In times of drought, the plant roots actively seek as much water as possible from the planting medium to support the plant's survival.The variance analysis revealed that both drought stress and plant variety had a substantial different on the wet and dry weight of the roots (Table 4).However, no interaction was observed between the two treatments.Treatment FTSW 1 showed superior wet and dry weight of roots compared to the FTSW 0.4 and FTSW 0.15.Drought-stressed plants experienced a notable reduction in root wet weight, up to 55%, and a decrease in root dry weight, up to 57%, compared to plants under field capacity conditions.Furthermore, the oil palm varieties also had a substantial impact on the weight of plant roots.DxP Simalungun displayed higher root wet weight and dry weight compared to the DxP PPKS 540 and DxP Langkat.

Palm Oil Variety Growth Response
Crops with more photosynthate are more likely to develop more leaves, as photosynthate is used to build various crops organs.The rate of photosynthesis has a direct impact on the growth capacity of plant cells, which impacts to crops height and the creation of new leaves.Oil palm seedlings' growth and development are hampered during periods of extreme drought stress, resulting in limited growth [12].Drought stress has a considerable influence on the quantity of plant leaves, according to this study (Table 1).Among the oil palm varieties tested, the DxP Simalungun variety exhibited the highest response in leaf growth compared to the DxP PPKS 540 and DxP Langkat.Plants prefer keeping their growth tip and young leafs, particularly spear leaves, when stressed by drought [13].However, the genetic characteristics of each variety play a crucial role in determining the overall increase in the number of seedling leafs [14].These genetic differences influence how the plant responds to drought stress and its ability to produce new leafs under adverse conditions.
Root characteristics are very important in determining plant response to drought.Morphologically, water stress will extend the root area of plants [15].This is related to the response of plants in finding a water supply for photosynthesis.
In this drought stress observation, there was no effect of plant variety on root length variables (Table 2).Research Wagino et al. [16] using the DyxP Dumpy variety showed the same results.Oil palm has shallow roots (fiber roots), so this causes oil palms to be sensitive to water shortages [17].Continued dryness can cause a decrease in cell turgor which results in decreased pressure towards the outside of the cell wall, thus disrupting the process of cell enlargement and ultimately decreasing cell division activity.Plants experiencing water stress will expand their absorption area to increase water uptake from the growing media.This study showed that drought stress could significantly reduce root volume up to 36% compared to seedling treatment under field conditions (Table 2).The DxP Simalungun variety showed the highest root volume among the DxP PPKS 540 and DxP Langkat varieties.The results of research by Nio and Torrey [18] show the same thing, where the volume of plant roots decreases in conditions of water shortage.This is probably due to genetic factors and morphological characters of oil palm roots where the root type of oil palm is fibrous roots.With this type of rooting, oil palm will adapt by expanding the root uptake area to find water in the growing media.
One of the parameters that can be used as an observation to see the response of plants to drought is plant biomass (dry matter production) [19].The greater the biomass of a plant, the greater the nutrient content in the soil that is absorbed by the plant.Plant biomass is closely related to plant growth.The canopy biomass includes stems and leafs which means the accumulation of photosynthetic results and is influenced by the availability of nutrients.Root biomass is the accumulation of photosynthate in the roots.Root biomass also describes the ability of plants to find available water in the growing media.The DxP Simalungun variety showed higher wet and dry plant weights, both Bud and root weights than DxP PPKS 540 and DxP Langkat varieties (Table 3 and Table 4).Research results [16] using DxP Langkat oil palm seeds showed that drought stress had an effect on canopy dry weight, but had no significant effect on seedling root dry weight.
Wet and dry weights of plants can decrease under drought stress conditions due to decreased photosynthetic rates during drought stress [20].This decrease in the rate of photosynthesis will affect stem enlargement, height growth and leaf development of plants due to lack of assimilate for use in cell division and enlargement.An interesting finding was the situation where the biomass of the DxP Simalungun variety was better than that of the DxP PPKS 540 and DxP Langkat varieties, which correlated with the state of leaf and root growth that was strongly influenced by photosynthesis.However, there was no significant difference in root length parameters for each variety.This is probably because there is an adaptation mechanism in these oil palm varieties by increasing the development of tertiary and quaternary roots to expand the area of root distribution around the surface of the planting medium compared to extending the roots to seek water at deeper soil depths.It is also possible when drought stress occurs, photosynthesis is directed for the development of meristem cells in tertiary and quaternary roots which are known to be the part of the root that absorbs the most water and nutrients from the growing media.

