Non-structural carbohydrates dynamics of Pinus yunnanensis seedlings under three levels of continuous drought stress

doi:10.21203/rs.3.rs-4698713/v1

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Non-structural carbohydrates dynamics of Pinus yunnanensis seedlings under three levels of continuous drought stress

https://doi.org/10.21203/rs.3.rs-4698713/v1

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There is limited understanding of how drought stress intensity and duration affect the dynamic changes of Non-structural carbohydrates (NSC) in various organs of seedlings, and there is a lack of consistent research results among different species. We performed experiments on the dynamics of NSC in different organs of Pinus yunnanensis seedlings under three continuous drought stresses from March 14 to May 12, 2021, respectively, with four levels of water gradients of suitable moisture (CK), light drought (LD), moderate drought (MD), and severe drought (SD). The results showed that the distribution of NSC in P. yunnanensis seedlings varied with drought stress intensity and duration. The NSC content of each organ (needles, stems, coarse roots and fine roots) showed different trends with the increase of drought stress intensity in different time periods, respectively. After 15d of drought stress, the intensity of drought stress had no effect on needle, stem and coarse root NSC contents, while the fine root NSC contents decreased significantly. At 30d and 45d, drought stress intensity had no significant effect on the NSC content of each organ. However, at 60d, the stem NSC content increased significantly under MD and SD conditions, while the fine root NSC content decreased significantly under SD conditions. With the extension of the drought duration, the coarse root NSC increased while the fine root NSC content decreased under SD conditions. The results showed that the drought duration played an important role in the dynamic change pattern of NSC, only a decrease in fine root was observed at the initial drought phase, and 60d was a turning point when significant changes in NSC occurred at the organ level. This is of great significance to better understand the dynamic changes of NSC in the organ level under drought stress.

Biological sciences/Ecology/Climate change ecology

Earth and environmental sciences/Environmental sciences/Environmental impact

Drought stress intensity

drought duration

Pinus yunnanensis seedlings

Non-structural carbohydrates

The dramatic changes in the global climate have enhanced the frequency and intensity of drought events, while forest decline due to extreme drought is a worrying phenomenon¹. In particular, rainfall patterns are changing across the globe, drought-induced forest mortality is increasing, plants in almost every forest biome are living at the edge of their survival hydrological limits, and forest ecosystem services have been severely impacted². Extreme drought inhibits tree growth, leading to tree mortality due to hydraulic damage, carbon starvation and a rise in forest degradation³. The increasing role of drought in community dynamics and forest mortality has led to a growing interest in understanding how to enhance drought resistance². Plant adaptation to different intensities of drought stress through the characteristics of gas exchange and leaf water potential regulation, and thus by altering the content of NSC and their components^4,5. In recent years, Yunnan province, China, a region seriously affected by winter-spring drought, has experienced a gradual increase in the duration and intensity, and forests are under threat of death from drought⁶.

Non-structural carbohydrates (NSC) is mainly composed of soluble sugar and starch, variation of composition can characterize the carbon budget and balance in plants and their resistance to stress⁷. NSC reserves have a potentially critical role in tree survival⁸. A comprehensive analysis of the death mechanism shows that there are two hypotheses related to drought induced death: carbon starvation and hydraulic failure, and hydraulic failure is more common⁹. Drought can directly cause trees to suffer from consequences such as embolism, hydraulic failure and cell failure, and can also affect the carbon balance of trees¹⁰. When plants face long-term drought stress, drought caused a decrease in photosynthesis through leaf loss and stomata closure, resulting in increasing the consumption of stored NSC to meet their metabolic needs. When carbon cannot maintain the basic functions of trees, "carbon starvation" will occur, which will eventually lead to the tree deaths^9,11. However, due to different tree species, ages, drought intensity and duration, the changes of NSC reserves under drought are diverse and complex¹².

