Several studies have shown environment and nutrition have effects on growth of lettuce. RGR, the relative increase in weight per day, changes slowly as plants grow in a constant environment. What are the components of plant growth that scale according to RGR? To what extent are these determined by irradiance or time of year and by conditions of the nutrient solution in hydroponics?
Irradiance is the primary factor affecting RGR (Gent, 2014). However, a function of light intercepted by the crop may be better related to RGR. Bierhuizen et al. (1973) showed that shoot dry weight of lettuce was linearly related to radiation corrected for soil cover and leaf area growth was related to degree days. Additive Gomperz functions in soil temperature and radiation correlated with head weight better than either factor alone (Salomez and Hofman, 2007). The rate of photosynthesis of lettuce increased with light intensity (Park and Yong Beom, 2001). When leaf area index was less than one, there was a linear relation between CO2 consumption and leaf area of lettuce crops in a plant factory (Choi et al., 2014). How would such a function be changed to predict RGR for leaf area index greater than one?
Temperature may affect plant growth. The most rapid growth of lettuce in a controlled environment was at 25/25 °C day/night temperature (Knight and Mitchell, 1983). The maximum dry weight of lettuce in a greenhouse was at 24/24 °C air/root temperature (Thompson et al., 1998), and a 17 °C air or root temperature resulted in less growth. When temperature was varied during the day, it did not affect growth, except when the minimum temperature was at dawn (Miller and Langhans, 1985). There was no interaction of effects of day and night, or air and root temperatures in hydroponics (Hicklenton and Wolynetz, 1987), but the leaf area-to-dry weight ratio increased with both day and night temperatures to 23/19 °C day/night. Under tropical conditions, the longer the period with temperature controlled at 20 °C, the higher the biomass (Qin et al., 2002). There was no effect of day to night temperature differences on lettuce growth when it changed because of light intensity (Eguchi et al., 1997).
The pH or concentration of various elements could affect growth in hydroponics. The concentration of elements is related to EC. A solution EC of 1.2 to 4.8 dS·m−1 did not affect photosynthesis of lettuce (Park and Yong Beom, 2001). Yield decreased when EC increased from 2.8 to 3.8 or 4.8 dS·m−1 (Scuderi et al., 2009). However, lettuce had faster growth in summer than in winter when nitrate was raised from 2.5 to 10 mm (Van der Boon et al., 1990). Of the limiting nutrients for lettuce, the relation of nitrate to growth is most complex. Lettuce grown in sand was N deficient at 2 mm nitrate, 28 mg·L−1 nitrogen, and suffered salt toxicity at 36 mm nitrate, 504 mg·L−1 N (Huett and White, 1992). In hydroponics, there was a linear relation between RGR and tissue P or K, but there was rapid growth until tissue nitrate was depleted (Burns, 1992). This may be due to the relation with organic N in tissue (Burns, 1994). Hydroponic lettuce switched to a no-nitrate solution at various times had changes in growth such that there was no growth at 1% organic N by dry weight, up to control growth at greater than 4% N (Broadley et al., 2003). The change in nitrate concentration in plant tissue with irradiance was much greater than the effect on total nitrogen (Demsar et al., 2004). This resulted in an interaction of effects of nitrate and irradiance on growth (De Pinheiro Henricques and Marcelis, 2000). There was a linear relation between the maximum rate of nitrate inflow into lettuce roots and RGR, as affected by radiation, temperature, or plant size, but there was little relation with the nitrate concentration in the plant (Steingrobe and Schenk, 1994).
There are fewer reports on the effect of environment on growth of spinach. The effects of solution nitrate on spinach growth were similar to those for lettuce. Nitrate withdrawal for 2 d did not affect fresh weight, but tissue nitrate dropped from 3000 to 600 ppm at 4 d (Fukuda et al., 1999). Spinach growth decreased at 4 d with nitrate withdrawal under high light and at 2 d when switched from high to low light (Buysse et al., 1996). RGR of spinach was proportional to the reduced N in leaf tissue minus the minimum concentration required for growth (Smolders and Merckx, 1992). Solution concentrations of 0.8 or 4 mm nitrate gave an RGR of 0.17 and 0.25 for spinach (Ter Steege et al., 1998). The nitrate concentration in solution affected nitrate in leaves. It was 2-fold higher at 4.0 compared with 0.8 mm nitrate in solution (Ter Steege et al., 1998). When spinach was grown in solutions with different nutrient proportions, the solution with highest nitrate and K and total nutrients gave the highest fresh weight (Rivera et al., 2009).
Previously, I studied the effects of environment on composition of metabolites in lettuce and spinach (Gent, 2014, 2016). In lettuce, much of the variation in tissue composition on a fresh weight basis was related to NDLI (Gent, 2014). Except for nitrate, metabolite concentrations on a fresh weight basis increased with irradiance, and the changes in sugars due to irradiance were greater when plants were harvested in the afternoon rather than in the morning. The NDLI also explained much of the variation in composition of spinach (Gent, 2016). However, unlike in lettuce, sugars in spinach decreased more with an increase in temperature than expected from NDLI. This was related to a decrease of sugars in spinach petioles with temperature up to 20 °C. From 1996 to 1998, I changed the ratio of nitrate to other nutrients to see if this affected the growth of lettuce (Gent, 2003). In those experiments, lettuce required more nitrate for growth in summer than in winter. Is the RGR for lettuce and spinach predicted by the same environmental variables for all these experiments?
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