Drought Stress Treatment on Oil Palm Seed Growth
Water is a vital component of leaf development, and Kuswandi and Sugiyarto [21] highlighted how its shortage has a substantial influence on the quantity of leaves produced.Given that all three of these components contribute to the overall biomass of the plant, the leaf growth rate is correlated with the growth in plant height and stem diameter.The quantity of seedling leaves was shown to be significantly affected by drought stress in the current study, which became apparent 42 days to 70 days following the drought stress treatment (Table 1).However, these findings contrast with others study [16] [22], who used the DyxP Dumpy and DxP Avros and found that the drought stress treatment did not significantly impact the number of seedling leafs.Firda [23] mentioned that under consistent temperature and light intensity conditions, leaf formation tends to remain relatively constant.This suggests that other factors, such as genetic variations between the plant varieties and specific environmental conditions, may have contributed to the differing outcomes observed in the studies.In summary, water availability plays a vital role in leaf growth, and drought stress was found to significantly affect the number of leafs in the current study.However, the results differed from other studies involving different plant varieties, indicating the complexity of factors influencing leaf formation in response to drought stress.
One form of inhibition of cell formation and development due to water stress is the formation of few plant roots, small size and relatively narrow distribution area [15].This study showed that there was no effect of drought stress on the seedlings' root length (Table 2).Root length and volume are related to plant resistance during drought stress.Oil palm plants have fibrous root types, so that when stressed by drought, oil palm will expand its root area and increase tertiary roots to expand the area of root absorption.Research by Febrianto et al. [22] showed that water deficit stress had significant impact on seedling root length.
One form of inhibition of cell formation and development due to water stress is the formation of few plant roots, small size and relatively narrow distribution area (Palupy and Dedywiryanto, 2008).Drought stress had a significant effect on root volume, where it was found that roots in FTSW 1 treatment (field capacity) were greater than root volume in FTSW 0.4 (adequate drought) and FTSW 0.1 (extreme drought) treatments (Table 2).Root volume describes the area of distribution of roots.Root volume in plants will decrease in response to water shortages.This is due to the process of partitioning assimilate slightly to the roots so that root development is hampered [18].
Because the availability of nutrients is significant as an energy source, the amount of nutrient and water adequacy influences the biomass of a plant.The fresh weight of the bud includes stems and leafs which means the accumulation of photosynthetic results and is influenced by the availability of nutrients.The fresh weight of the bud is also a reflection of photosynthesis during the growth process, and 90% dry weight is the result of photosynthesis.Root biomass is the accumulation of photosynthate in the roots.Drought will have a greater influence on the canopy compared to the roots [24].The drought stress treatment had a substantial influence on the wet and dry weight of the plant bud in this investigation (Table 3).This study also found that drought stress treatment had a substantial influence on root wet and dry weight (Table 4).
The wet weight of the plant is a description of the photosynthetic ability of the plant and the absorption of plant nutrients and water.When drought stress occurs, the wet weight of the plant will decrease which causes inhibition of root development, stem enlargement, height growth and plant leaf formation.Plant dry weight describes the accumulation of photosynthetic products contained in plants.If photosynthesis is disturbed, food assimilation and photosynthate transport will be disrupted, resulting in a decrease in the wet and dry weight of the plant.The reduction in plant dry weight corresponds to the decrease in plant wet weight, where plant dry weight and wet weight reflect plant biomass connected to photosynthesis.Drought stress reduces the dry biomass weight of oil palm seedlings by disrupting biochemical and physiological processes in the plant body.Abscisic acid produced as a result of drought stress will induce stomatal closure, affecting plant photosynthetic processes.A reduction in plant dry weight results in a reduction in photosynthate production.The reduction in photosynthesis in plants will stifle plant morphological development and growth.The reduction in plant dry weight corresponds to the decrease in plant wet weight, where plant dry weight and wet weight reflect plant biomass connected to photosynthesis.Drought stress reduces the dry biomass weight of oil palm seedlings by disrupting biochemical and physiological processes in the plant body.Abscisic acid formed due to drought stress will stimulate stomata closure, which will affect the photosynthetic process of plants [25] [26] [27].A decrease in plant dry weight means a decrease in photosynthate yield.The decrease in photosynthesis in plants will inhibit the development and growth of plant morphology.

Conclusion
The study revealed that DxP Simalungun exhibited superior bud and root development beside DxP PPKS 540 and DxP Langkat under drought conditions.This suggests that DxP Simalungun possesses a more effective adaptation mechanism and a greater ability to withstand drought stress.The results highlight the importance of genetic variability among oil palm varieties in responding to adverse environmental conditions.The significant differences observed in bud and root development between the varieties under drought stress underscore the potential for selecting and breeding drought-tolerant oil palm varieties to enhance resilience and productivity in water-limited environments.To gain a comprehensive understanding of oil palm varieties' abilities to cope with drought stress, further research can be conducted from various perspectives.For instance, investigating the physiological and biochemical responses of different varieties to drought stress, studying their water-use efficiency and drought resistance mechanisms, and exploring genetic traits associated with drought tolerance could provide valuable insights for sustainable oil palm cultivation in water-scarce regions.Such research efforts can contribute to the development of climate-resilient oil palm varieties and support the agricultural sector in mitigating the impact of drought on crop productivity.

Table 1 .
Effect of drought stress and variety differences on leaf number of oil palm seedlings at observations from 7 DAT (Days After Treatment) to 70 DAT.The Tukey's test indicate that numbers next to the same character in the same column aren't substantially different at 0.05.DAT = Days After Treatment.

Table 2 .
Effect of drought stress and varieties on root length and volume of oil palm seedlings.

Table 3 .
Effect of drought stress and varieties on wet and dry weight of oil palm seedling bud.The Tukey's test indicate that numbers next to the same character in the same column aren't substantially different at 0.05.

Table 4 .
Effect of drought stress and varieties on wet weight and dry weight of oil palm seedling roots.Note: The Tukey's test indicate that numbers next to the same character in the same column aren't substantially different at 0.05.