The dynamics of NSC stored in different tree organs under different drought intensities and duration forms the basis of drought adaptation in trees. However, this topic still plagues the majority of ecologists due to its complex and diverse physiological processes, and no consistent trends have been observed thus far¹³. Different drought intensities will induce various NSC dynamics of trees¹³. It is speculated that Pinus edulis and Juniperus osteosperma under severe drought stress may not induce NSC consumption like mild or moderate drought stress, because it may cause trees to die of irreversible xylem cavitation⁹. O'Brien found that under moderate drought stress, the NSC in woody tissue decreased, while it increased under extreme drought in 10 shade-tolerant tropical tree seedlings¹⁴. In the experiment of conifer species¹⁵, it was found that the hydraulic conductivity and carbohydrate accumulation decreased under extreme drought in Pseudotsuga menziesii (Mirb.) Franco, Pinus ponderosa Douglas ex C. Lawson, and Pinus lambertiana Douglas.

It is reported that the dynamics of NSC varies with the duration of drought stress¹¹. At the early stage of drought stress, growth tends to respond more rapidly than photosynthesis to water stress^16,17, which may subsequently induce an increase in NSC content in trees. At the early stage of drought stress, NSC concentrations in Quercus coccifera, Arbutus andrachne, Pistacia lentiscus and Olea europaea increased¹⁶. With the prolongation of drought stress, the growth rate of trees may slow down or even stop, and the photosynthesis and respiration rate will decrease, with the former decreasing more than the latter. This may eventually lead to a decrease in NSC concentrations in trees¹¹.

In addition, the NSC dynamics under drought stress vary with the tree organ type¹⁸. As the water stress increases, root NSC is reported to decrease in Ulmus minor and Quercus ilex, yet the leaf and stem NSC increase in Quercus ilex, while leaf NSC decreases in Ulmus minor¹⁹. Under severe drought conditions, observed NSC concentrations in Picea abies roots to decrease significantly, yet this was not the case for all leaves and branches²⁰. Furthermore, drought-induced changes in carbon allocation, utilization, and transport differ between above- and below-ground tree tissues²¹. In a word, it is important to compare the response of NSC concentration in different tree tissues to drought stress of different intensity and progress.

Pinus yunnanensis, an endemic species and a major timber species in southwestern China, is a pioneer tree species for the reforestation of barren mountains, the area accounts for about 52% of the forest area in Yunnan Province²². Numerous studies have been conducted over the past 20 years on drought stress and NSC, typically including the effects of drought stress on Robinia pseudoacacia²³, Laurus nobilis²⁴, Ulmus minor and Quercus ilex¹⁹, Pinus massoniana²⁵, Pinus sylvestris, and Picea abies²⁶. The lack of studies on the intensity and duration of NSC in P. yunnanensis in response to drought stress limits our understanding of the drought resistance mechanisms of this tree species, especially in the context of the current global climate change. We make the following hypotheses that 1) The NSC content decreases only in severe drought and is related to the duration of drought. 2) NSC does not change during the initial stages of drought stress, while it decreases in the later stages.

Experimental site. The experimental site is located in the Arboretum of Southwest Forestry University (Kunming, Yunnan province, E102°46′, N25°03′). It is in the subtropical plateau monsoon climate zone, with an altitude of 1964 m, short frost period, mild climate, average annual temperature of 16.5°C, average annual precipitation of 1035 mm, average annual relative humidity of 67%, and red loam soil type. The temperature inside the shelter is 18.5–37.0℃, and the relative humidity is 22.3–48.0%.

