Photosynthetic and Growth Responses in a Pioneer Tree (Japanese White Birch) and Competitive Perennial Weeds (Eupatorium sp.) Grown Under Different Regimes With Limited Water Supply to Waterlogging

Leaf acclimation to soil flooding and light availability underlies photosynthetic capacity of Lindera melissifolia, an endangered shrub of bottomland forests in the Mississippi Alluvial Valley, USA

Lindera melissifolia is an endangered shrub indigenous to the broadleaf forest of the Mississippi Alluvial Valley (MAV). In this region, extant colonies of the species are found in periodically ponded habitats where a diversity of broadleaf trees can form well-developed overstory and sub-canopies-these habitat characteristics suggest that soil flooding and light availability are primary drivers of L. melissifolia ecophysiology. To understand how these two factors affect its photosynthetic capacity, we quantified leaf characteristics and photosynthetic response of plants grown in a large-scaled, field setting of three distinct soil flooding levels (no flood, 0 day; short-term flood, 45 days; and extended flood, 90 days) each containing three distinct light availability levels (high light, 30% shade cloth; intermediate light, 63% shade cloth; and low light, 95% shade cloth). Lindera melissifolia leaves showed marked plasticity to interacting effects of flooding and light with lamina mass per unit area (Lm/a) varying 78% and total nitrogen content per unit area (Na) varying 63% from the maximum. Photosynthetic capacity (A1800-a) ranged 123% increasing linearly with Na from low to high light. Extended flooding decreased the slope of this relationship 99% through a reduction in N availability and metabolic depression of A1800-a relative to Na. However, neither soil flooding nor light imposed an additive limitation on photosynthetic capacity when the other factor was at its most stressful level, and the A1800-a-Na relationship for plants that experienced short-term flooding suggested post-flood acclimation in photosynthetic capacity was approaching the maximal level under respective light environments. Our findings provide evidence for wide plasticity and acclimation potential of L. melissifolia photosynthetic capacity, which supports active habitat management, such as manipulation of stand structure for improved understory light environments, to benefit long-term conservation of the species in the MAV.

Exposure to strong irradiance exacerbates photoinhibition and suppresses N resorption during leaf senescence in shade-grown seedlings of fullmoon maple (Acer japonicum)

Leaves of fullmoon maple (Acer japonicum) turn brilliant red with anthocyanins synthesis in autumn. Based on field observations, autumn coloring mainly occurs in outer-canopy leaves exposed to sun, whereas inner-canopy leaves remain green for a certain longer period before finally turn yellowish red with a smaller amount of anthocyanins. Here, we hypothesized that outer-canopy leaves protect themselves against photooxidative stress via anthocyanins while simultaneously shading inner canopy leaves and protecting them from strong light (holocanopy hypothesis). To test this hypothesis, we investigated photoinhibition and leaf N content during autumn senescence in leaves of pot-grown seedlings of fullmoon maple either raised under shade (L0, ≈13% relative irradiance to open) or transferred to full sunlight conditions on 5^th (LH1), 12^th (LH2), or 18^th (LH3) Oct, 2021. Dry mass-based leaf N (N_mass) in green leaves in shade-grown seedlings was ≈ 30 mg N g^-1 in summer. N_mass in shed leaves (25^th Oct to 1^st Nov) was 11.1, 12.0, 14.6, and 10.1 mg N g^-1 in L0, LH1, LH2, and LH3 conditions, respectively. Higher N_mass was observed in shed leaves in LH2, compared to other experimental conditions, suggesting an incomplete N resorption in LH2. F_v/F_m after an overnight dark-adaptation, measured on 19^th Oct when leaf N was actively resorbed, ranked L0: 0.72 > LH3: 0.56 > LH1: 0.45 > LH2: 0.25. As decreased F_v/F_m indicates photoinhibition, leaves in LH2 condition suffered the most severe photoinhibition. Leaf soluble sugar content decreased, but protein carbonylation increased with decreasing F_v/F_m across shade-grown seedlings (L0, LH1, LH2, and LH3) on 19^th Oct, suggesting impaired photosynthetic carbon gain and possible membrane peroxidation induced by photooxidative stress, especially in LH2 condition with less N resorption efficiency. Although the impairment of N resorption seems to depend on the timing and intensity of strong light exposure, air temperature, and consequently the degree of photoinhibition, the photoprotective role of anthocyanins in outer-canopy leaves of fullmoon maple might also contribute to allow a safe N resorption in inner-canopy leaves by prolonged shading.

Root-Zone Heat Priming Effects on Maximum Quantum Efficiency of PSII, Productivity, Root Morphology and Nutritional Quality of Two Aeroponically Grown Leafy Greens in a Tropical Greenhouse

This study investigated the effects of root zone (RZ) heat priming on Eruca sativa (cv. Arugula) and Lactuca sativa (cv. Canasta) in a tropical greenhouse by exposing them to three different RZ temperatures (RZTs). For the first group, plants were grown at 25 °C-RZT for 36 days. The second group of plants was grown at 25 °C-RZT for 10 days before transferring to 38 °C-RZT for 10 days and then to 42 °C-RZT for another 16 days (primed). For the third group, plants were grown at 25 °C-RZT for 20 days and for the last 16 days were transferred to 42 °C-RZT (non-primed). RZ heat priming did not affect the leaf expansion of Arugula while the leaf expansion was slower in RZ heat primed Canasta compared to 25 °C-RZT plants. After transferring to 42 °C-RZT in the later stage, RZ heat primed Arugula had similar productivity but a larger root system with higher nutritional quality at harvest and during postharvest storage compared to those of 25 °C-RZT plants. After subjection to 42 °C-RZT, although RZ heat priming enhanced its nutritional quality, the productivity of Canasta was compromised. The results of this study suggest that it is feasible to enhance productivity and nutritional quality of leafy greens at lower production cost through RZT management.