Photosynthetic gas exchange in seedlings of Hopea odorata Roxb. (South Vietnam)

The paper presents the results of studies related to the study of photosynthetic gas exchange at the leaf level in situ of three-year-old seedlings of Hopea odorata Roxb. during the dry season (South Vietnam). The results obtained will contribute to a better theoretical understanding of the growth and development of plants of this species. The obtained quantitative values of the daily fluxes of photosynthetic gas exchange, as well as the physiological reactions of the plant to environmental conditions, will allow a more qualitative approach to the assessment of carbon fluxes in the corresponding ecosystems. OBJECTS AND METHODS OF RESEARCH The research was conducted from January to April 2020 on the territory of the Cat Tien National Park (South Vietnam) (11.41530 s.w., 107.42460 v.d.) during the dry season. Three-year-old H. odorata seedlings planted in mid-January 2020 were selected as the object of the study. 25 seedlings were selected for observation. The average height of seedlings is 110.0 0.5 cm (SD = 14.4 cm), and their average diameter at a height of ~10 cm is 8.3 0.1 mm (SD = 0.6 mm). According to the illumination conditions of the site and the location of the seedlings, the site was divided into three experimental sites (SA1, SA2, SA3), Fig. 1. The SA1 site (seedlings № 1-16) was located in a relatively open space. The total value of photosynthetically active radiation (FAR) per seedling of this site was 25.71.2 molm-2. The SA2 site (seedlings № 17-20) was located under the crowns of adult trees. The total value of FAR is 10.80.5 molm-2. The SA3 site (seedlings № 21-25) was adjacent to an untouched part of the forest. The total value of FAR is 9.20.4 molm-2. During planting, as well as on 12.02 and 19.03, the seedlings were watered. On 17.02 there was heavy rain at the site. To clarify the question of the effect of the moisture content in the soil on the condition of the studied plants, seedlings № 4-9 were watered from 26.03 to 5.04. The processes of photosynthesis were considered from the standpoint of CO2 gas exchange. Photosynthesis was measured in situ using the Portable Photosynthesis System LI-6800 (Li-Cor, USA). The formed intact leaves in the upper part of the crowns were used for the study. The moisture content in the soil was determined in a 12 cm surface layer using the HydroSense II soil moisture meter (Campbell Scientific, Inc. USA). Soil moisture below 10% corresponded to withering humidity. To study the growth of seedlings in thickness, the stem diameters were measured at a height of 10 cm. The MichaelisMunten equation was used as a basis for the mathematical description of the dependence of photosynthesis on FAR. We used this equation in a modified form [Kaibeyainen, 2009]: A = AmQ/(Q + KM) + Ad. (1) To evaluate the efficiency of photosynthesis, we used the angular coefficient of the tangent (a) to the curve of the function (1) at the point corresponding to KM. From a physical point of view, this coefficient reflects the rate of change in photosynthesis when the headlights change by one unit. RESULTS Figures 3, 4, 5 show graphs showing the daily dynamics of photosynthesis and FAR of seedlings growing on SA1, SA2, SA3. Figure 6 shows graphs showing the daily dynamics of photosynthesis and FAR of watered and non-watered seedlings. The soil moisture under the watered seedlings was ~25.1%, under the non-watered ones - ~8.0%. Photosynthesis of watered seedlings was well associated with FAR, r = 0.84 (for non-watered seedlings, r = -0.34). Fig. 7 shows the values of photosynthesis depending on the FAR, the curves approximating these values obtained according to (1), and tangents to these curves at points corresponding to KM. The indicators characterizing the photosynthetic features of the leaves of seedlings obtained according to (1) are summarized in Table.1. Figure 8 (a) shows the dynamics of the growth of the studied plants by the diameter of the trunk, and Figure 8 (b) shows its derivative, showing the rate of growth per day. DISCUSSION In the morning, the daily dynamics, Fig. 3-5, were characterized by a high degree of association of photosynthesis with FAR, as well as a rapid increase in the values of photosynthesis to maximum values. At the same time, saplings growing on SA1 had a high degree of association of photosynthesis with FAR and in the evening hours, with the decline of FAR. In the midday hours, except for SA2 seedlings, the values under consideration were not associated, and the midday depression of photosynthesis was clearly traced on the daily curves. At the same time, on SA3 seedlings, midday depression was traced until the end of the day. In SA2 seedlings, photosynthesis was well associated with FAR throughout the day. It should be noted that the maximum values of the FAR on SA2 were ~ 640 mmolm2s-1, whereas on other sites - 1600 mmolm-2s-1 and more. The analysis of environmental factors showed that in our case, the main factors that could have inhibitory effects on the photosynthesis of seedlings are FAR and their water supply, determined by soil moisture. To find out which of these factors had the greatest inhibitory effect on photosynthesis, an experiment was conducted with additional watering of seedlings. As can be seen from Fig. 6, FAR did not have any noticeable inhibitory effect on the photosynthesis of the watered seedlings. However, as follows from the comparison of the dynamics of photosynthesis with the dynamics of soil moisture, with a decrease in soil moisture, there is an increasingly depression of photosynthesis. The depression of photosynthesis caused by lack of water has a close effect on its net productivity. Net productivity can be estimated by the increase in plant biomass. Indirectly, the increase in plant biomass can be estimated by the growth of the plant in the thickness of the trunk. When comparing the dynamics of the growth of the studied seedlings in thickness with the dynamics of soil moisture, it can be seen that with a decrease in soil moisture, the increase also decreases, up to a negative value observed in plants at SA3, which we associate with a certain shrinkage of wood. The maximum values of photosynthesis for saplings on SA1 and SA3, Table 1, were approximately the same and were limited by insufficient moisture content in the soil. Nevertheless, saplings on SA1, in comparison with SA3, were characterized by better parameters of leaf growth and development, and, accordingly, biomass growth, Fig. 8. For SA2 seedlings, Table 1, the maximum values of photosynthesis intensity were 5.0 mmolm2s-1 and were mainly limited by a limited FAR. Photosynthesis indicators of these plants were better than those of others, which suggests that SA2 seedlings had a certain shade tolerance. The watered seedlings were not subjected to any noticeable inhibitory effects from environmental factors. The maximum value of photosynthesis for them was 10.5 mmolm2s-1, and the values of photosynthesis efficiency indicators showed that the leaves of these plants were still in the development stage. Based on the analysis, we can make an assumption explaining the high degree of association of photosynthesis of seedlings with FAR, as well as the rapid growth of photosynthesis to the maximum values observed in the morning. This assumption is that during the dark time of the day, seedlings could restore their water balance. At the same time, the restoration of the water balance was possible until the moisture content in the soil corresponding to the withering humidity was reached. Thus, the depression of photosynthesis of H. odorata seedlings revealed during the study was a consequence of their insufficient water supply, which was regulated by the moisture content in the soil. The greatest net productivity of photosynthesis was distinguished by seedlings growing in more illuminated conditions. Seedlings growing in shaded conditions were less exposed to lack of moisture. Seedlings growing in competitive relationships were subjected to the greatest depression of photosynthesis.