Seedling preparation. Seeds for seedlings were obtained from P. yunnanensis seed orchard in Midu County, Dali City, Yunnan Province. The seeds were cultivated in seedling bags and grown in nursery for 2 years. 200 two-year-old P. yunnanensis seedlings from Yiliang Garden Forestry were selected on August 15, 2020, and transplanted into plastic flowerpots with a diameter of 26.5 cm, a base diameter of 21 cm, and a height of 21 cm. Each pot was filled with an equal amount of 6 kg of sieved soil mixture (red loam: humus = 3:2), with a tray was placed at the bottom, 1 seedling per pot. The transplanted seedlings were placed on the experimental plot, which was covered with mulch to prevent underground moisture from affecting the potted plants, and a rain shelter was built to ensure good ventilation and no shade. The soil water content of the potted plants was under completely controlled conditions. The soil moisture content of the seedlings was maintained at field holding capacity after planting to ensure the healthy growth of the seedlings. Field water holding capacity was determined by ring knife method, The field water holding capacity of the soil used in the experiment was 25.94%, and soil bulk density was 1.14g·cm-3. After the seedlings were transplanted, the root collar diameter and height of all the seedlings were measured with vernier calipers and tape measures with an accuracy of 0.01 mm and 0.1 cm, respectively. Before the experiment started, seedling height, ground diameter and biomass were 20.84 ± 1.70 cm, 18.60 ± 0.47 mm.

Application of drought treatments. All the seedlings with consistent growth were selected, numbered and divided into four groups The difference of seedling height and root collar diameter between the four groups were tested to ensure that the grouping is uniform, 40 plants per group. When there is no significant difference in seedling height and ground diameter between the four groups, hang a tag and record the corresponding number. Soil moisture and density were measured by the ring-knife method before the experiment, and then soil mass moisture content and volumetric moisture content were calculated, and real-time soil volumetric moisture content was detected with a soil moisture meter during the experiment. The potted weighing control method was used to simulate natural drought conditions, and four moisture gradients were set, namely, suitable moisture (CK), light drought stress (LD), moderate drought stress (MD) and severe drought stress (SD), with soil moisture gradients were set at 80% ± 5%, 65% ± 5%, 50% ± 5% and 35% ± 5% of the field water holding capacity, respectively; The actual moisture content was maintained at 19.45–22.05%, 15.56–18.16%, 11.67–14.27%, and 7.78–10.38%, respectively²³.The height and ground diameter of all viable seedlings were measured and recorded, and the water supply was then stopped to allow the soil water content to fall naturally to a predetermined range, namely, weighed and sampled as the initial value. The actual soil water content was measured using a Procheck handheld multifunctional soil moisture meter (Decagon, USA), controlled by the potted weighing method^23,27, and all potted plants were weighed daily at 17:00, the current weight was noted, and water control or watering was performed according to the target weight. The drought stress experiment started on March 14, 2021, and ended on May 12, 2021, lasting for 60 days in total. 40 seedlings in each of the 4 treatments, after calculating the weight of each pot.

Measurements and sampling. Measurements were performed from March 14, 2021, including growth measurements and destructive sampling on the 15th, 30th, 45th, and 60th days after the the start of the drought stress experiment (i.e., March 29, April 13, April 28, and May 12, respectively). The sampling time is fixed at 17:00 on the sampling date, because the NSC concentration in the leaves fluctuates with the photosynthesis activity²⁸. 4–6 seedlings were selected per treatment at each sampling. They were divided into needles, stems, coarse roots (root diameter > 2 mm) and fine roots (root diameter < 2 mm), and washed separately with distilled water. The seedlings were placed in an oven at 105°C for 30 minutes to prevent enzymatic carbohydrate reactions, and then dried at 80°C to a constant dry weight. For the NSC measurements, all dry samples were ground with a grinder until they could pass through a 60 mesh screen. The NSC was divided into soluble sugars and starch measurements, with the sum equal to the NSC content. Weighing 0.300 g of each dry sample for the determination of soluble sugar and starch. Both soluble sugar and starch contents were determined by the phenol colorimetric method²⁹. Firstly, the weighed dry sample was put into a 10 mL centrifuge tube, 4 mL of 80% ethanol was added and centrifuged at 80℃ for 30 min at 3000 r/min for 10 min, the supernatant was poured into a graduated test tube, the residue was added to 2mL of 80% ethanol and the extraction was repeated twice, the supernatant was decolorized by activated charcoal at 80℃, and the soluble sugar was determined by anthrone colorimetry. The absorbance at 625 nm was measured by anthrone colorimetric method, and the sugar content in the extract was obtained according to the standard curve; secondly, the precipitate after the extraction of soluble sugar was pasteurized with distilled water for 15 min, and then extracted with 9.2 mol/L perchloric acid 2 mL for 15 min and centrifuged, and the supernatant was collected. The absorbance at 625 nm was measured by phenol colorimetric method, and the sugar content in the extract was obtained according to the standard curve, and the obtained sugar content was multiplied by 0.9 as the actual starch content after deducting water when calculating the starch content.

