Factors Influencing Diurnal and Seasonal Fine Root Growth in Grapevines

Introduction: Grapes are one of the world’s most economically important fruit crops. Most of the grape cultivars are grafted onto rootstocks derived from one Vitis species or hybrids of two or three species. Commonly used rootstock varieties in vineyards do not only contribute to the management of aboveground parts such scion vigour, fruit composition and yield, but also allow for the genotype specific root system to adapt to a particular soil environment. The genotypic variation in root distribution provides an opportunity to select rootstocks based on suitability of root traits to certain environmental conditions. The aim of this study was to ascertain whether grapevine root systems have a clear diurnal and seasonal growth dynamics and whether it is varied across different rootstocks in response to soil temperature, air temperature or a circadian clock, or a combination of these factors.

Material and methods: Three separate pot experiments were carried out over a two year period and two field experiments were conducted over a seven year period. The pot experiments utilized one-year-old non-grafted vines of Shiraz (Vitis vinifera), and the rootstocks Ramsey (V. champinii), 140 Ruggeri (V. berlandieri x V. rupestris) and Schwarzmann (V. rupestris x V. riparia). These species were grown in 3.2 L tapered rectangular pots with two sides made of transparent acrylic. Each pot was in turn placed into a larger container filled with wet sand in order to exclude light from the transparent sides. Each pot could be temporarily removed so that root growth could be imaged using a flatbed scanner. Pot Experiment 1 was undertaken outside under natural conditions while Experiment 2 was conducted in controlled environment chambers under a constant temperature of 22 °C with progressively shifting and shortening day length. The vines were exposed to a typical ambient photoperiod and a consistent temperature of 22 °C in the first two days and the last three days of the experiment. However, temperature was increased to 32 °C from day 3 to day 7 for monitoring root growth responses to soil temperature. Pot Experiment 3 utilized six-year-old Shiraz vines grown under field-like conditions in 780 L soil-filled plastic bins. Root growth was monitored using minirhizotron tubes installed in each bin. The experiments ran for 5 to 10 days, with root growth assessments carried out 3 to 5 times daily. In the field rootstock trial, root growth was monitored in the same four genotypes, grafted to Shiraz
vines, and 169,600 images were collected by using a minirhizotron camera over the 2007/2008, 2008/2009, 2009/2010, 2012/2013 and 2013/2014 growing seasons. The trial was located at the National Wine and Grape Industry Centre, Charles Sturt University, Wagga Wagga, NSW Australia (35o
05'S 147o 35'E). Air temperature was recorded over the seven-year period while soil temperature and soil moisture sensors were monitored at the soil depths of 10, 30 and 60 cm over the 2012/2013 and 2013/2014 growing seasons. For the post-harvest study, root growth in response to three post-harvest irrigation strategies were compared using mature field-grown Shiraz. These included no post-harvest irrigation (NPHI), early post-harvest irrigation (EPHI) and late
post-harvest irrigation (LPHI). The EPHI treatment maintained soil moisture in the readily available range for a period of 15 days after harvest. The LPHI treatment was applied at 30 days after harvest, and similarly maintained soil moisture for a period of 15 days. Minirhizotron tubes were used to monitor root growth across all three treatments. The data of the five experiments were then analysed using Asreml in R software (version 3.2).

Results: Under both pot and field-like conditions of these root growth studies, the elongation rate of actively growing roots was found to have a pronounced diurnal dynamics. Maximum growth rates, which ranged from 0.15 to 0.38 mm hour-1across the three experiments, were highest in the afternoon and two hours after darkness, growth rates declined through the night and reached a minimum the next morning. This dynamics was observed across all genotypes with different levels of growth between them. In addition, these growth dynamics were evident in the small potted vines and the larger container grown Shiraz. Under the naturally fluctuating environmental conditions of Experiment 1, root growth was positively correlated with temperature. However, under the controlled environmental conditions of Experiment 2, a diurnal growth
dynamics was observed during a typical photoperiod, regardless of temperature
(constant or increased). Interestingly, the maximum root extension rate decreased when the soil temperature was increased to around 31 °C. However the rate of shoot growth rate increased in this higher temperature zone, suggesting that shoot dynamics are dominated by genotype, but root dynamics are dominated by temperature related variables, in the context of the typical photoperiod treatment. Under the conditions of progressively shortening days, continuous darkness was reached after 7 days. This caused a concurrent reduction in daily root growth rate, although root growth did not completely cease until early on day 10. During the last three days, a reduced diurnal
fluctuation was still present despite continuous darkness. Under the progressively shortening photoperiod conditions, genotype played a dominant role only on average root length, while soil temperature was a main factor regulating average root length/growth rate. These findings suggest that the decline in root growth rates in response to a decreasing photoperiod may potentially be related to the dependence of fine root growth on carbohydrate supply from photosynthesis. The maintenance of a reduced diurnal growth dynamics into the period of continual darkness may also suggest a contribution from mechanisms that maintain physiological processes in alignment with the diel cycle.

Under field conditions, vertical root distribution dynamics through the soil profile were genotypic specific. Roots of 140 Ruggeri were found in the top 10 cm to 60 cm while most of the roots of Ramsey were scattered between 20 cm to 40 cm. The root system of Schwarzmann was mainly distributed between 10 cm and 40 cm while own rooted Shiraz was dominantly found growing at depths between 20 cm to 60 cm. In terms of root population, 140 Ruggeri had the highest number of roots followed by Ramsey, Schwarzmann and own rooted Shiraz had the lowest number. The seasonal root growth dynamics varied between genotypes. While there was some seasonal variation in the periods and extent of root growth, some overall trends were apparent. Strong new root
flushes occurred around bud break, and were most pronounced at flowering with the least growth occurring after harvest. Surprisingly, some root growth activity was still apparent in winter when the aboveground components of the vine were dormant. These seasonal dynamics were significantly related to air temperature, development stage and genotype.

Conclusion: In summary, fine root growth rates of grapevines were found to have a pronounced diurnal dynamics that was independent of genotype. Soil temperature was found to modify the amplitude of this dynamics, but it was hypothesised that carbon supply from photosynthesis and possibly the circadian clock may play more dominant roles in its regulation. The variation in seasonal root growth dynamics and distribution was dependent on genotype, air temperature, development stage and growing season. The mechanisms linking diurnal root growth dynamics at different phenological stages and its relationships with seasonal root growth dynamics and carbon supply needs
further investigation. In addition, the elucidation of genotypic variation in root growth behaviour is also relevant to determining their suitability for particular environments.