GrainGenes

II.18. Effects of kinetin on photosynthesis and growth response of barley trisomics.

R. G. McDaniel. Department of Agronomy and Plant Genetics, University of Arizona, Tucson, Arizona 85721, U.S.A.

Photosynthetic rates of barley trisomics have been found to be inferior to those of normal diploids (McDaniel, BGN 1:29, 1971). Phenotypic characteristics of barley plants having extra chromosomes 1 and 2 resembled those of plants characterized by hormonal imbalances. Trisomics 1 or 2, under appropriate environmental conditions, exhibit aerial roots and epinasty, characteristic of high auxin concentration (Imber and Tal, Science 169:592, 1970). Additionally, leaves of these trisomics unroll slowly, and are more narrow than those of normal diploids; these effects may relate to cytokinin concentration in leaves. I tested this hypothesis by watering trisomics with exogenous kinetin (10^-6 M) and observing physiological and biochemical responses after 24 hours.

Kinetin affected both degree of leaf unrolling and rate of leaf expansion of plants trisomic for chromosome 1 or 2. These effects could be noted within 4 hours on treated trisomics: observations were then continued for at least 3 days. No significant response of leaf morphology was noted in the kinetin-treated diploid. These data are summarized in Table 1.

Table 1. Width of young leaves (in mm) of kinetin-treated (K) trisomics and diploids of Hordeum vulgare L. cv. 'Betzes'.

Young leaves of barley trisomics 1 or 2 present a somewhat triangular cross-section. This is most noticeable in leaves of trisomic 1, which remain tightly "rolled under a variety of photoperiods and light intensities. Kinetin treatment causes a rapid "unrolling" of leaves of these trisomics. This serves as an easily monitored physiological response to kinetin and resembles the growth regulation-controlled leaf unrolling effects previously described (Poulson and Beevers, Plant Physiol. 46:509, 1970; Beevers, et al. Planta 90:286, 1970).

Effects of exogenous kinetin on plant photosynthetic rates have been described (Wareing, et al. Nature 220:453, 1968; Meidner, J. Exptl. Bot. 18:556, 1967). These experiments have pointed out the relation between cytokinin concentration and rate of protein synthesis in young leaves. The inferior photosynthetic rates I previously observed with barley trisomics prompted me to examine the possible effects of kinetin on barley leaf photosynthetic rates. Data indicated that kinetin treatment elicited a marked increase (27%) in photosynthetic rates of these trisomics. as illustrated in Table 2. Plants trisomic for chromosome 1 gave a similar response.

Table 2. Photosynthesis of leaf disks of kinetin-treated trisomics and diploids of barley. Data are presented in mu liters O₂ evolution per leaf disk per hour at 25°C + the standard error of three experiments, three replications. (See McDaniel, BGN 1:29, 1971, for methods).

Photosynthetic rates of kinetin-treated trisomics greatly exceeded that of untreated trisomics, and approached the rates of the diploids, expressed on an area, unit protein or unit chlorophyll basis. Kinetin treatment also appeared to increase dark respiration of trisomics, although significance has not been established. Photosynthetic measurements using attached leaves monitored in a closed system by I. R. spectrophotometry have verified the data of Table 2 (McDaniel, unpublished). Kinetin treatment also increased the transpiration rate of the trisomics. These and other physiological responses of trisomics to hormone treatment, including stomatal response (Pailas and Box, Nature 227:87, 1970) and senescence (Adedipe et al., Physiol. Plant 25:151, 1971) are presently being investigated. The experiments reported here indicate that barley trisomics are excellent systems for biochemical studies on hormonal plant growth regulation.

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