Statistical analysis. All statistical analyses were performed with SPSS 19.0 (IBM). Values are expressed as mean ± standard deviation, with 4–6 replicates per treatment. The data were tested for normality and homogeneity of variance prior to analysis of variance. Two factor analysis of variance (ANOVA) was used to analyze the effects of drought intensity (CK, LD, MD, and SD) and drought duration (March 14, March 29, April 13, April 28, and May 12) on the NSC (sugar and starch) content in each tissue. The differences in soluble sugars, starch, NSC content and biomass were examined by one-way ANOVA with different water treatment levels and different treatment times, and Duncan's method was used for multiple comparisons, at a 0.05 level of significance. The graphs were plotted using Origin (2021 Edition, Origin Lab Company,US).

The effect of drought stress on NSC. The drought duration exhibited a highly significant effect on the soluble sugar, starch, NSC content and soluble sugar:starch values (Table 1). Drought intensity exerted significant effects on fine root soluble sugar, stem starch, stem and fine root NSC, and needle soluble sugars:starch values. Furthermore, a significant interaction effect was observed for drought stress duration and intensity on the coarse root NSC, needle soluble sugar:starch values.

Table 1

Two-way ANOVA (F-value) of different drought stress intensities, durations and organs on soluble sugar, starch, NSC and soluble sugar to starch ratios of *P. yunnanensis* seedlings.
	Fixed factors	Soluble sugar	Starch	NSC	Soluble sugar:starch
Needles	Drought intensity	2.10	2.04	2.46	3.66*
	drought duration	10.60**	24.81**	10.88**	18.76**
	Drought intensity×progression	1.67	0.60	0.99	2.69**
Stems	Drought intensity	2.00	3.88*	4.05*	1.56
	drought duration	4.08**	16.80**	5.45**	9.98**
	Drought intensity×progression	1.75	1.54	1.44	1.87
Coarse roots	Drought intensity	1.73	1.07	1.53	1.58
	drought duration	6.26**	9.45**	4.31**	6.76**
	Drought intensity×progression	1.58	1.40	2.12*	1.13
Fine roots	Drought intensity	5.77**	1.95	5.83**	0.55
	drought duration	42.82**	7.48**	19.15**	6.86**
	Drought intensity×progression	1.76	1.16	0.69	1.35
Note: P < 0.05, *P < 0.01.

Soluble sugar concentrations in different organs. Different seedling tissues (needles, stems, coarse and fine roots) exhibited different responses to drought intensity and progression in terms of sugar concentration (Fig. 1A-D). The soluble sugar content of needles, fine roots and coarse roots in each treatment showed a trend of increasing at 15 d and then decreasing. At 60 d, the concentration of soluble sugar in needles and coarse roots under SD increased, 23.50% and 31.63%, respectively, whereas decreased in fine roots by 24.61%.

Starch concentrations in different organs. The needle starch content of each treatment showed a trend of first decreasing and then increasing, and the stem starch content of MD and SD showed a trend of increasing at 45 d, and MD decreasing at 60 d (Fig. 2A-D). At 60 d, the needles starch concentration under LD was 40.31%, and the stem in SD was 119.26% higher than that of CK, respectively.

