Responses of temperate plants to drought conditions

Effects on: photosynthesis

Overview of photosynthesis

Photosynthesis is essential for the growth and survival of virtually all plants on earth. In the presence of light, photosynthesis allows plants to combine water and CO2 into oxygen and energy the plant can use for maintence and growth. For a review of photosynthesis look at photosynthesis diagrams.

Photosynthesis is sensitive to a number of environmental conditions, including light, temperature, CO2 concentrations, nutrient supply and water supply. In the case of drought, photosynthesis can be greatly reduced. The primarily means for this are through:

• stomatal limitations
• non-stomatal limitations
  

Stomatal limitations

Stomates are constantly reacting to environmental stimulii, with responses that can occur on the order of seconds. The primary environmental factors that influence stomatal conductance are similar to those that affect photosynthesis: light, temperature, humidity, and internal CO2 concentration. Stomatal control is often thought to be the first line of defense against water stress.

What signals stomates to close in response to soil drought?

There are two major ideas as to what actually causes stomates to close:

• hydraulic signals (leaf water potential, cell turgor)
• chemical signals (abscisic acid (ABA))

The earliest idea for control of stomates in response to soil dryness was that as water supply decreased, leaf water potential and cell turgor declined, and stomatal closure was promoted. However, examples began emerging in which stomatal conductance was reduced, but no reduction in leaf water potential was observed. In these cases, the response of stomates seemed to follow soil water potentials more closely than leaf water potentials. Research came out demonstrating that for some cases, the closing correlated well with abscisic acid (ABA) concentrations in leaves. This ABA is thought to be produced in plant roots and tranported via the xylem stream to stomatal guard cells (Zhang and Davies 1989; Zhang and Davies 1990). It is not entirely clear what triggers the production of the ABA within the roots, but it may be related to cell turgor there. Currently most researchers seem to find evidence for a combination of both types of signalling occurring together or at different times (Comstock 2002; Aasamaa et al. 2002). For more information on the effect of ABA on a variety of plant processes.

Stomatal limitations can easily be detected by a measured decrease in stomatal conductance.

Non-stomatal limitations

Non-stomatal limitations refer to reductions in photosynthetic capacity. These can include reductions in:

• carboxylating enzymes
• electron transport
• chlorophyll content

Each of these things are integral to the photosynthetic process. Electron transport as well as Rubisco activity (carboxylation) are both considered to be fairly resistant (Yordanov 2000) to water stress. However, chlorophyll destruction, and a decrease in photophosphorylation have been observed.

In measurement situations, non-stomatal limitations are implied when internal CO2 concentrations are high (suggesting a decrease in carboxylation efficiency) and can be measured as decreases in photosystem 2 activity (decrease in electron transport) (Ramanjulu et al. 1998).

Relative importance of stomatal vs. non-stomatal limitations

Stomatal limitations are often thought to be the short term response to drought stress, whereas the non-stomatal effects are usually considered to be more important during longer and more severe water stresses. However, many researchers have found that even in the short term, depression of photosynthesis cannot be attributed entirely to stomatal limitations (Ni and Pallardy 1992; Ramanujulu et al. 1998; Yordanov et al. 2000). The relative importance of stomatal vs non-stomatal limitations seems to depend both on the severity and length of the stress and on the drought tolerance of the species or genotype, with non-stomatal limitations generally being less important for plants with greater drought tolerance or during milder stresses (Ramanjulu et al. 1998).

Potential for other reductions in photosynthesis

Depending on the species, and the severity of the drought, the largest reductions in photosynthesis may actually be from leaf area reductions, due to leaf shedding, or by the delay of leaf growth and development (Jones 1992).

When drought stress is finally relieved, photosynthesis may not return immediately to pre-drought levels. The recovery time can vary greatly by species, with the harmful effects of the drought sometimes lasting from weeks to months. This can occur if stomates are slow to regain their conductance, or photosynthetic machinery is damaged (Kozlowski and Pallardy 1997). Of course, whole plant photosynthesis can also be slow to recover if significant leaf area was lost as a result of the drought.