Leaf temperature

Fermentation. The term fermentation arose from the misconception that black tea production is a microbial process (73). The conversion of green leaf to black tea was recognized as an oxidative process initiated by tea—enzyme catalysis circa 1901 (74). The process, which starts at the onset of maceration, is allowed to continue under ambient conditions. Leaf temperature is maintained at less than 25—30°C as lower (15—25°C) temperatures improve flavor (75). Temperature control and air diffusion are faciUtated by distributing macerated leaf in layers 5—8 cm deep on the factory floor, but more often on racked trays in a fermentation room maintained at a high rh and at the lowest feasible temperature. Depending on the nature of the leaf, the maceration techniques, the ambient temperature, and the style of tea desired, the fermentation time can vary from 45 min to 3 h. More highly controlled systems depend on the timed conveyance of macerated leaf on mesh belts for forced-air circulation. If the system is enclosed, humidity and temperature control are improved (76). 

Fig. 1. Rates of CO2 assimilation, A (/miol s ) leaf conductance, g (mol m s ) intercellular partial pressure of CO2, Pi (Pa) soil water potential and leaf water potential, xp (MPa) during gas-exchange measurements of a 30-day-old cotton plant, plotted against day after watering was withheld. Measurements were made with 2 mmol m sec" photon flux density, 30 °C leaf temperature, and 2.0 kPa vapour pressure difference between leaf and air (S.C. Wong, unpublished data).

Fig. 2. Rates of CO2 assimilation,. 4, and leaf conductances, g, as functions of intercellular partial pressure of CO2, p in Zea mays on various days after withholding watering. Measurements made with 9.5,19.0,30.5, and 38.0 Pa ambient partial pressure of CO2, 2 mmol m" s" photon flux density, 30 °C leaf temperature, and 2.0 kPa vapour pressure differences between leaf and air. Closed symbols represent measurements with 30.5 Pa ambient partial pressure of COj. Leaf water potentials were 0.05, - 0.2, - 0.5 and - 0.8 MPa on day 0, 4, 11 and 14, respectively (after Wong et al., 1985).

It is desirable to prevent leaf temperature from rising above 35°C during the maceration process to preserve quality. 

The barbaqua step may be carried out in a specially constructed room with a frame above the floor to contain the leaves that are dried with hot air conducted from a fire. Leaf temperatures reach 80 to 100°C. Some caffeine is lost at the higher temperature. 

In desert areas of southern California fruit are often injured but leaves are seldom injured by sulfur dust. In coastal areas fruit burn is less marked but leaf burn may be acute. Where the air-vapor density is high, leaf temperatures in the sun may sometimes become higher than fruit temperatures. The leaf, a better absorber of radiation and a better radiator than the fruit, has a higher surface-mass ratio and appears to be very sensitive to the heat trap effect of high vapor density its temperature changes with great rapidity, but fruit temperature may lag until the danger period is passed (18). 

FIGURE 6.25 Effect of willow leaf temperature on isoprene emission rate (adapted from Fall and Wildermuth, 1998 and Fall, 1999). 

Gates DM (1968) Transpiration and leaf temperature. Ann Rev Plant Physiol 19 211-238 Gates DM (1976) Energy exchange and transpiration. In Water and Plant Life. Lange OL, Kappan L, Schulze E-D (eds) Springer Verlag, Berlin, p 135-147... 

Leaf temperature (T,eaf) Temperature in °C at the leaf surface 27—37, varies3 Monti et al., 2005... 

The tops of Jerusalem artichoke can also be cut off at 1.5 m to reduce the likelihood of wind damage (Wood, 1979), which in this instance prevented the plants from flowering. Wind also affects leaf temperature and the rates of photosynthesis and transpiration, with resulting but minor effects on growth rate (Meyer et al., 1973). 

Equation 7.1 describes the case in which the leaf temperature is greater than the temperature of the air when the leaf temperature is less than that of the surrounding turbulent air, heat moves into a leaf. Also, when water condenses onto a leaf, the leaf gains heat. In such cases, the appropriate energy terms in Equation 7.1 change sign. 