Figure 2 The effects of different durations on the content of starch in needles (A), stems (B), coarse roots (C) and fine roots (D) of P. yunnanensis under four kinds of stresses: suitable moisture (CK), light drought (LD), moderate drought (MD) and severe drought (SD). Different lowercase letters indicate significant differences between different sampling dates; different capital letters indicate significant differences among different drought stress intensity (P < 0.05).

NSC concentrations in different organs. The drought process had a significant effect on the NSC concentration in different seedling tissues in SD, and the drought intensity had a significant effect on stem and fine root NSC concentrations in seedlings (Table 1). At 15 d, the fine root NSC concentrations under LD was 31.11% lower than CK. At 60 d, compared with CK, NSC concentrations under MD and SD increased by 47.92% and 48.23% in stems, whereas fine root NSC concentrations decreased by 23.38% under SD (Fig. 3A-D).

Figure 3 The effects of different durations on the content of NSC in needles (A), stems (B), coarse roots (C) and fine roots (D) of P. yunnanensis under four kinds of stresses: suitable moisture (CK), light drought (LD), moderate drought (MD) and severe drought (SD). Different lowercase letters indicate significant differences between different sampling dates; different capital letters indicate significant differences among different drought stress intensity (P < 0.05).

Soluble sugar to starch ratio in different organs. Significant effects were observed for the drought duration on the ratio of soluble sugars to starch in each tissue (P < 0.05; Table 1). At 15 d, soluble sugar to starch ratios of needles under LD, MD and SD were significantly higher than those under CK. During the whole experiment, the value in each organ was greater than 1, and the ratio of MD and SD in the stem decreased at 45 d (Fig. 4A-D).

Figure 4 The effects of different durations on the ratio of soluble sugars to starch in needles (A), stems (B), coarse roots (C) and fine roots (D) of P. yunnanensis under four kinds of stresses: suitable moisture (CK), light drought (LD), moderate drought (MD) and severe drought (SD). Different lowercase letters indicate significant differences between different sampling dates; different capital letters indicate significant differences among different drought stress intensity (P < 0.05).

NSC distribution pattern. With the increase of drought process, under severe drought, the distribution proportion of NSC in organs under SD was in the order of: needle > stem > coarse root > fine root for 15 d, and stem > needle > coarse root > fine root for 30 d to 60 d (Fig. 5A-L). At day 60, compared to day 15, the LD treatment was observed to increase NSC concentrations in the needles and stems by 8% and 3%, respectively, and decrease coarse and fine roots by 10%, and 1%, respectively (Fig. 5A-L); the MD treatment increased needle and stem NSC by 3% and 14%, respectively, and decreased coarse and fine root NSC by 11% and 6%, respectively (Fig. 5A-L); and the SD treatment decreased needle, coarse root and fine root NSC by 10%, 1% and 2%, and increased stem NSC by 12% (Fig. 5A-L).

Effects of drought intensity on NSC dynamics. The effects of drought intensity on NSC concentrations may arise from changes in carbon sources or sinks and potential regulatory mechanisms. Other studies showed that in the presence of carbohydrate deficiency, trees may prioritize investment in NSC storage over in growth^13,27. Furthermore, Wiley found that NSC remained stable under moderate drought stress and significantly decreased under severe drought stress³⁰. In our study, significant changes in NSC of fine roots were only found at 15 d and at 60 d of drought stress, but no significant changes in NSC of needles, stems, and coarse roots were found at the beginning of drought (Fig. 3A-D). Moreover, the NSC of stems increased and that of fine roots decreased under severe drought conditions at the 60 d of drought (Fig. 3A-D). This confirms the hypothesis (1) of this study. After 60 days of moderate drought treatment, Deng observed that the root NSC content of 2-year-old Pinus massoniana seedlings increased³¹, this is contrary to the results of our study. It is possible that the decrease in NSC of fine roots at 15 d of drought is a result of the high rate of fine root growth. On the one hand, severe drought may significantly limit tree growth and development, leading to a reduction in the use of photosynthetic products and consequently starch accumulation³². On the other hand, prolonged severe drought may cause mechanical damage leading to metabolic disorders and blocked transport of stored NSC⁹. Thus, our study differs from Deng that carbon transfer from leaves to roots may be blocked under prolonged severe drought stress, therefore more NSC accumulation in stems and less NSC in fine roots³¹.

