Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

RHIZOSPHERE MICROORGANISMS 361
the utilization of biofertilizers a feasible practice of production. Vessey (2003) proposed
that biofertilizer be defined as a substance that contains living microorganisms which, when
applied to seed, plant surfaces, or soil, colonizes the rhizosphere or the interior of the plant
and promotes growth by increasing the supply or availability of primary nutrients to the
host plant. Marketable biofertilizers are mainly based on Rhizobium, PGPRs, and AMF.
Probably, the most ancient are those based on Rhizobium, which in addition to enhance
yield, improve grain quality by increasing protein content and in some cases the lipid content
(e.g., soybean) due to their nitrogen-fixing activity (Achakzai et al., 2003; Hayat and
Ali, 2004). Not all the beneficial rhizosphere microorganisms exert their positive effect on
plant growth via increasing nutrient status of host plants. PGPR are long known to promote
growth through suppression of plant disease acting as bioprotectants (Kloepper, 1993), or
through production of phytohormones and peptides acting as biostimulants (Glick et al.,
1998; Jiménez-Delgadillo, 2004).
Although the beneficial influence of AMF on the growth of crops is mostly due to the
better mineral nutrition, it is known that they protect plants against cultural and environmental
stresses (Smith and Read, 1997). AMF-plant symbiosis can also alter plant water
relations and responses to drought (Augé, 2001), photosynthesis rate (Aguilera-Gómez
et al., 1999), and provide host plant with growth hormones like auxins, cytokinins, gibberellins,
and growth regulators such vitamin B (Gupta et al., 2000).
Product quality as determined by the ultimate consumer is affected by both pre- and
postharvest factors (Schreiner et al., 2000). A wide variety of preharvest factors influence
the quality of the harvested product. These include (1) biological factors (pathological,
entomological, animal); (2) physiological factors (physiological disorders, nutritional
imbalances, maturity); (3) environmental (e.g., climate, weather, soils, water relations,
and light intensity), cultural factors (fertilization, growth regulators); (4) extraneous matter
(growing medium, vegetable matter, chemical residues); and (6) genetic (cultivars,
aberrations) (Kays, 1999; Mattheis and Fellman, 1999; Sams, 1999). It is evident that
a diverse range of biotic and abiotic factors can alter agricultural product quality. We
propose rhizosphere microorganisms to be considered as preharvest biotic factors affecting
fruit and vegetable quality. This is why some interactions in the rhizosphere are of
current concern and their effects on crop yield and quality are described and discussed
here.
17.2 Plant growth–promoting rhizobacteria
Rhizobacteria that exert beneficial effects on plant development are referred to as plant
growth–promoting rhizobacteria (Kloepper et al., 1980), because their application is often
associated with increased rates of plant growth. PGPR enhance plant growth by direct and
indirect means, but the specific mechanisms involved have not been well characterized
(Glick et al., 1998).
With regard to the indirect mechanisms for growth promotion, one of the most important
is biological control, and consequently the applied work with PGPR has mainly been
based on their role in biological control. Bacteria in the genera Streptomyces, Burkholderia,
Agrobacterium, Pseudomonas, and Bacillus are the most studied and commercialized
biological control agents. They suppress plant disease through at least one mechanism:
induction of systemic resistance, antibiotic production, depletion of iron from rhizosphere,