We will now consider the amount of energy that can be stored because of changes in leaf temperature. For purposes of calculation, we will assume that a leaf has the high specific heat of water (4.18 kJ kg-1 °C 1 at 20° C Appendix I), where specific heat is the energy required to raise the temperature of unit mass by one degree. We will further assume that the leaf is 300 pm thick (e.g., Fig. 1-2) and has an overall density of700 kg m-3 (0.7 g cm-3) — a leaf is often 30% air by volume. Hence, the mass per unit leaf area in this case is... 

To estimate the IR radiation emitted by a leaf, we will let Cir be 0.96 and the leaf temperature be 25°C. By Equation 7.7, the energy loss by thermal radiation then is... 

Certain plants have silvery or shiny leaves, which increases the amount of solar irradiation reflected. For instance, the fraction of S reflected by the leaf may increase from typical values of 0.1 or 0.2 (see Fig. 7-4) to 0.3 for silvery leaves, with an accompanying reduction in the absorptance from 0.6 to 0.5 or lower. This reduction in absorptance can have a major influence on T eaf. Other conditions remaining the same, a reduction of the absorptance a by only 0.1 can cause the leaf temperature to decrease from 24°C to 13°C (Table 7-1). Such a reduction in leaf temperature can have substantial effects on transpiration and photosynthesis, which can be particularly important for desert plants in hot environments. For instance, seasonal variations in... 

Leaf temperature Air temperature Relative humidity Wind speed... 

Many observations and calculations indicate that exposed leaves in sunlight tend to be above air temperature for T a up to about 30°C and below Fta for air temperatures above about 35°C. This primarily reflects the increasing importance of IR radiation emission as leaf temperature rises [see Eq. 7.7 . /ir = 2eiRcr(Tleaf)4] and the increase with temperature of the water vapor concentration in the leaves, which affects transpiration (discussed in Chapter 8, Section 8.2D). Such influences often lead to temperatures for exposed leaves that are more favorable for photosynthesis than is the ambient air temperature. We can readily extend our analysis to include leaves shaded by overlying ones. For certain plant parts, heat storage and heat conduction within the tissues are important. We will conclude this section with some comments on the time constants for changes in leaf and stem temperatures. 

D. If the net radiation balance for the leaf is 300 W m-2, what is the transpiration rate such that the leaf temperature remains constant ... 

Leuning, R. 1987. Leaf temperatures during radiation frost part II. A steady state theory. Agric. Forest Meteorol. 42 135-155. 

In Equation 8.27, Vmax and, to some extent, Kcch depend on the photosynthetic photon flux (PPF), temperature, and nutrient status. For instance, Vmax is zero in the dark because photosynthesis ceases then, and it is directly proportional to PPF up to about 50 jimol m-2 s-1. If we continually increase the PPF, Fmax can reach an upper limit, its value for light saturation. This usually occurs at about 600 junol m-2 s-1 for most C3 plants, whereas photosynthesis for C4 plants is generally not light saturated even at full sunlight, 2000 pmol m-2 s-1 (see Chapter 6, Section 6.3D for comments on C3 and C4 plants also see Fig. 8-20 for responses of leaves of C3 plants and a C4 plant to PPF). Photosynthesis is maximal at certain temperatures, often from 30°C to 40° C. We note that Vmax increases as the leaf temperature is raised to the optimum and then decreases with a further increase in temperature. 

Equation 8.28 summarizes the overall steady-state balance of CO2 fluxes for leaves. These fluxes depend on temperature in different ways (Fig. 8-16). Net photosynthesis and respiration plus photorespiration can occur at leaf temperatures slightly below 0°C. Inhibition of gross photosynthesis can occur at temperatures above 35 to 40°C for many C3 species, often primarily reflecting damage to Photosystem II (e.g., Chapter 5, Section 5.5A). In such cases, net CO2 uptake (Eq. 8.28) can be optimal near 30°C (Fig. 8-16). 

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