Soluble sugars, play an important role in osmotic adjustment, vascular transport, and embolism refilling³³. Starch is an important energy storage material. In our study, under SD treatment, fine root soluble sugars decreased at 45 d and 60 d of treatment, needle and coarse root sugars increased at 60 d (Fig. 1A-D). Our results are consistent with the fact that soluble sugar content in leaves and stems of gymnosperms increases under severe drought conditions¹³. Under drought conditions, soluble sugars in each organ of the tree increase to improve osmotic regulation and cope with stress, moreover, severe drought can impede the transfer of available carbon from leaves and stems to roots^12,34. Therefore, the increase in stem starch under SD at 60d (Fig. 2B) is also due to the accumulation of material because it is continuously obtained from photosynthesis but not transported out. And the decrease in soluble sugars of fine roots may also be due to the fact that fine roots are still growing but the transport pathway is blocked, while the intensity of drought is proportional to the degree of blockage and the material is unevenly replenished and consumed²³.

The variation patterns in NSC of gymnosperms and angiosperms under drought stress are different¹³. Moreover, there is still no consistent conclusion on the effect of drought stress for NSC, with some studies finding an increase in stored NSC in trees under drought stress^35,36, a decrease in stored NSC under drought stress³⁷, and unchanged stored NSC under drought stress³⁸. This suggested that plants coordinate photosynthesis, growth and respiration through complex internal stabilization mechanisms to maintain the relative stability of NSC during drought. Further understanding of the organ distribution of NSC in arid environments required the use of carbon isotope tracer technology.

Effects of drought duration on NSC dynamics. Research has demonstrated that the timescale of various physiological processes in different drought stress stages plays an important role in NSC dynamics³⁹. In the short term, tree growth are more sensitive than photosynthesis may increase the accumulation of NSC^16,17.This may be due to the role of stored carbon in the maintenance of various functions of the plant, maintaining homeostasis among and within the organs. Storage NSC play a role in maintaining essential functions during prolonged drought, such as osmoregulation and maintaining cellular tension¹⁴, possibly prolonging the time to death and maintaining xylem water potential².

In this study, needle, stem and coarse root NSC remained constant at 15 d, but fine root NSC decreased at the beginning of drought stress, whereas at 60d, stem NSC increased and fine root NSC decreased with increasing drought intensity. These results are consistent with all our initial hypotheses. This suggested that trees may exhibit resistance stability in the early stages of drought, more carbon allocation in storage organs to maintain their function¹⁴. In the medium term, the decline in photosynthesis, causing a decrease in reserve NSC as they are used for metabolism^11,17. However, the NSC of this study did not decrease at 30d and 45d (Fig. 3). In the long term, the complex adaptive mechanisms of trees may be fine-tuned between photosynthesis and growth to ensure sufficient stored NSC to maintain physiological regulation^39,40. Our study differs from the increase in soluble sugar content of drought-tolerant tree species under prolonged drought conditions⁴¹.

