Abstract
Leaves of various ages and positions in a plant's canopy can present distinct physiological, morphological and anatomical characteristics, leading to complexities in selecting a single leaf for spectral representation of an entire plant. A fortiori, as growth rates between canopies differ, spectral-based comparisons across multiple plants – often based on leaves' position but not age – becomes an even more challenging mission. This study explores the effect of differential growth rates on the reflectance variability between leaves of different canopies, and its implication on physiological predictions made by widely-used spectral indices. Two distinct irrigation treatments were applied for one month, in order to trigger the formation of different growth rates between two groups of grapevines. Throughout the experiment, the plants were physiologically and morphologically monitored, while leaves from every part of their canopies were spectrally and histologically sampled. As the control vines were constantly developing new leaves, the water deficit plants were experiencing growth inhibition, resulting in leaves of different age at similar nodal position across the treatments. This modification of the age-position correlation was characterized by a near infrared reflectance difference between younger and older leaves, which was found to be exponentially correlated (R2 = 0.98) to the age-dependent area of intercellular air spaces within the spongy parenchyma. Overall, the foliage of the control plant became more spectrally variable, creating complications for intra- and inter-treatment leaf-based comparisons. Of the derived indices, the Structure-Insensitive Pigment Index (SIPI) was found indifferent to the age-position effect, allowing the treatments to be compared at any nodal position, while a Normalized Difference Vegetation Index (NDVI)-based stomatal conductance prediction was substantially affected by differential growth rates. As various biotic and abiotic factors may form distinctions in growth, future precision agriculture studies should consider its spectral effect on physiological predictions.
Highlights
Over the last few decades, improvements in the spatial and spectral capabilities of remote sensors have significantly promoted the precise monitoring of crop growth and development, from the field scale to the satellite level [1,2,3,4]
Significant physiological distinctions between the irrigation groups were noticed on the 7th day of experiment (DOE), as the stomatal conductance (Figure 1B) and net assimilation rates (Figure 1C) of the water deficit (WD) vines were lower than those of the control vines by 0.23 mol H2O m22 s21 (a 75% reduction) and 5.35 mmol CO2 m22 s21 (a 37% reduction), respectively
While the average control plant began the trial 2 nodes shorter than the average WD plant, the former concluded the experiment with 11 nodes more than the latter (Figure 2A)
Summary
Over the last few decades, improvements in the spatial and spectral capabilities of remote sensors have significantly promoted the precise monitoring of crop growth and development, from the field scale to the satellite level [1,2,3,4]. Leaves of various ages and positions in a single plant were previously documented to have distinct levels of water content [6,7,8], water use efficiency [9,10], stomatal conductance [11,12], nitrogen content and allocation [13,14,15], photosynthesis [16,17,18,19], chlorophyll content [19,20,21], assimilation rates [12,22,23], and cellular structures and processes [24] These within-canopy variations highlight the difficulty of representing an entire plant and, more so, comparing between different plants using a single leaf – normally chosen on the basis of the ‘age’ and ‘position’ concepts, which many studies refer to as equals (e.g., [25,26,27,28]). The specific control mechanism for this phenomenon, which is likely genetically- and environmentally dependent, is yet to be elucidated
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