Morphological and Physiological Responses of Pinus massoniana Seedlings to Different Light Gradients

Haoyun Wang,Min Li,Xiaokun Zhu,Guijie Ding,Feng Wu,Changshuang Shi

doi:10.3390/f12050523

Abstract

Light intensity is a critical factor regulating photosynthetic capacity in plants. However, the effects of varying light intensity on morphological and photoprotective mechanisms in Pinus massoniana seedlings have not been explored in depth, especially those in the first seedling growing season. We measured the growth, photosynthetic physiology, biochemistry, and chlorophyll fluorescence of P. massoniana seedlings at four light gradients: 100% relative irradiance (RI, full sunlight), 70% RI, 50% RI, and 20% RI. The seedling height at 70% RI was 9.27% higher than that at 100% RI. However, seedling height was inhibited under low light intensity; at 20% RI, all seedlings died. The decreasing light intensity inhibited ground diameter growth but increased the height-diameter ratio. The secondary needle emergence rate was 53.4% higher at 70% RI than at 100% RI but was only 2% at 50% RI. The chlorophyll and carotenoid contents increased significantly with decreasing light intensity. The increased Chl b and Car contents promoted the photoreceptor potential of the violet (400~420 nm), blue (440~480 nm), and yellow-orange (597~655 nm) regions in leaves. Among the chlorophyll fluorescence parameters, Fv/Fm, Fv′/Fm′, Y(II), qp, and ETR all reached maximum values at 70% RI but were significantly lower at 50% RI than at 100% RI. However, decreasing the light intensity caused a reduction in NPQ. The 70% RI level increased POD and SOD activity and the contents of osmotic regulation substances and slowed MDA accumulation. Seedlings at 70% RI had a higher growth rate, higher photosynthetic activity and potential, and significantly greater stress resistance than the other seedlings. Therefore, appropriate shading measures were beneficial to the cultivation of vigorous seedlings. Furthermore, spectral reflectance indexes were found to be a suitable tool for monitoring the photosynthetic physiological characteristics, stress resistance characteristics, and growth status of P. massoniana seedlings in real time.

Highlights

Light is the environmental factor that most directly affects seedling survival and early growth [1] and is an important ecological factor in forest community succession [2]
We investigated the initiation of secondary needles in current-year seedlings under a light gradient and performed quantitative analyses of the pigment contents, photosynthetic characteristics, protective enzyme activity, and osmotic regulation substance contents during seedling growth to test the following hypotheses: (1) during the whole growth season, from cotyledon germination to the end of the seedling growth period, the seedlings would show different responses and regulatory mechanisms in morphology and metabolic systems along the light gradient, and (2) the change trend between the vegetation index and the measured values would capture the intrinsic differences among seedlings, indicating the feasibility of using spectral reflectance measurement technology to monitor the physiological status of seedlings in real time
Compared with that at 100% relative irradiance (RI), ground diameter growth slowed with the decrease in light intensity, especially at extremely low light intensity; all the seedlings died at the lowest light intensity, indicating that P. massoniana seedlings cannot adapt to low light conditions

Summary

Introduction

Light is the environmental factor that most directly affects seedling survival and early growth [1] and is an important ecological factor in forest community succession [2]. The adaptation of plants to light gradients limits the regulation of the photosynthetic capacity of individual leaves and the overall photosynthetic characteristics of seedlings. In both saplings and mature trees, the light adaptation strategy of the leaves will directly determine the carbon acquisition and energy distribution modes [4,5]. Species tolerance to light gradients in early ontogenetic development has important guiding significance for forest management, ecosystem restoration and horticultural cultivation

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Discussion

Conclusion