The soluble sugar content of needles and fine roots in SD and MD showed first increased and then decreased (Fig. 1A-D), the starch content of needles of all treatments first decreased and then increased, while the starch content of stems, coarse roots and fine roots remained stable the first 15 days, and after that, first increased and then decreased (Fig. 2A-D). This may be due to the fact that when P. yunnanensis seedlings encounter drought stress, growth is first inhibited and soluble sugars of all organs increase first in response to the organ's need for osmoregulation^18,42. Whereas, the soluble sugar content of each organs subsequently decreased, probably due to the decrease in photosynthesis that reduced soluble sugars and, more importantly, the prolonged drought increased the consumption of soluble sugars in coarse and fine roots, what's more, the inter-organ transport is hindered and only stored NSC could be consumed. In contrast, needle starch decreased first, probably because starch was converted to soluble sugars to maintain osmotic pressure, while needle starch subsequently increased probably because starch transport was blocked. While stem, coarse and fine root starch increased first probably because of mobilization of stored carbon to maintain organ homeostasis, and later decreased probably because trees closed some stomata to maintain water balance⁹, resulting in reduced photosynthesis, reduced sugar accumulation in leaves, and conversion of consumed starch to soluble sugars to maintain osmotic pressure.

In this study, at 60 d of drought stress, the starch content of stems and fine roots of Pinus yunnanensis seedlings ranged from 20.47 mg·g^− 1 to 48.95 mg·g^− 1 and 9.10 mg·g^− 1 to 21.20 mg·g^− 1 (Fig. 2A-D), respectively. The NSC content of stems and fine roots ranged from 77.14 mg·g^− 1 to 132.55 mg·g^− 1 and 43.08 mg·g^− 1 to 61.87 mg·g^− 1 (Fig. 3A-D), respectively. This is consistent to previous studies where the NSC content did not approach zero³⁴. It is probably due to the fact that during drought stress, the NSC content in plants may maintain a certain threshold to maintain metabolic processes and hydrodynamic integrity⁴³. Failure of osmoregulation or other factors leading to tree death can occur when NSC levels are reduced to a certain level⁴³. Therefore, the mortality mechanism of P. yunnanensis seedlings under drought stress needs to be further investigated.

The drought duration had a significant effect on the content of NSC in all organs of P. yunnanensis seedlings. The drought intensity only had a significant effect on the content of NSC in stems and fine roots of P. yunnanensis seedlings. With the increase of drought stress, there was no significant difference in NSC content of different organs in the first 45 days of the experiment, except for fine roots of LD at d 15, which showed a significant decrease. While at d 60, the fine root NSC content of SD decreased and the stem NSC content of MD and SD increased. Our study is of great significance to better understand the dynamic changes of NSC in different organs under drought stress. However, only changes in potted seedlings were discussed in this study. Whether such a pattern exists in seedlings and young trees under field conditions needs further study. In addition, this study did not control carbon input to study the effects of drought stress. How stored NSC varies between organs under drought stress and complete carbon limitation could be a direction for future research. Furthermore, effects of the circadian/light regulation of carbohydrates in trees under drought stress conditions is also a direction that could be studied in the future.

Author Contributions statement

Xin Deng and Xin Chen co-wrote the manuscript and completed the final article. Junwen Wu, Yaocong Liu and Li Zheng designed the experiments, provided critical revisions and final approval of the article. Ping Lan, Tianyu Li, Jingwen Yang and Hang Zhang carried out the experiments and run the data. All authors also helped to write, read and approved the final manuscript.

Conflicts of Interest

There are no conflicts of interest to declare.

Author Contribution

"Xin Deng and Xin Chen co-wrote the manuscript and completed the final article. Junwen Wu, Yaocong Liu and Li Zheng designed the experiments, provided critical revisions and final approval of the article. Ping Lan, Tianyu Li, Jingwen Yang and Hang Zhang carried out the experiments and run the data. All authors also helped to write, read and approved the final manuscript."

Acknowledgments

This study was supported by the National Natural Science Foundation of China (31960306).

Data Availability

"The datasets used during the current study are available from the corresponding author on reasonable request".

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Non-structural carbohydrates dynamics of Pinus yunnanensis seedlings under three levels of continuous drought stress

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