Читать книгу Cucurbits - James R. Myers - Страница 20

Most cucurbits grow rapidly in warm weather, with stem extension growth outpacing leaf development in the tuberous perennials. In a single growing season, a wild buffalo gourd plant produced 360 shoots covering an area 12 m in diameter with a total vine length of over 2000 m (Dittmer and Talley, 1964). Among the annuals, bottle gourd stems can elongate up to 60 cm in 24 h, and wild cucumber (Echinocystis lobata), which is adapted to the short growing seasons of southern Canada, is considered one of the fastest growing vines. Holroyd (1914) reported that a single annual squash (C. pepo) plant produced 450 leaves on a vine measuring 43 m. Cucurbit root growth is also rapid, occurring at a rate of 6 cm per day for squash when conditions are favourable. Elite cultivars of pickling cucumber will go from seed planting to first harvest in 39 days if grown at the optimum growth temperature (32°C).

In large-fruited vining squash plants, total leaf area increases exponentially throughout the season until fruit set creates a large reproductive sink and vegetative growth is suppressed. During the period between flower primordia initiation and the start of fruit development in West Indian gherkin, differentiation of new vegetative organs decreases, the growth rate of existing vegetative organs increases, and water and nutrient intake drops; soon after fruit development begins, vegetative differentiation and water and nutrient intake resurge (Hall, 1949).

Biomass productivity differs among species and cultivars and is influenced by cultural practices (e.g. planting density, irrigation, fertilizer application) as well as local environmental conditions. Environmental factors most affecting growth rates are photoperiod and ambient temperature, either of which can affect the intake and effective utilization of water and nutrients. Many investigations of the relationship between growth and environmental factors have been carried out on species of Cucumis, particularly cucumber. The results of these studies, which were discussed in detail in Whitaker and Davis (1962) and Wien (1997), are summarized below.

1. The growth rate curve for a single leaf under continuous light is generally an S curve, but is affected by light intensity.

2. The rate of stem elongation is greater during 8 h days than during 16 h days, and plants grown under short-day conditions produce more nodes and leaves, but smaller total leaf and root areas.

3. Overall stem length may be greater under a long-day versus a short-day regime when nitrogen levels are high.

4. Under low-nitrogen conditions, plants grown during long days contain more carbohydrates at anthesis than plants grown during short days, but this carbohydrate relationship is reversed at fruit maturity.

5. Stem extension and leaf area growth rates are linearly dependent on mean ambient temperature during periods of optimum temperatures for growth (20–30°C, depending on other environmental conditions). However, in the range of 15–27°C, Grimstad and Frimanslund (1993) found that plant dry weight of cucumber had a sigmoidal response curve with inflections at about 17°C and 24°C.

6. When temperature rises above the optimum, leaf growth rate in young plants declines as material is redistributed to the stems, and cell division in developing leaves is reduced.

7. At below-optimum temperatures, relative leaf growth rate is independent of temperature and is controlled instead by light intensity.

8. Stem extension rates are lower than normal when night temperatures exceed day temperatures.

9. Low temperatures slow the development of apical buds.

In C. pepo, bushy plants with short internodes possess an allele that reduces biosynthesis of endogenous gibberellin. When these plants are treated with a high concentration (2.9–4.3 mmol l⁻¹) of gibberellic acid, internode lengths become as long as those of naturally viny squash plants.

Breeders have also selected for bush cultivars in C. maxima. Research on bushy versus viny plants of this species indicates that bush cultivars have a more uniform growth pattern, respond better to high-density planting, and produce a greater percentage of fruit versus vegetative biomass during short growing seasons (Loy and Broderick, 1990). This last effect is partly due to the fact that fruiting begins sooner in bush cultivars, which in turn suppresses vegetative growth. However, photosynthetic rates in bush plants increase during fruit set in response to increasing sink demand, which may be possible because of a proportionally thicker palisade layer in bush plant leaves (Loy and Broderick, 1990).

In climbing cucurbits, the stems often revolve, twist and extend upwards. Darwin (1906) made several interesting observations on the revolving nature of cucurbit stems and tendrils. He noted that the average rotation rate was 100 min per revolution in wild cucumber. Light affects this movement, with stem tips, including the two uppermost internodes subtending the apical meristem, following the sun throughout the day.

Generally, the slightly curved tendril becomes sensitive to mechanical stimulus on its concave side when it is one-third grown. At that stage, it reacts quickly (in under 2 min), with coiling caused by the elongation of parenchymatous cells on the convex side of the tendril. The revolving movement of a tendril does not stop after it has coiled, but its ability to coil is limited after it stops revolving.