1. Introduction
Teak (
Tectona grandis Linn.) is naturally distributed in India, Thailand, Myanmar, and Laos [
1,
2]. Its desirable hardwood properties, fine grain, and durability have made teak the luxury timber for furniture making, carving, and building around the world [
3,
4]. Due to its economical importance, teak has been introduced widely in the tropical regions since the 19th century, especially in Asia, Africa, Central America, and South America [
5].
As one of the most valuable wood species in international markets, teak plantations have developed rapidly in the recent decade. Developing high productivity and uniform clones that can be used for plantations in different regions has become an important objective of teak breeding. Information on variation of photosynthetic parameters and their relationship with growth traits help us understand underlying processes and responses, and will be useful in tree improvement programs. During the growth process of plants, organic compounds are generated by photosynthesis, and gradually accumulate in trunks. The photosynthetic characteristics are the main measurable indicators of plant growth rates [
6]. Numerous studies on breeding for high photosynthetic ability in crops have been conducted, to improve the yield [
7,
8,
9], but studies on forest trees are limited [
10,
11,
12]. Chu et al. 2010 [
10] studied gas exchange and chlorophyll fluorescence parameters, as well as their relationship with the growth of
Populus nigra, and found that the species originating in Serbia, southern and east Europe can be regarded as a resource with high light-use efficiency for future breeding. Teak has broad leaves and prefers warmth and sunlight, and developing clones with high productivity and uniformity by evaluating photosynthetic characteristics can be an important goal in teak breeding. In the past, teak breeding was mainly focused on the analysis of growth indices in field experiments [
13,
14], and studies on photosynthetic physiology of teak are limited to those on photosynthetic responses of a single clone to simulated acid rain stress [
15], photosynthetic physiological characteristics under different disturbance intensities among teak plants [
16], and diurnal and seasonal photosynthetic characteristics in teak clones [
17]. However, studies on teak germplasm or clones, which systematically estimate photosynthetic characteristics and correlation with growth, have not been reported.
The purpose of this study was (1) to investigate the genetic variation of photosynthetic parameters and growth traits of teak clones, (2) to reveal the correlation, if any, between photosynthetic characteristics and growth traits within the gene resources of teak, and (3) to evaluate and select superior teak resources possessing high photosynthetic efficiency for breeding.
4. Discussion
Plant growth and yield depend largely on photosynthesis [
23,
24]. Plant photosynthesis is not only affected by environmental factors, but also affected by plant genetic characteristics. It is the complex process of interaction between plant genetic and environmental factors that influences photosynthetic activity [
25]. To date, ecophysiological studies on photosynthesis in forest trees were those that examined the effects of stress on photosynthetic physiology [
26,
27,
28,
29], and the photosynthetic responses to light intensity [
30] and CO
2 concentration [
31]. The present study chiefly focused on systematically measuring photosynthetic gas exchange and chlorophyll fluorescence parameters, correlating the photosynthetic characteristics with growth, and providing a means of rapid evaluation of teak germplasm, for introduction, utilization, and improvement of teak resources in future breeding programs.
Our study showed that teak clones had high variation and high heritability (h
2) for many growth and physiological traits. The results were generally consistent with the findings reported for
Populus trichocarpa by McKown [
32]. The gas exchange, chlorophyll fluorescence, and growth parameters of teak clones were highly controlled by genetic factors, especially for the actual quantum yield (Yield) of PSII. Therefore, such a parameter has high practical significance and can be effectively used for improving the efficiency of teak breeding. However, it is worth emphasizing that intercellular CO
2 concentration (
Ci) and non-photochemical quenching (NPQ) were greatly influenced by the environment.
Further analysis showed that teak clones and resources from different regions vary in their photosynthetic characteristics. In this study, teak clones 7029, 71-5, 7219, 7412, 7122, and provenances 3070, 3074, 3071, which had higher
Pn, can be regarded as the key resource in future breeding and management programs. However, more teak clones from different provenances and countries need to be included in this kind of study in the future. Huang et al., 2016 [
33] had suggested, after SSR molecular marker testing, that the Nigerian provenance 3078, investigated in this paper, may have originated from India. The present studies, that reveal their similar photosynthetic characteristics, further corroborates this conclusion.
The significantly positive correlation that the net photosynthetic rate has with seedling height, individual volume,
Fv/
Fm,
Gs, and
Tr, is an interesting finding of this study. In addition, seedling height was significantly and positively correlated with plant height and individual volume. Both results indicate that teak clones of high
Pn and high seedling height result in fast-growing clones. However, it is known that photosynthetic processes are influenced by environmental conditions such as light, temperature, water, and nutrients [
25]. Photosynthetic rate is not the only limiting factor for growth [
34]. These factors may affect growth differently for different clones, resulting in no significant relationship between
Pn and plant height or DBH of field growth at 4 years, the result being similar to previous reports [
12,
35].
Correlation analysis also revealed that water use efficiency was significantly but negatively correlated with
Tr, suggesting that teak clone
WUE may decrease when transpiration rate is high in daytime. Such results were consistent with the study by Huang et al., 2016 [
17], in that diurnal variation possessed a double peaked curve, with a “midday depression” phenomenon in summer, when strong sunshine often accompanied by high temperature produces excessive transpiration, followed by decline of water use efficiency. There was no significant correlation between seedling collar diameter and other parameters, except for seedling height, consistent with the results of the study on photosynthesis and growth of
Populus nigra [
10]. At the same time, the coefficient of genetic variation of
Ci and
Fv/
Fm were lower than other photosynthetic indices in the present study, similar to photosynthetic characteristics of the clones [
10]. The variation coefficients of
Fv/
Fm were small in this study (0.028) and in
Populus nigra clones (0.024) [
10]. This may be due to CO
2 concentration, leaf temperature, and relative humidity fluctuating significantly under natural conditions, reducing the
Fv/
Fm compared to conditions where they remain constant [
25].
Farquhar et al., 1982 [
36] concluded that photosynthetic rate was controlled by stomatal factors when
Pn,
Ci, and
Gs increased or decreased at the same time. In this study, correlation analysis indicates that there was significant positive correlation between
Pn and
Gs, a positive but not significant correlation between
Pn and
Ci,
Gs, and
Ci, suggesting that the photosynthetic rate of teak was mostly controlled by stomatal factors. Plant dynamic photosynthesis was affected by many environmental factors such as light intensity, CO
2 concentration, leaf temperature, and relative humidity. Fluctuating environments would have a large impact on photosynthesis. Plants have a highly responsive regulatory system to make rapid photosynthetic responses to fluctuating environments, and a number of photoprotective mechanisms allow plants to maintain photosynthesis under stressful fluctuating environments [
25].
For further research, the following points need to be considered in the future studies on teak. Firstly, it is desirable that more clones from different provenances be included in this kind of study in order to analyze variation among teak resources of different provenances more efficiently. Secondly, the differences in
Pn among teak clones in this study was greater than that seen in
Populus nigra [
10] and
Populus deltoides clones [
11]. It is to be ascertained whether such a difference was caused by inherent differences in photosynthetic characteristics between the tree species, or if is due to other reasons. Thirdly, further evaluation of differences in leaf area between teak clones is needed since tree growth is restricted not only by photosynthetic efficiency, but also by photosynthetic leaf area [
37,
38]. Lastly, we found that photosynthetic rates of teak plants in the field trial measured at the age of 2 years were higher than that of the potted seedlings and, therefore, correlation analysis among photosynthetic parameters, photosynthetic leaf area, and growth traits in field trials need to be executed in future teak breeding programs.
5. Conclusions
Our findings have at least three important implications. First, photosynthetic parameters other than intercellular CO2 concentration (Ci) are highly controlled by genetic factors. In addition, photosynthetic parameters and growth traits in different clones revealed abundant genetic variation. Second, the net photosynthetic rate (Pn), seedling height, and individual volume of wood significantly correlated between each other, and seedling height was significantly correlated with plant height in field trials, suggesting Pn and seedling height can help us in teak breeding. Third, teak clones 7029, 71-5, 7219, 7412, and 7122, and provenances 3070, 3074, and 3071, revealed to have higher photosynthetic rate, can be regarded as key resources for future breeding and germplasm resource management.